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Geological Survey of Western Australia

RECORD 2014/9

THE APPLICATION OF PASSIVE SEISMIC TO ESTIMATE COVER THICKNESS IN GREENFIELDS AREAS OF WESTERN AUSTRALIA — METHOD, DATA INTERPRETATION AND RECOMMENDATIONS

by

AJ Scheib

Government of Western AustraliaDepartment of Mines and Petroleum

Record 2014/9

THE APPLICATION OF PASSIVE SEISMIC TO ESTIMATE COVER THICKNESS IN GREENFIELDS AREAS OF WESTERN AUSTRALIA — METHOD, DATA INTERPRETATION AND RECOMMENDATIONS

byAJ Scheib

Perth 2014

MINISTER FOR MINES AND PETROLEUM

Hon. Bill Marmion MLA

DIRECTOR GENERAL, DEPARTMENT OF MINES AND PETROLEUM

Richard Sellers

EXECUTIVE DIRECTOR, GEOLOGICAL SURVEY OF WESTERN AUSTRALIA

Rick Rogerson

REFERENCE

The recommended reference for this publication is:

Scheib, AJ 2014, The application of passive seismic to estimate cover thickness in greenfields areas of Western Australia — method,

data interpretation and recommendations: Geological Survey of Western Australia, Record 2014/9, 67p.

National Library of Australia Card Number and ISBN 978-1-74168-567-1

Grid references in this publication refer to the Geocentric Datum of Australia 1994 (GDA94). Locations mentioned in the text are

referenced using Map Grid Australia (MGA) coordinates, Zones 51 and 52. All locations are quoted to at least the nearest 100 m.

Disclaimer

This product was produced using information from various sources. The Department of Mines and Petroleum (DMP) and the State

cannot guarantee the accuracy, currency or completeness of the information. DMP and the State accept no responsibility and disclaim

all liability for any loss, damage or costs incurred as a result of any use of or reliance whether wholly or in part upon the information

provided in this publication or incorporated into it by reference.

Published 2014 by Geological Survey of Western Australia

This Record is published in digital format (PDF) and is available online at <www.dmp.wa.gov.au/GSWApublications>.

Further details of geological products and maps produced by the Geological Survey of Western Australia

are available from:

Information Centre

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Telephone: +61 8 9222 3459 Facsimile: +61 8 9222 3444

www.dmp.wa.gov.au/GSWApublications

iii

Figures

1. The Tromino at passive seismic acquisitions sites near Mulga Rock and in the Eucla Basin ....................3

2. Map of southern Western Australia showing the three study areas ..............................................................4

3. Summary of the passive seismic acquisition, method and analysis .............................................................5

4. Average H/V spectral ratio for data acquired at site 1425, stability plot, and input parameters ..................7

5. H/V spectral ratios for site PNC18 showing effect of engine noise on signal .............................................9

6. Stability plots and corresponding H/V spectra for MAD002 before and after cleaning ............................10

7. H/V spectra of three acqusitions recorded at drillhole sites in the Eucla Basin .........................................11

8. Graphs comparing the H/V spectra acquired in 2012 and 2013 at sites RC1302 and CD1252 .................11

9. Distribution of passive seismic acquisitions in the Mulga Rock trial area .................................................12

10. Generalized cross-section at Mulga rock along the PNC Baseline ............................................................16

11. Selected H/V spectral ratios and Fourier plots for six sites at Mulga Rock ...............................................18

12. Resonance frequencies corresponding to the peaks in H/V spectral ratios from the PNC Baseline

and Emperor traverse in 2012 and 2013 .....................................................................................................20

13. Cover thickness estimates for sites at Kakarook corner .............................................................................22

14. Plot of resonance frequencies and depth to basement for nine control sites along the PNC Baseline

and Emperor traverse ..................................................................................................................................25

15. Cross-section showing depth to basement for 89 sites along the PNC Baseline and Emperor traverse ........25

Contents

Abstract ..................................................................................................................................................................1

Introduction ............................................................................................................................................................1

Materials and methods ...........................................................................................................................................2

What is passive seismic? .................................................................................................................................2

The single-station H/V method .......................................................................................................................2

The Tromino: technical details and acquisition settings .................................................................................4

Data transfer and analysis ...............................................................................................................................6

Data quality .....................................................................................................................................................6

Shear-wave velocity measurements ................................................................................................................8

Results and data analysis ......................................................................................................................................10

Mulga Rock ...................................................................................................................................................10

Geology ..................................................................................................................................................12

Passive seismic data ...............................................................................................................................14

Shear-wave velocity ...............................................................................................................................20

H/V analysis and data interpretation ......................................................................................................20

The Eucla Basin ............................................................................................................................................25

Geology ..................................................................................................................................................26


Shear-wave velocity ..............................................................................................................................30


The Boorabbin sand resource survey – an application ..................................................................................37



Conclusions ..........................................................................................................................................................47

Acknowledgments ................................................................................................................................................47

References ............................................................................................................................................................47

Appendices

1 Horizontal to vertical (H/V) spectral ratios for sites at Kakarook Corner at Mulga Rock.........................49

2 Horizontal to vertical (H/V) spectral ratios for sites along the PNC Baseline at Mulga Rock ..................50

3 Horizontal to vertical (H/V) spectral ratios for sites along the Emperor Traverse at Mulga Rock ............58

4 Horizontal to vertical (H/V) spectral ratios for sites in the Eucla Basin ....................................................60

5 Horizontal to vertical (H/V) spectral ratios for sites along the Transline at Zanthus at the western

edge of the Eucla Basin ..............................................................................................................................64

6 Photographs of drillcore of sedimentary rocks from hole Empress 1/1A, FOR004 and FOR011 .............65

7 Selection of waveforms of elastic pulse transmitted through core samples ...............................................67

iv

16. Distribution of passive seismic acquisition sites in the Eucla Basin trial area ...........................................27

17. Distribution of passive seismic acquisition sites along the Transline access road at Zanthus ...................29

18. Simplified stratigraphic correlation between seven drillholes from the Haig and Forrest areas,

Eucla Basin .................................................................................................................................................30

19. Selected H/V spectral ratios and Fourier plots for four sites in the Eucla Basin .......................................31

20. Resonance frequencies from peaks in the H/V spectral ratios of acquisitions taken in the Eucla Basin .......33

21. Resonance frequencies from peaks in H/V spectral ratios for acquisitions taken at Zanthus,

western Eucla Basin ...................................................................................................................................33

22. Regression plots for passive seismic and depth data for six drillhole sites in the Eucla Basin..................34

23. Depth to basement based on cover thickness estimates for sites in the Ecula Basin .................................36

24. Cover thickness estimates and the depths of basement for seven sites along a 4-km section at Zanthus ......37

25. Location of passive seismic acquisition sites across the Boorabbin sand resource survey ........................38

26. H/V spectral ratios and Fourier plots for control sites in the Boorabbin area ............................................40

27. Selected H/V spectral ratios and Fourier plots for sites 47, 113, 72 and 124, Boorabbin area ..................43

28. H/V spectral ratio plots of four 14 minute recordings at site 48 using various settings ............................44

29. Two power-law regression models based on site resonance frequencies and sand thickness from

drill logs at control sites .............................................................................................................................45

Tables

1. Summary of published average shear-wave velocities .................................................................................5

2. Approximate values for the Poisson ratio and bulk density .........................................................................7

3. Locations of 89 passive seismic acquisition sites in three areas at Mulga Rock recorded during

2012 and 2013 ............................................................................................................................................13

4. Summary of drillhole log descriptions for sites along the PNC Baseline and Emperor traverse ...............15

5. Lowest resonance frequencies derived from peaks in the H/V spectra for 89 sites at Mulga Rock ..........17

6. Laboratory-determined shear-wave velocities for core samples from Empress 1/1A, FOR004

and FOR11 .................................................................................................................................................21

7. Modelled shear-wave velocities at drillhole sites that have depth to basement information, PNC

Baseline and Emperor traverse ...................................................................................................................23

8. Summary of resonance frequencies for sites along the PNC Baseline and Emperor traverse ...................23

9. Site locations of passive seismic recordings in the Eucla Basin and Zanthus areas ..................................28

10. Lowest resonance frequencies derived from H/V spectral ratios for 42 sites in the Eucla Basin ..............32

11. Modelled shear-wave velocities for sedimentary rocks and underlying basement at six drillhole

sites in the Eucla Basin...............................................................................................................................34

12. Cover thickness and depth of basement estimates for 35 sites in the Eucla Basin ....................................35

13. Known sand thickness and corresponding resonance frequencies for control sites in the Eucla Basin ........39

14. Resonance frequencies based on H/V spectral ratios of 26 sites in the Boorabbin study area ..................42

15. Estimates of sand thicknesses for the nine control sites in the Boorabbin study area ...............................46

16. Estimates of sand thicknesses for the 26 test sites in the Boorabbin study area ........................................46

GSWA Record 2014/9 The application of passive seismic to estimate cover thickness in greenfields areas of WA

1

The application of passive seismic to estimate

cover thickness in greenfields areas

of Western Australia — method, data

interpretation and recommendations

by

AJ Scheib

AbstractPassive seismic has been used to estimate the thickness of cover in parts of the Gunbarrel and Eucla Basins in Western

Australia, and in the eastern Yilgarn Craton, using a three-component, broadband, single-station seismometer (Tromino;

Micromed, 2009, 2012). At each measurement site, the horizontal (H) and vertical (V) component of the ambient subsurface

noise was recorded. Peaks in the H/V spectral ratio correspond to impedance contrasts between subsurface units. The

frequency at which these impedance contrasts are found can then be modelled in terms of depth, providing an estimate of

cover thickness. Results from both the Gunbarrel and Eucla Basins provide cover thickness estimates consistent with drilling

data, and there is broad agreement between estimated and augered thickness of sand cover in the eastern Yilgarn Craton.

In the Gunbarrel Basin, the thickness of a range of cover units has been determined from 5 m-thick Quaternary sandplain

deposits through to >700 m of Cenozoic to Permo-Carboniferous sedimentary rocks overlying crystalline basement. In the

Eucla Basin, cover thickness estimates indicate a depth to basement of between 200 and 400 m. In the Boorabbin area of the

eastern Yilgarn Craton, passive seismic has been used to provide estimates of thicknesses for a sand resource study.

Single-station passive seismic is a rapid, non-invasive and low-cost technique, and provides a viable alternative to conventional

methods of estimating cover thickness, such as drilling, reflection seismic, or airborne electromagnetics. Furthermore, data

can be easily processed, meaning that cover thickness information is available immediately for planning drilling campaigns or

geochemical surveys. However, some degree of depth control, ideally from drillholes, is necessary for effective interpretation

of passive seismic data.

KEYWORDS: regolith, sedimentary basins, geophysical exploration, shear-wave velocity, seismic

IntroductionThis Record describes the application and assessment of passive seismic to estimate cover thickness in three areas of Western Australia. As near-surface mineral deposits are increasingly hard to find, future exploration efforts must expand into areas under cover (UNCOVER, 2012). In these areas, information on the thickness of cover (taken to include both regolith and Phanerozoic sedimentary rock successions) is essential. However, in these greenfields areas, information on the depth of cover from, for example, drilling, airborne electromagnetics, or reflection seismic, is unevenly distributed or non-existent, and is costly and time-consuming to obtain. An alternative approach is passive seismic, which is a rapid, non-invasive approach, utilizing relatively cheap technology and requiring only basic data processing. The passive seismic approach has been successfully used

to estimate the thickness of glacial sediments in North America (Lane et al., 2008), sedimentary rocks in Spain (Delgado et al., 2000) and Germany (Parolai et al., 2002; Ibs-von Seht and Wohlenberg, 1999), and the depth to basement in central Australia (Smith et al., 2013). It is also used in hydrocarbon exploration, commonly in conjunction with 3D reflection seismic (Rode et al., 2010). In that case, passive low-frequency techniques are particularly efficient as a screening tool to identify areas of hydrocarbon potential, which are then investigated by more expensive and time-consuming methods (Graf et al., 2007). In contrast to the single-station method discussed in this Record, the passive seismic approach in the oil and gas industry is based on multi-station monitoring systems and arrays (n = 20 to 100; Duncan 2005). These array-based methods are also more frequently used in seismic ambient noise tomography studies to help delineate deep crustal structures (Saygin and Kennett, 2012).

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2

The Geological Survey of Western Australia (GSWA) has recently acquired the Tromino single-station, passive seismic system (Fig. 1). This has been used to estimate the thickness of cover (or depth to basement) in three trial areas: the Gunbarrel Basin, the central part of the Eucla Basin, and the Boorabbin area in the eastern Yilgarn Craton (Fig. 2). The application and evaluation of this technology is the subject of this Record.

Materials and methods

What is passive seismic?

Passive seismic involves the measurement of ambient (natural) sound waves in the ground, as opposed to reflection seismic which measures sound waves generated by a controlled source, such as a hammer blow or vibration. Accordingly, passive seismic requires a different sensor environment and different equipment from active seismic (Rode et al., 2010), uses lower frequency data (regarded as noise in active seismic applications; Graf et al., 2007), and requires a different approach to data processing.

The passive seismic signal has been described variously as ambient noise (Lane et al., 2008; Claprood, 2012), ambient vibrations (Fäh et al., 2001), microtremors (Nakamura, 1989; Ibs-von Seht and Wohlenberg, 1999; Delgado et al., 2000; Okada, 2004; Smith et al., 2013), or seismic background noise (Rode et al., 2010). It includes weak, low-amplitude and continuous noise or vibration in the Earth’s surface found at usually low frequencies (<10 Hz), resulting from ocean waves, rainfall and wind, as well as distal anthropogenic activities (Okada, 2004; Claprood, 2012; Ibs-von Seht and Wohlenberg, 1999; Lane et al., 2008). Okada (2004) used data from Peterson (1993) to show that, even in the middle of large continents, background seismic vibrations could be detected, although in a passive seismic program in central Australia, Smith et al. (2013) found it beneficial to amplify the ambient noise using the vehicle engine.

The ambient noise signal generated from natural sources is dominated by both Rayleigh and Love waves and is present at more than one wavelength (Arai and Tokimatsu, 2004). The ambient noise spectrum is not only influenced by the source, but also by subsurface conditions, which can cause amplification of seismic waves. This amplification is a function of the thickness and shear-wave velocities of the subsurface layers (Ibs-von Seht and Wohlenberg, 1999). Measuring this amplification has been widely used to mitigate earthquake damage by assessing subsurface conditions, especially the seismic stability of the ground. For example, during a seismic event, which results in a strong ambient noise signal, the subsurface conditions affect the degree of ground shaking. This ground movement is affected by the amplitude, frequency and duration of the horizontal and vertical components of the vibration of seismic waves (Fäh et al., 2001), and

is particularly well amplified where adjacent layers have markedly different shear-wave velocities (e.g. soil and rock). This is reflected in the resonance frequency of a site, which can be measured by recording the horizontal (H) and vertical (V) components of the ambient noise over a certain time, and then calculating the H-to-V ratio. This is referred to as the H/V method, or Nakamura technique (Nakamura, 1989; SESAME European Research Project, 2004), and has been successfully used to derive the shear-wave velocity of the subsurface and predict the ground stability response to earthquake seismicity (Konno and Ohmachi, 1998). The same method can also be applied to estimate the thickness of cover units, whose boundaries with unweathered basement rocks correspond to an impedance contrast (e.g. Lane et al., 2008; Parolai et al., 2002; Ibs-von Seht and Wohlenberg, 1999).

The single-station H/V method

The following summary of the H/V method is based on guidelines and recommendations made by the Site Effects Assessment using Ambient Excitations project (SESAME European Research Project, 2004), and recent publications that have used the H/V method to estimate thickness of soils and sediments. The H/V method is based on a simplified two-layer system, such as regolith overlying bedrock (Fig. 3). In this example, the impedance contrast relates to the regolith–rock boundary, and results from the different shear-wave velocities (Vs1, Vs2) of the two layers (Fig. 3c). The shear-wave velocity (Vs1 or Vs2) is a measure of how fast the ambient noise ‘waves’ travel through a unit. A summary of shear-wave velocities for different materials ranging from soft soils to hard rocks is given in Table 1. The greater the difference in shear-wave velocity between the two units, the larger the impedance is and the greater the peak in the H/V spectral ratio. The frequency (fz) at which the impedance contrast is measured (Fig. 3b) is a function of the depth of the boundary between the two units (Fig. 3d). This relationship between this frequency (fz), the shear-wave velocity of the unit in question (Vs1), and the thickness of the unit (h1; Fig. 3d) is the basis of the passive seismic approach to estimating cover thickness. Clearly, the validity of any thickness estimate relies on a strong impedance contrast across the boundary of the two units (resulting in a sharp peak in Figure 3b), as discussed by Perret (2012) and SESAME European Research Project (2004). Where adjacent units have similar rheological properties, or there is a gradational contact between them, the resulting weaker impedance contrast means that the resonant frequency, and therefore the H/V peak, is less well defined. An example of this is provided by Lane et al. (2008) in a discussion on the determination of till thickness in North America. They pointed out that passive seismic estimates of till thickness showed poor agreement with drilling data in areas where till overlaid weathered bedrock, resulting in a low impedance contrast between the two units. In Western Australia this is an important issue, as large areas of the State are characterized by deep weathering (e.g. Yilgarn Craton; Anand and Butt, 2010).


3

Figure 1. The Tromino at typical passive seismic acquisition sites: a) and b) PNC07 near Mulga Rock; c) MAD002 in the Eucla

Basin, where proposed drillhole location is indicated by the pink stake

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Figure 2. Map of southern Western Australia showing the two trial areas at Mulga Rock in the southern Gunbarrel Basin

and in the central Eucla Basin, and the third area near the Boorabbin sand resource survey. Brown lines are major

roads. Other solid and dashed lines delineate the boundaries of the main sedimentary basins in the area, as labelled

(GSWA, 2007)

The Tromino: technical details and

acquisition settings

In 2012, GSWA acquired the passive seismic system Tromino (Fig. 3a), an ultra-light, compact instrument for high-resolution digital seismic noise measurements (Micromed, 2007). The device weighs 1.1 kg and has dimensions of 14 x 10 x 8 cm. It has very low energy consumption, requiring only two AA batteries. The Tromino is a three-component seismometer that measures ambient seismic noise in the subsurface in the range of 0.1 to 200 Hz. The device has three channels connected to three orthogonal, high-resolution electrodynamic velocimeters, which feed the bare signal to a low-noise digitization circuitry achieving a resolution better than 23 bits. This provides an accuracy to better than 10-4 on the spectral components down to 0.1 Hz. Each digital ambient noise recording is saved to a flash memory card, and site measurement data can be downloaded to a PC or laptop for storage and analysis using proprietary software. The instrument settings and device set-up in the field were

chosen in accordance with recommendations supplied by Micromed (2009 and 2012).

At each acquisition site, the uppermost layer of soil was removed to ensure a solid and level base for the device (Fig. 1a,b). To ensure firm contact with the ground and optimal signal transfer, three 6-cm spikes were screwed into the bottom of the unit, which were then pushed into the ground until the lower plate of the Tromino touched the surface. A leveling bubble ensured the device was positioned horizontally. For all passive seismic acquisitions during this study, the acquisition length was set to 20 minutes and the sampling frequency range to 128 Hz, as recommended for stratigraphic applications (Micromed, 2012). This is consistent with the SESAME European Research Project (2004) recommendations of a minimum 20 minute recording time if a minimum resonance frequency peak of 0.5 Hz is expected, indicating an impedance contrast at a depth of more than 100 m. During acquisition, a bucket or plastic box was placed over the Tromino to reduce the effect of wind noise during the recording. Stored data were downloaded at least once a day.


5

Figure 3. Summary of passive seismic acquisition, method, and analysis: a) Tromino three-component seismometer

indicating the directions of recording; b) schematic H/V spectral ratio plot with peak at resonance frequency (fz)

indicating an impedance contrast; c) simple 2-layer regolith–bedrock model with an impedance contrast at depth h1;

d) Equation 1 relating the resonance frequency from the peak in the H/V spectral ratio plot to the shear-wave

velocity, Vs1, and depth, h1

Lithology/material Vs (m/s) Source

Clays and sandy silts 200 Micromed, 2009

Sand and gravel 300

Gravel and altered/soft rocks 400

Soft/layered sedimentary rocks 500

Stiff soil 180–360 Hunter and Cow, 2012; NRC, 2010

Very dense soil and soft rock 360–760

Rock 760–1500

Hard Rock >1500

Very soft soil <180 Ambraseys et al., 1996

Soft soil 180–360

Stiff soil 360–750

Rock >750

Alluvium 257–338(a) Collins et al., 2006

Archean granite (in WA) 474–1232(a)

Sandstone (Sydney Basin) 536–804(a)

Cretaceous shale 517–760 Smith et al., 2013

Cretaceous sandstone 350–516

Permo-Carb. diamictite 1068–1571

NOTE: (a) values are average shear-wave velocities to 30 m depth (Vs30)

Table 1. Summary of published average shear-wave velocities (Vs) for various

lithologies and materials

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Data transfer and analysisData transfer, storage and analysis are facilitated by Micromed’s proprietary Grilla software, which was developed to closely follow recommendations and guidelines provided by the European Commission-supported SESAME project (SESAME European Research Project, 2004). Prior to analysis, the Tromino is connected via a USB cable to a PC to transfer the passive seismic data into a predefined database location. Once import is completed, individual acquisitions can be selected for analysis. One further click on a selected 20 minute acquisition starts the H/V modelling routine, which produces the H/V spectral ratio from the three measured ambient noise components. At this point the software requires input of three parameters prior to analysis of the H/V data. Firstly, an appropriate window size is selected; this groups acquired data into manageable amounts, thereby reducing processing time. Based on Micromed (2009) recommendations, a 20 second window was selected, equating to 60 windows per acquisition. The second parameter is the degree of triangular smoothing of the data for the 60 windows, which is important to reduce the effects of narrow spikes (often of anthropogenic origin), and subsequently make the H/V curve easier to interpret. A smoothing degree of 10% is a good compromise for most stratigraphic applications (Micromed, 2009). However, if a peak is suspected to be of anthropogenic origin, Micromed (2009) recommends reducing the smoothing to 1–5%, at which point any nonstratigraphic peaks should become thin spikes in the H/V spectrum. For all H/V analyses the smoothing was initially set to 10%, but where a possible anthropogenic peak (see Data quality section below) was identified, the H/V analysis was rerun with a smoothing of <5%. The last input parameter relates to the frequency range within which the H/V analysis is carried out, for which a default setting of 0–64 Hz was selected.

A typical H/V spectrum (site 1425; Fig. 4a) shows the acquired H/V spectral ratio (red line) and the 95% confidence limits (grey lines). The blue line is the modelled synthetic curve (discussed below). The corresponding Fourier plot (Fig. 4b) shows the three individual ambient noise components (i.e. two horizontal and one vertical). The H/V spectral ratio for site 1425 shows a well-defined peak at a resonance frequency of 22.5 Hz indicating an impedance contrast within the subsurface. The peak also corresponds to a lens-shaped feature in the three-component plot (Fig. 4b). The consistency of the noise signal at about 22 Hz is shown throughout the acquisition period in Figure 4c (coloured pixels).

The interpretation of the passive seismic data using the Grilla software involves fitting a synthetic curve to the measured peaks in the H/V spectrum (Fig. 4a). In most applications there may be only one or two peaks (corresponding to two or three subsurface layers), but the software is capable of modelling multiple subsurface layers. For H/V spectral analysis, only peaks with an amplitude greater than two (>2) are considered for modelling (Micromed, 2012). Furthermore, modelling peaks with resonance frequencies >30 Hz is not

recommended (Micromed, 2012) as such peaks represent either impedances within one metre of the surface, or result from anthropogenic noise (e.g. generators).

Fitting a synthetic curve to the calculated H/V spectral ratio relies on selection of a set of parameters (Fig. 4d) for both units (e.g. regolith and bedrock, Fig. 3a), including the unit thickness, its shear-wave velocity, the Poisson variable (a ratio that converts the shear-wave velocity into the primary-wave velocity (Vp); see likely values in Table 2), and density.

Where the thickness of the unit in question is known (e.g. from drilling), the relationship

fz = Vs/(4h) Equation 1 (Fig. 3d)

can be used to estimate the shear-wave velocity. For site 1425 (Fig. 4), the thickness of the surface layer is known to be five metres. Because the unit is unconsolidated sand, a Poisson ratio of 0.4 and a density of 1.8 g/cm3 were selected. Using the equation relating frequency, shear-wave velocity and thickness (Fig. 3d), the shear-wave velocity (Vs) of the uppermost unit can be calculated: Vs = 4 x 5m x 22.5Hz = 450 m/s. To calculate the synthetic curve for site 1425 (Fig. 4a), similar parameters must be entered for the underlying (bedrock) unit. Because there are no clear peaks at lower frequencies, this unit is assumed to have an infinite thickness, indicated by 0 in the thickness field (Fig. 4d). The shear-wave velocity for the infinite layer is calculated by multiplying the shear-wave velocity of the layer above the impedance contrast by a factor of 2.2 to 2.6. This factor is taken from Micromed (2012) and is based on the peak amplitude. The peak amplitude is an indication of the difference between shear-wave velocities; i.e. the higher the peak, the greater the shear-wave velocity difference and the greater the multiplication factor. The Poisson ratio and density were set to 0.35 and 2.5 g/cm3, respectively. In most cases, the first modelled result does not provide the best fit to the measured H/V spectrum, and small changes must be made to the shear-wave velocities of the two layers, while maintaining the known thickness, Poisson ratio, and density values constant. However, when the shear-wave velocity of the layers is well defined (such as from drilling), the best fit is achieved by changing the unit thickness. Once the best fit for the synthetic curve has been achieved (blue line in Fig. 4a), Grilla allows the observed and modelled H/V peaks to be checked against the SESAME European Research Project (2004) guidelines, particularly with regard to appropriate length of time partitions (windows) and statistical significance of the peak (Micromed 2009, 2012; SESAME European Research Project, 2004).

Data quality

In addition to only modelling peaks with an amplitude of >2 at frequencies less than 30 Hz, the effects of anthropogenic noise or other transient disturbances must be considered in relation to data quality. Anthropogenic noise, for example, might relate to nearby industrial activities or vehicle traffic. The effect of an internal


7

Figure 4. Summary of H/V analysis in the proprietary Grilla software: a) plot of the

average horizontal and vertical (H/V) spectral ratio (red line) of a 20-minute

acquisition at 128 Hz for site 1425. Grey lines are 95th percentile confidence

limits; b) corresponding Fourier plot showing the two horizontal and one vertical

components; c) stability plot showing the amplitude of the resonance frequency

for each of the 60 windows over the whole 20-minute acquisition. Red indicates

high amplitude, blue indicates low amplitude; d) input table for the thickness,

shear-wave velocities, Poisson ratio, and approximate density necessary to

model the synthetic curve (blue line) in a)

Description Poisson ratio Density (g/cm3)

Mulga Rock

Largely unconsolidated surface sand 0.40 1.8

Cenozoic to Permo-Carboniferous sand- to clay-dominated sedimentary rocks 0.38 2.4

Basement 0.35 2.5

Eucla Basin

Succession of Cenozoic carbonate sedimentary rocks and Cretaceous weakly consolidated

sedimentary rocks

0.38 1.8 – 2.4

(2.1)

Basement 0.35 2.5

Table 2. Approximate values for Poisson ratio and bulk density used in the analysis of H/V spectral ratios from Mulga Rock

and the Eucla Basin

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8

combustion engine during data acquisition is illustrated in Figure 5. Without engine noise, there is a strong low-frequency peak at around 1 Hz and possibly two peaks above 30 Hz (Fig. 5a). All of these peaks correspond to lens-shaped features in the associated Fourier plot and an overall stable signal throughout the 20-minute acquisition period (Fig. 5a). In comparison, acquisition results with the vehicle engine running show that although the low-frequency peak at around 1 Hz is preserved, there is considerable noise above 10 Hz (Fig. 5b). This also causes peaks in all three components of the Fourier plot, in contrast to the lens-shaped features for natural ambient noise (Fig. 5a). Furthermore, the effect is also shown in the corresponding stability plots (Fig. 5c,d) where the engine noise adds a constant high-frequency signal at around 37 Hz. Transient disturbances can occur during any acquisition period, and appear as dispersed warm colours (Fig. 6a,b) away from the linear features typical of constant, coherent noise (e.g. Fig. 5d). Transient noise results in a wider confidence interval around the H/V spectral ratio. The Grilla software has a function to remove these unwanted noise signals. The data are identified as 20-second intervals (black blocks, Fig. 6b), which can be removed prior to reprocessing of data, resulting in an improved H/V spectral ratio (Fig. 6d) compared to the original data (Fig. 6c).

To address the precision of the passive seismic technique, multiple acquisitions at selected sites in the Eucla and Gunbarrel Basins were captured. Three 20-minute acquisitions were collected at each of the drillhole sites HDDH02, MAD002, and FOR004 in the Eucla Basin. Acquisitions at HDDH02 and MAD002 were carried out at a single location in each case, whereas two acquisitions at FOR004 were taken approximately 30 m to the south and north of the original reading, which was taken at the actual drill site. The three H/V spectra for HDDH02 and MAD002 are very similar (Fig. 7a,b), and show a single low-frequency peak at more or less the same position. The more prominent peaks are found at higher frequencies (>30 Hz) and are related to near-surface sources of interference. The variations in the middle part of the H/V spectra may result from temporal changes in the ambient noise signal. Compared to HDDH02 and MAD002, the three H/V spectra acquired at FOR004 (Fig. 7c) are broadly similar, but lack strong peak features seen in the other two plots. Particularly at the lower frequencies, the three H/V spectra indicate broad and weak peaks between 0.53 and 0.66 Hz. This variation may be due to small differences in ambient noise and subsurface conditions at the different locations.

Passive seismic data were acquired at two different times (2012 and 2013) at two sites (RC1302 and CD1252) in the Mulga Rock (Gunbarrel Basin) area (Fig. 8). At RC1302 (Fig. 8a), there are two noticeable peaks corresponding to resonance frequencies of 0.56 Hz (2012) and 0.53 Hz (2013). However, the 2013 acquisition fails to reproduce the peak at 13.9 Hz, which is apparent in the data from 2012. The 2012 H/V spectrum of CD1252 (Fig. 8b) has subtle peaks at 1.18 Hz and 33 Hz. The 2013 acquisition has a peak at 1.13 Hz, but no peaks at higher frequencies. Although spatial and temporal variations related to ambient noise have been reported by Okada (2004), the

Eucla and Gunbarrel Basins data reported here show that the Tromino is capable of replicating results in both space and time, in particular at lower frequencies, with most variations seen at higher frequencies.

In those cases where a passive seismic response in the H/V spectra is lacking, this can be attributed to an insufficiently strong impedance contrast between the subsurface units, which may reflect similar rheology of the units, progressive cementation, or deep weathering of basement. Although it is recommended that the analysis of passive seismic data should only be carried out at resonance frequencies corresponding to peaks in the H/V spectral ratio with amplitudes >2, in a few cases where peaks are broad and less pronounced, and have an amplitude <2, the data can be processed. However, in these cases, the Fourier spectra should be examined. If a lens-shaped feature is present in the Fourier spectra, it is commonly only expressed by one of the horizontal components (typically the E–W component), or by an offset of the horizontal components.

Shear-wave velocity

measurements

As stated above, the analysis of the H/V spectral ratio requires two essential types of information, the thickness of the upper layer and its shear-wave velocity. In those cases where thickness information is available from drillhole logs, the shear-wave velocity can be estimated. However, if the thickness is not known, shear-wave velocities can be obtained from the literature for various lithologies (Table 1). However, using those values is problematic as the shear-wave velocities for particular lithology types, such as sedimentary rocks and unconsolidated sediments, are affected by the degree of compaction, consolidation or cementation, pore volume, pressure, and moisture content. A more robust approach is to derive shear-wave velocities through experimental determination. This approach has been taken for a selection of drillcore samples for the study areas at Mulga Rock and in the Eucla Basin.

Before laboratory measurements were carried out, the selected core samples were trimmed to ensure opposing faces were parallel. Subsequently, shear-wave velocities were measured at ultrasonic frequencies and under room condition pressure and temperature. The technique consists of measuring the arrival time of an elastic pulse transmitted through a specimen of known length. The pulse is generated by a pulser-receiver, transmitted via two piezoelectric transducers acting as source and receiver, and recorded by a digital oscilloscope. The arrival time is then manually picked from the digitized waveforms. A selection of these waveforms is listed in Appendix 7, which indicates the estimated arrival time as vertical reference bars. The calculated shear-wave velocities for the selected samples are presented in the results sections below.

It should be noted that the reported velocities are for high frequencies (c. 0.5 MHz) and may not be directly comparable to those estimated from the recorded H/V


9

Figure 5. H/V spectral ratios and corresponding three-component (Fourier) plots for site PNC18: a) without

a noise source; b) with vehicle engine running; c) and d) corresponding frequency stability

plots demonstrate the effect of engine noise on the ambient noise signal. Measurements were

recorded at the same location and using the same settings (20 minutes, 128 Hz)

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10

Figure 6. Stability plots and corresponding H/V spectra for site MAD002: a) and c) before cleaning;

b) and d) after cleaning. Black bars in b) indicate excluded data of several 20-second windows

at the start and end of the 20-minute acquisition.

spectrum in the field, which are normally acquired in the sub-kHz range. Moreover, the propagation direction of the experimentally measured velocities was arbitrarily chosen to be parallel to the core axis; therefore, no possible effect due to anisotropy is taken into account. Because of this, the experimentally derived shear-wave velocities are likely to be higher than those estimated by the H/V method at sites where the thickness is known.

Results and data analysisApproximately 150 passive seismic recordings were acquired in three test areas: at Mulga Rock in the southern part of the Gunbarrel Basin, in the central part of the Eucla Basin, and in the Boorabbin area west of Kalgoorlie (Fig. 2). In all cases passive seismic data were acquired with the aim to estimate cover thickness. The data interpretations are also based on the same approach: 1) assessing the H/V spectral ratios at drill sites for which

complete drillhole logs are available, to estimate the shear-wave velocity for the cover materials; 2) using this shear-wave velocity in the H/V analysis of the remaining sites to determine cover thicknesses.

Mulga Rock

The Mulga Rock area in the southern Gunbarrel Basin, located approximately 250 km east-northeast of Kalgoorlie (Fig. 2), is being actively explored by Energy and Minerals Australia (EMA). There are three separate, uranium-bearing, polymetallic mineral deposits — Ambassador, Emperor, and Shogun. The area was chosen for this study for several reasons. First, it is the site of ongoing study on anomalous gold concentrations in transported regolith (Morris, 2013). Second, there is a well-established network of tracks. Third, there are available drillhole data necessary for the interpretation of passive seismic data.


11

Figure 7. Graphs for H/V spectra of acqusitions recorded at three drillholes in the Eucla

Basin: a) HDDH02; b) MAD002; c) FOR004. See Fig. 16 for drillhole locations.

Confidence limits are not displayed

Figure 8. Graphs comparing the H/V spectra of acquisitions recorded in 2012 (green) and

2013 (red) at two sites in the Mulga Rock area: a) site RC1302; b) site CD1252

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12

Fourth, this is a greenfields area of known prospectivity. Passive seismic data were acquired at 89 sites (Fig. 9; Table 3), 70 of which are located along a 51 km section of the northwesterly trending PNC Baseline. A further eight sites are located near Kakarook Corner and 11 along a 6.7 km long traverse parallel to the PNC Baseline and crossing the Emperor deposit. The average spacing between sites is 650 m. The area was first visited in October 2012 and revisited in April 2013 to extend the coverage of measurements along the PNC Baseline towards the northwest, and to include the traverse across the Emperor deposit (Fig. 9). In total, 41 measurements were taken at known drillhole locations, and the remaining

Figure 9. Distribution of passive seismic acquisitions in the Mulga Rock area. Blue stars indicate drillholes with information

of depth to basement. Base map is derived from the 1: 250 000 topographic map and the 1:250 000 Interpreted pre-

Mesozoic bedrock geology of the east Albany–Fraser Orogen and southeast Yilgarn Craton (Spaggiari and Pawley,

2012). See Table 2 for site coordinates and Table 4 for log descriptions. The eastern limit of the shallow basement is

inferred from drilling and regional gravity data (Energy and Minerals Australia, unpublished data).

48 sites taken as infill measurements. For most of the drillhole locations, drill logs are available, which show that only 11 drillholes penetrate the basement (Fig. 9, blue stars; Table 3).

Geology

A simplified geological map of the Mulga Rock area shows three main lithological associations (Fig. 9). Archean granitic rocks and greenstones of the Yilgarn Craton are found to the northwest, and granitic and mafic rocks of the Northern Foreland of the Albany–Fraser Orogen lie to the southeast (Spaggiari et al., 2009).


13

Site ID Area year Easting Northing Site ID Area year Easting Northing

200069(a) Kakarook 2012 584025 6687503 OF06 PNC 2013 550884 6698324

1331 Kakarook 2012 590875 6688285 PNC10 PNC 2013 549444 6699076

1345 Kakarook 2012 585993 6688650 PNC11 PNC 2013 550258 6698595

1419 Kakarook 2012 585387 6686316 PNC12 PNC 2013 551661 6697760

1420 Kakarook 2012 585022 6686580 PNC12A PNC 2013 551072 6698106

1421 Kakarook 2012 584693 6686767 PNC12B PNC 2013 551397 6697919

1422 Kakarook 2012 586017 6685978 PNC13 PNC 2013 552495 6697268

1425 Kakarook 2012 584344 6686989 PNC14 PNC 2013 553349 6696761

AC1033 PNC 2012 579928 6679971 PNC15 PNC 2013 554165 6696247

AC1036 PNC 2012 577989 6682026 PNC16 PNC 2013 555074 6695732

AC1000 PNC 2012 573873 6684523 PNC17 PNC 2013 555932 6695221

AC999 PNC 2012 572462 6685323 PNC18 PNC 2013 556776 6694727

CD1252 PNC 2012 577411 6682387 PNC18A PNC 2013 556776 6694727

NNA5503 PNC 2012 570213 6686715 PNC19 PNC 2013 557639 6694204

NNA5509 PNC 2012 570645 6686423 PNC20 PNC 2013 558475 6693704

NNA5501 PNC 2012 572980 6685053 PNC21 PNC 2013 559347 6693199

NNA5502 PNC 2012 573235 6684900 PNC22 PNC 2013 560203 6692670

OF09 PNC 2012 568786 6687564 PNC23 PNC 2013 561061 6692163

OF11 PNC 2012 579164 6680417 PNC24 PNC 2013 561958 6691626

OF10A PNC 2012 575153 6683737 PNC25 PNC 2013 562839 6691245

PNC1 PNC 2012 569125 6687286 PNC26 PNC 2013 563842 6690863

PNC2 PNC 2012 574469 6684172 PNC27 PNC 2013 564822 6690585

RC10 PNC 2012 571887 6685675 PNC28 PNC 2013 565741 6690170

RC1034 PNC 2012 576028 6683211 PNC29 PNC 2013 566521 6689465

RC11A PNC 2012 582511 6679338 PNC3 PNC 2013 541612 6703743

RC1169 PNC 2012 576578 6682911 PNC30 PNC 2013 567104 6688499

RC1296 PNC 2012 578442 6681241 PNC31 PNC 2013 567868 6687785

RC1302 PNC 2012 576733 6682798 PNC32 PNC 2013 583611 6678710

RC1460 PNC 2012 577756 6681214 PNC33 PNC 2013 584415 6678368

RC151 PNC 2012 580773 6679725 PNC34 PNC 2013 585286 6677907

RC339 PNC 2012 569558 6687069 PNC4 PNC 2013 540754 6704258

RC340 PNC 2012 571133 6686164 PNC5 PNC 2013 543168 6702817

RC70 PNC 2012 570287 6686636 PNC6 PNC 2013 544107 6702256

EMP1 Emperor 2013 556903 6688806 PNC7 PNC 2013 546654 6700738



EMP4 Emperor 2013 554552 6690118 RC118 PNC 2013 547890 6700000


EMP6 Emperor 2013 552290 6691705 CD1252 PNC 2013 577421 6682397


EMP8 Emperor 2013 554191 6690428 SSP1 PNC 2013 577296 6682469

NNA5709 Emperor 2013 553779 6690693 SSP2 PNC 2013 577175 6682546

RC128 Emperor 2013 551729 6691888 SSP3 PNC 2013 577033 6682627

RC905 Emperor 2013 553500 6690823 SSP4 PNC 2013 576909 6682706

OF05 PNC 2013 545948 6701715

NOTE: (a) location of regolith sample site corresponding to anomalous gold concentration (Morris et al., 2013)

Table 3. Locations of the 89 passive seismic acquisition sites in the three areas at Mulga Rock recorded during 2012 and 2013.

Site IDs highlighted grey correspond to drillholes that reached crystalline basement. All sites in 2012 were positioned

at known drillhole locations except PNC1 and PNC2. Map zone is 51J

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14

The contact between rocks of the Yilgarn Craton and the Albany-Fraser Orogen is defined by a fault zone, the Cundeelee Fault, located approximately 5–10 km southeast of the Mulga Rock exploration camp (Fig. 9). The contact between these two basement units forms a graben-like feature, which corresponds to the southwestern extent of the Gunbarrel Basin comprising Permo-Carboniferous claystone, sandstone, and conglomerate of the Paterson Formation (Figs 2 and 9). To the east of the study area, the Gunbarrel Basin overlies the Officer Basin (Fig. 2), which contains Mesoproterozoic to Devonian sedimentary rocks, but these are absent in the Mulga Rock area. The Mulga Rock area is further covered by recent Cenozoic sedimentary rocks (not shown in map of Fig. 9), which are likely an outlier of the Eucla Basin.

Detailed information on Cenozoic sedimentary succession and lithologies of the Gunbarrel Basin is available from well completion reports, including Lancer 1 (Haines et al., 2004) and Empress 1 and 1A (Stevens and Apak, 1999), located in the northern part of the Officer Basin. In those reports, the interpreted thickness of the Cenozoic strata varies between 17 and 79 m in Lancer 1 and the two Empress drillholes, but exceeds 400 m in Minigwal 2A (Perincek, 1998). In Empress 1 and 1A, Cenozoic rocks comprise poorly sorted, fluvial clastic rocks and poorly to moderately sorted, lacustrine clay deposits. The sediments are unconsolidated to moderately consolidated in parts, and are weathered to a depth of about 60 m. Lower parts of this succession could be very poorly consolidated, Early Cretaceous sedimentary rocks (Stevens and Apak, 1999). The uppermost interval (1–5 m) consists of fine-grained, medium-red to brown clay and sand. In Lancer 1, the upper 5 m consists of ferricrete and brown to yellow ferruginous mudstone with minor fine-grained sandstone and Cenozoic rocks below have been described as fluvial-lacustrine in origin (Haines et al., 2004).

The Permo-Carboniferous Paterson Formation extends beneath the Cenozoic cover to 131.9 m below surface in Empress 1A (Stevens and Apak, 1999) and 169.1 m in Lancer 1 (Haines et al., 2004). In Empress 1A, the Paterson Formation consists of three facies: diamictite; cross-bedded, coarse-grained pebbly sandstone; and well-bedded claystone, siltstone, and fine-grained sandstone (Stevens and Apak, 1999). These are interpreted as tillites, fluvioglacial outwash, and lacustrine deposits with some possible marine influence, respectively (Jackson and van de Graaff, 1981). The basal 20 m in Empress 1A is interpreted as fluvioglacial outwash, and is either tillite or reworked, glacigenic sediments (Stevens and Apak, 1999). The Paterson Formation in Lancer 1 is interpreted to be mainly of fluvial origin, and glacial diamictites, as seen in Empress 1 and 1A, are absent (Haines et al., 2004). In the Lancer drillhole, the Paterson Formation is approximately 152 m thick with its base at 169 m. The lower part of the unit, below approximately 96 m, is dominated by sandstone with significant mudstone intervals at a depth of 155 m, followed by a 2 m-thick conglomerate unit at the base below 167 m (Haines et al., 2004). The upper part is dominated by claystone with minor sandy beds and extends from 17–94 m, of which the top 4.5 m is conglomeratic. At Mulga Rock, rocks of the

Paterson Formation are >700 m thick, and are overlain by up to 100 m of fluvial to lacustrine Cenozoic sedimentary rocks (Douglas et al., 2005), which commonly have a ferruginous or siliceous cement. Drillhole logs for a selection of sites (Fig. 9) are summarized in Table 4. These are largely based on legacy data, and are of variable reliability. From these data, Cenozoic lithologies are fine-grained to pebbly sandstones (locally ferruginized or silicified), with total thicknesses ranging from 36 m (drillhole OF05) to 119 m (drillhole AC1036). These rocks are in turn covered by 1–16 m of Quaternary surface sand. In some areas, in particular to the northwest and southeast, the Cenozoic rocks unconformably overlie highly weathered basement rocks of the Albany–Fraser Orogen and Yilgarn Craton, and rocks of the Paterson Formation are absent. Information on the Paterson Formation is limited and only drillhole logs for OF09, RC340, and AC999 recorded the intersection of Permian claystone or mudstone at depths of 40–68 m. However, because drillholes did not reach the bases of these units, only minimum unit thicknesses are recorded, and the depth to basement is unknown. A schematic cross-section (Fig. 10) summarizes the stratigraphy, indicating a trough-like shape for the Gunbarrel Basin infilled with the largely glacigenic sedimentary rocks of the Paterson Formation, which is confined to the east by the Cundeelee Fault. It also shows that rocks of the Paterson Formation do not extend across the whole basin, whereas Cenozoic sedimentary rocks are laterally extensive.

The stratigraphy at Kakarook Corner to the northeast of Mulga Rock is somewhat different. Based on the log for drillhole 1425, a very thin veneer of about 5 m of partly consolidated sand directly overlies crystalline basement of the Albany–Fraser Orogen. This thin cover is likely to be of Quaternary age because optically stimulated luminescence dates for samples collected at 1.8 m depth nearby indicated a Pleistocene age (90.9 ka; Morris, 2013). Recent eolian sands cover nearly the entire study area of Mulga Rock, concealing the underlying bedrock geology, resulting in less than 3% outcrop in this area. The surface is characterized by westerly to northwesterly trending sand dunes 5–10 m high and spaced at approximately 500–1000 m.

Passive seismic data

A representative selection of H/V spectral ratios and corresponding Fourier spectra based on 20-minute acquisitions at 128 Hz from the three areas at Mulga Rock are shown in Figure 11a–f. The remaining H/V spectra are presented in Appendices 1 to 3. At site 1425 (Kakarook Corner) the H/V spectral ratio comprises a single, strong peak with an amplitude of 5.6 at a resonance frequency of 22.5 Hz (Fig. 11a). The associated 3-component Fourier plot shows a lens-shaped feature at the same frequency. Spectra from the other seven sites at Kakarook Corner (Appendix 1) are similar; the peaks are much weaker compared to 1425, but in a similar resonance frequency range of 16 – 22.5 Hz (Table 5). In addition, three of the acquisitions indicate a second peak corresponding to frequencies of c. 33.6 – 43.8 Hz (Table 5).


15

Hole ID Depth (m) Log description

NNA5709 0–6 Fine to medium orange sand (Quaternary)

6–22 Fine to medium red sandstone

22–41 Normally graded, variably very fine to coarse-grained brown sandstone

41–57 Largely black claystone with carbonate and lignite (Eocene)

57–76 Alternating sequence of brown to grey claystone and sandstone

76–87 Grey claystone

87–88 Black, pyrite-rich, pebbly hard band

88–94 Variably grained clay and sandstone

94 Granite – basement

OF05 0–8 Red to yellowish orange sand

8–12 Silcrete

12–36 Largely medium-grained sand with light-grey clay seams

36 Schist – basement

OF06 0–1 Yellowish orange sand

1–17 Ferruginized dark reddish orange rock with silcrete seam at 6–7 m

17–53 Yellowish brown to grey muddy sands with seams of clay and quartz sand

53 Biotite granite

RC128 0–3 Yellowish brown to orange sand

3–15 Ferriginised, reddish brown, silty sand with some ferricrete

15–68 Partly silicified, largely well-sorted, pale-brown siltstone and sandstone

68–74 Weathered granite basement

74 Biotite-rich granite

RC118 0–4 Very fine to coarse yellow-brown sand, more clayey from 1 m

4–59 Partly ferruginized reddish brown to pale yellow, very fine to coarse-grained silty sandstone, with pale-brown

silcrete bands at 4 m, 6.5 m, and more clayey siltstone intervals at about 22 m and 43 m

59–85 Highly weathered granitic basement, increase of fragments and granitic texture with depth

85 Granite – basement

RC119 0–1 Ferricrete with surface sand

1–10 Poorly sorted sandy siltstone with fine to medium-sized quartz grains

10–48 Largely fine- to coarse-grained, yellowish grey to brown quartz sandstone with silicification between 26 and 36 m

48–77 Weathered biotite granite

>77 Basement

RC905 0–6 Surface sand

6–17 Orange-brown, ferruginous, silty, medium to coarse sand

17–43 Largely pale grey to yellow, medium to coarse sand, silicified in places

43–54 Black to very dark brown peaty clay, interbedded in places with carbonaceous medium to coarse sand

54–73 Dark brown, carbonaceous (pyritic) coarse sand, interbedded with clay; pale brown to grey; clayey sandy silt at

base

73 Granite basement

1425 0–1 Surface sand

1–5 Silicified sand more silty at base

5–35 Highly weathered graphitic schist, white to pink more indurated with depth, redox boundary at 20 m

OF11 0–10 Yellowish orange sand with quartz pebbles at base

10–30 Light-grey, clayey quartz sand, some quartz pebbles at base

30–89 Massive dark-grey silty clay (Permian?)

89 Black schist with quartz seams – basement

Table 4. Summary of drillhole log descriptions for a selection of sites along the PNC Baseline and Emperor traverse (see

Fig. 9 for locations) which intercepted basement. The sedimentary successions are assumed to be of Cenozoic age,

unless otherwise labelled.

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16

Hole ID Depth (m) Log description

AC1036 1–16 Surface sand

16–22 Orange-brown ferruginized coarse sand

22–47 Pale-pink, brown to grey silicified course sand, with some silty clay seams

47–95 Largely grey to brown carbonaceous clay with some layers of coarse sand and gravel

95–119 Grey, poorly sorted, coarse sand, pebbley in places

119 Dark-grey Proterozoic metamorphosed granitic rock – basement

RC11 0–13 Fine- to medium-grained, reddish brown well-sorted surface sand

13–22 Pale-yellow to light-grey silicified coarse (pebbly) sand

22–72 Grey claystone, light orange and more silty above Redox boundary at 42 m

72–76 Grey laminated claystone and siltstone with conglomerate of granitic gneiss fragments at base

76 Fine-grained dark schist – basement

OF09 0–3 Reddish orange surface sand

3–5 Pale reddish grey ferruginized rock

5–11 Silcrete

11–30 Silt with clay and sand, quartz pebbles at base

30–68 Light-grey, sandy clay; more grey to brown, clayey quartz sand with depth

68–88 Massive, dark-grey silty clay (Permian)

>88 Unknown basement contact

RC340 0–2 Surface sand

2–35 White/pink to light-brown/grey silt and sand, partly silicified in the upper half

35–46 Light-brown to white clayey silt

46–61 Light-brown to grey pebbly quartz sand in the upper half; more medium quartz sand with depth; quartz pebble

conglomerate at base

61 Blue/dark-grey sandy mudstone (Permian)


AC999 0–5 Surface sand, very fine to fine sand and partly silicified from 3 m

5–15 Largely medium to coarse sand, pebbly in places

15–40 Silt, slightly silicified at top and ferruginized at the base from 35 m

40–45 Silty to clayey sandstone (Permian)


Table 4. (continued)

Figure 10. Generalized cross-section at Mulga Rock along the PNC Baseline. Depths are approximate


17

Site IDResonance

frequency (Hz) Site ID

Resonance

frequency (Hz) Site ID

Resonance

frequency (Hz)

PNC Baseline

PNC4 2.20 PNC27 0.50 RC1296 0.90

PNC3 2.03 PNC28 0.44 OF11 1.67

RC119 2.12 PNC29 0.47 AC1033 1.56

PNC5 2.20 PNC30 0.50 RC151 1.85

PNC6 – PNC31 0.44 RC11A 1.55

OF05 2.90 OF09 0.44 PNC32 2.70

PNC7 – PNC 1 0.47 PNC33 –

PNC8 2.80 RC339 0.51 PNC34 –

RC118 2.50 NNA5503 0.51

PNC9 3.07 RC70 0.44 Kakarook Corner

PNC10 2.70 NNA5509 0.49 200069 19.1

PNC11 2.24 RC340 0.54 1425 22.5

OF06 2.80 RC10 0.51 1422 –

PNC12A 1.56 AC999 0.49 1421 16.3

PNC12B – NNA5501 0.45 1420 >30

PNC12 1.78 NNA5502 0.50 1419 17.9

PNC13 1.16 AC1000 0.49 1331 28.1

PNC14 1.26 PNC2 0.46 1345 >30

PNC15 1.25 OF10A 0.52

PNC16 1.22 RC1034 0.50 Emperor traverse

PNC17 1.06 RC1169 0.53 EMP1 0.81

PNC18 1.03 RC1302 0.56 EMP2 0.84

PNC18A 1.08 RC1302(a) 0.53 EMP3 0.91

PNC19 0.88 SSP4 0.56 EMP4 0.94

PNC20 0.88 SSP3 0.63 NNA5709 1.19

PNC21 0.75 SSP2 0.75 RC905 1.41

PNC22 0.69 SSP1 0.84 EMP5 –

PNC23 0.66 CD1252 1.18 RC128 1.84

PNC24 0.56 CD1252(a) 1.13 EMP6 –

PNC25 0.51 RC1460 0.93 EMP7 2.49

PNC26 0.47 AC1036 1.19 EMP8 1.30

Table 5. Lowest resonance frequencies derived from peaks in the H/V spectra for the 89 sites at Mulga Rock. Sites are

ordered from northwest to southeast and grouped into PNC Baseline, Emperor traverse and Kakarook Corner

(data of the former two are displayed in Figure 12). Resonance frequencies for peaks >30 Hz are not listed and

resonance frequencies from drillhole sites with complete depth information are highlighted in grey (see Figure 9

for locations and Appendices 1 to 3 for corresponding H/V spectra).

NOTE: (a) data collected at the same site in 2013

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18


19

The H/V spectral ratios and corresponding 3-component Fourier plots for acquisitions along the PNC Baseline (Fig. 11b–e) are from four sites with drillhole data, for which there is control on the depth to basement (apart from RC340). Results for RC11 (Fig. 11b), the most southeasterly site, has a peak at a resonance frequency of 1.55 Hz. The broad peak at >30 Hz likely corresponds to interference within the uppermost metre, and was excluded. The H/V spectrum for AC1036 (Fig. 11c) shows two pronounced peaks indicating two impedances at depth (i.e. three subsurface units). The upper layer corresponds to a resonance frequency of 6.3 Hz, and a deeper layer is indicated by the resonance frequency at 1.16 Hz. Both peaks are expressed by lens-shaped features in the Fourier spectrum. In the Mulga Rock area, this was one of only three sites that displayed two peaks below 30 Hz.

The H/V spectrum for RC340 displays only one peak and one of the lowest resonance frequencies at 0.54 Hz (Fig. 11d; Table 5). This site is located in the centre part of the PNC Baseline over the Gunbarrel Basin. Site OF05 (Fig. 11e), which lies farther to the northwest along the PNC Baseline, displays only a subtle, broad peak (amplitude >2) at a resonance frequency of 12.4 Hz. The H/V spectrum for RC128 (Fig. 11f) located along the Emperor traverse, displays a peak at a resonance frequency of 1.85 Hz, as well as a peak at 25 Hz, which possibly corresponds to a near-surface impedance contrast. However, the latter signal is only visible in the E–W component of the two horizontal components (Fig. 11f, Fourier plot). H/V spectra from other sites along the southeastern Emperor traverse mostly display strong peaks at or below 1 Hz (Appendix 3), whereas responses in the H/V spectra from sites along the northwest part of the traverse, particularly for sites EMP5, EMP6 and EMP7, are either weak or absent. For all H/V spectra obtained at Mulga Rock (Table 5), if more than two peaks were detected by the Tromino, the derived resonance frequencies correspond to the lowest frequency peaks, only. Results from the PNC Baseline and Emperor traverse are similar and range from 0.44 to 3.07 Hz, and the lowest frequencies are between PNC19 and CD1252 in the middle part of the PNC Baseline (Fig. 12).

Three broad features are apparent along the PNC Baseline and Emperor traverse. At the western end, resonance frequencies range between 2 and 3 Hz (Fig. 12). There is then a steady decrease in resonance frequency to the east, where frequencies are around 0.5 Hz. At the eastern end of the traverse east of the Mulga Rock exploration camp, resonance frequencies increase markedly over a distance of <1 km to greater than 1 Hz. This is highlighted by the results of infill measurements (SSP1 to 4) collected at an average spacing of 150 m (Fig. 12).

Figure 11. (facing page and left) Selection of H/V spectral ratios

and corresponding three-component (Fourier) plots

for six sites at Mulga Rock. a) site 1425 at Kakarook

Corner; b) RC11, c) AC1036, d) RC340, and e) OF05

from the PNC Baseline; f) RC128 from the Emperor

traverse. H/V traces from the rest of the study area

are displayed in Appendices 1 to 3

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20

Figure 12. Distribution of resonance frequencies corresponding to main peaks (amplitude >2) in H/V spectral ratios

(Appendices 2 and 3) of measurements taken along the PNC Baseline and Emperor traverse in 2012 and 2013

(see Figure 9 for locations). The Empress traverse is offset to the southwest by approximately 5 km. Labels

indicate drillholes for which completed logs were available (Table 4)

Results from the Emperor traverse, which are offset c. 5 km to the southwest, parallel the distribution shown by those from the PNC Baseline. Furthermore, peaks in H/V spectra that correspond to sites at the southeastern and northwestern part of the distribution are much less pronounced, typically only just reaching an amplitude of 2 (Appendices 1 to 3). Conversely, the lower resonance frequencies of the central part of the traverse (Fig. 12) correspond to stronger peaks that are well defined, with amplitudes >2. The only data excluded from this traverse are from three sites located southeast of RC11, where no peaks were detected in the H/V spectral ratios (PNC32, PNC33 and PNC34; Appendix 2). Results for Kakarook Corner are not included in the traverse (Fig. 12), because observed peaks have a much higher frequency range (16.3 and 22.5 Hz; Table 5).

Shear-wave velocity

Shear-wave velocity data, which are essential for modelling of passive seismic data, can be sourced from the literature, estimated when depth to the impedance contrast is known (Fig. 3d), or measured in a laboratory. Shear-wave velocities have been measured for six samples taken from Empress 1/1A drillcore (Zone 51, MGA 713910E 7005786N). Empress 1/1A is located in the Officer Basin approximately 500 km north of Mulga Rock. The drillhole intersects approximately 50 m of the Paterson Formation, which is thicker in the Gunbarrel Basin at Mulga Rock. Of that 50 m section, approximately 30 m of core are archived and were sampled at various depths (Table 6). The heterogeneous nature of the glacigenic sedimentary rocks of the Paterson Formation is evident (see photos in Appendix 6). Following analytical procedures described above, shear-wave velocity measurements were

determined for five samples from the Empress 1/1A drill core (Table 6). All but one of the samples are mudstones and have shear-wave velocities of around 1240 m/s. One sandstone sample (212448) was noticeably brittle and poorly consolidated, resulting in a much lower shear-wave velocity (Table 6).

H/V analysis and data interpretation

Kakarook Corner

Resonance frequencies derived from H/V spectra acquired at eight sites near Kakarook Corner are assessed separately from other Mulga Rock measurements because they are characterized by peaks in the H/V spectra at higher frequencies, between 16.3 and 28.1 Hz (Table 5). At three of the eight sites (1420, 1422 and 1345), recordings could not be modelled, because either no peak was detected, or peaks were above 30 Hz. Qualitatively, the high-frequency response indicates a very thin regolith cover of perhaps less than 1 m. At site 1425, drillhole information indicates 5 m of eolian sand, which is silicified from 1 m downwards, overlying weathered graphitic schist, interpreted to be Proterozoic rocks of the Albany–Fraser Orogen (Table 4). This is consistent with a two-layer subsurface, which causes one substantial peak in the H/V spectrum at a resonance frequency of 22.5 Hz (Table 5; Fig. 4a). To model these data, the shear-wave velocity (Vs) of the upper unit was calculated from the known thickness of 5 m and a frequency of 22.5 Hz (Fig. 4a), using the relationship of Equation 1 (Fig. 3d), assuming densities of 1.8 g/m3 for the upper sand layer and 2.5 g/ m3 for the basement, and Poisson ratios of 0.4 for the sand layer and 0.35 for the basement (Table 2).


21

Table 6. Laboratory-determined shear-wave velocities (Vs) for samples of the Paterson Formation from core of drillhole

Empress 1/1A, and for samples of the Wilson’s Bluff, Madura and Loongana formations in drillholes FOR004 and FOR011

Sample ID Formation name Rock type Log description Depth (m)Average Vs

(m/s)

Empress 1/1A

212444 Paterson Mudstone Dark-grey, very fine to fine-grained, minor sand and

pebbles

105.6 1340

212445 Paterson Mudstone As above 107.0 1200

212446 Paterson Mudstone Moderate brown, as above 108.9 1240

212448 Paterson Sandstone(a) Light-grey, fine to medium grained, good porosity,

with minor, light-grey mudstone

126.7 730

212449 Paterson Mudstone Light-grey, minor sandstone 129.4 1170

FOR004

212455 Wilson’s Bluff Chalk/limestone White to grey 141.5 1909



212458 Madura Claystone/shale Grey to black 212.2 778

212462 Madura Mudstone Grey 232.1 684

212459 Madura Claystone/shale Grey and sandy 242.1 736

212460 Madura Mudstone Grey and sandy 248.8 771

212461 Loongana Sandstone Grey bluish 388.8 816

FOR011


212464 Madura Mudstone Grey 221.5 –



NOTE: (a) core plug very brittle

Through an iteration process, the best fit was achieved using a shear-wave velocity of 425 m/s for the sand cover and 1015 m/s for the basement (blue line in Fig. 4a). The shear-wave velocity for the upper sand unit is consistent with average shear-wave velocities for similar units (Table 1). A shear-wave velocity of 1015 m/s for the basement was derived from the peak amplitude and the shear-wave velocity of the sand cover.

The shear-wave velocity data, Poisson ratio, and density values from this model have been applied to the remaining passive seismic measurements at Kakarook Corner to estimate the cover sand thickness (Fig. 13). In this area, the sand cover shows a limited range in thickness of 5.0 – 6.5 m over a distance of approximately 2.5 km. The sand thickness at site 1331 (approximately 2.5 km to the northeast; Fig. 9), at 3.8 m, is slighter thinner than at site 1425. The H/V spectra for sites 1419, 1420, and 1421 (Appendix 1) show a second, higher frequency peak indicating a shallower impedance contrast, which may correspond to a depth of about 1 m. This is possibly related to the boundary between the unconsolidated surface sand and silicified sand recorded in the drillhole log for 1425 (Table 4). Additionally, the shape of some peaks in the H/V spectra of the sites along this short traverse (e.g. 1419) suggest that the boundary between the sand cover and basement is not as distinct as at site 1425, possibly indicating stronger weathering of the basement.

PNC Baseline and Emperor traverse

Although the stratigraphy in each of the PNC Baseline and Emperor areas is similar (i.e. sedimentary rocks overlying crystalline basement), it can be broadly divided into two sequences. In the southeastern and northwestern parts of the PNC Baseline and the northwestern part of the Emperor traverse, Cenozoic sedimentary rocks directly overlie Albany–Fraser Orogen and Yilgarn Craton basement, respectively. In contrast, in the middle part of the PNC Baseline and the southeastern part of the Emperor traverse, a thicker package of Permo-Carboniferous and Cenozoic sedimentary rocks overlies basement, presumed to be the Yilgarn Craton. These two sequences are reflected in the resonance frequency values for peaks with amplitudes >2 in H/V spectral ratios (Fig. 12): higher frequencies (i.e. impedance contrasts at shallower depths) are found at either end of the PNC Baseline traverse and at sites at the northwestern end of the Emperor traverse, and lower frequencies (i.e. impedance contrasts at greater depths) are found along the middle section of the PNC Baseline and southeastern part of the Emperor traverse. Drillhole information is available for nine sites (Table 5; Fig. 12) along the PNC Baseline and Emperor traverse. Four sites are located in the northwest and two sites in the southeast of the PNC Baseline. A further three sites correspond to the Emperor traverse (Table 5). Using the resonance frequencies from the H/V spectra for these nine

Scheib

22

Figure 13. Cover thickness estimates for sites at Kakarook corner. The result for site 1331 is excluded from the plot as it is

not located along this short linear traverse.

sites, depth to basement information from drill logs, and Poisson ratio values and densities from Table 2, shear-wave velocities have been estimated and listed in Table 7. As the nine sites are located in areas where Permian strata are absent, the shear-wave velocities correspond to the Cenozoic cover and range from 415 to 605 m/s with an average of 500 m/s, similar to values listed for stiff soils and soft rocks (Table 1). These estimated values also agree well with values for Cretaceous sandstone from Prominent Hill in central Australia (Smith et al., 2013). Variations in the calculated shear-wave velocities at the nine sites may reflect a different degree of cementation within these Cenozoic sedimentary rocks, as well as some variation in their thickness.

A plot of resonance frequency versus depth to basement for these nine sites (Fig. 14) shows a consistent frequency to depth trend, meaning that a fitted power law can be used to directly convert resonance frequencies of >1 Hz (i.e. corresponding to results for sites beyond the area of Permo-Carboniferous strata) into depth to basement estimates without the need to go through the modelling process described above. This approach was used by Parolai et al. (2002) and Ibs-von Seht and Wohlenberg (1999) to derive the thickness of sediments over basement in two basin successions from Germany. This approach relies on the assumption that there is a common stratigraphy throughout the area of interest, so the relationship may not hold for the middle part of the PNC Baseline. Unfortunately, in this part of the traverse there are no drillhole data with information on the total thickness of the sedimentary rock cover. This is required to estimate the shear-wave velocities to derive thickness estimates from resonance frequencies. However, laboratory measurements of core samples1 provided an average shear-wave velocity of 1238 m/s (Table 6), which can be used to determine thickness for the middle part of the traverse.

The results from the H/V analysis of data from drill sites, mentioned above, can now be applied to derive cover thickness estimates for the remaining sites along the PNC Baseline and Emperor traverse. The average shear-wave velocity of 500 m/s and the power-law model (Fig. 14) have been used to derive cover thickness estimates for sites PNC4 to PNC12A and NNA5709 to EMP8 from the northwestern PNC Baseline and Emperor traverse, respectively, and sites southeast of CD1252 on the PNC Baseline. Resulting cover thickness estimates and corresponding actual depths of the basement above sea level using the surface elevation are listed in Table 8. The thicknesses indicate a relatively thin cover of Cenozoic sedimentary rocks, corresponding to H/V peaks at resonance frequencies of >1 Hz. The good drillhole control in these areas of the PNC Baseline and Emperor traverse means that thickness estimates from both approaches are very similar, but the power-law-based results were used to calculate the actual depth of the basement (Table 8).

Most sites in the central part of the area have resonance frequencies <1 Hz indicating a much deeper impedance. As there are no available drill hole data, the synthetic H/V curves have been generated using a shear-wave velocity of 1238 m/s (Table 6). The resulting cover thickness estimates range from 179 to 703 m (Table 8), which are also displayed as depths of basement.

Figure 15 shows all depth-to-basement estimates in cross-section for the PNC Baseline and Emperor traverse. The distribution of the depth to basement estimates confirm the geological model for this area (personal communication with Energy and Minerals Australia; Fig. 10), in particular the asymmetric trough-like shape of the Gunbarrel Basin. The basin is characterized by two main features. Firstly, the depth to basement increases over a distance of 14 km by approximately 570 m from the edge of the Gunbarrel Basin near site PNC12A towards the centre of the basin.

1 Work carried out at CSIRO laboratory, Kensington WA 6151


23

Area Site ID fz (Hz)Depth to

basement (m)Vs1 (m/s) Vs2 (m/s) Features in the H/V spectra

PNC SE

part

AC1036 1.19 118 545 1255 Good peak with a second peak (>2) at 6.3 Hz ≈ 10 m depth

RC11A 1.55 76 495 1020 Peak amplitude just at 2; strong second peak >40 Hz

PNC NW

part

RC119 2.12 48 425 822 Peak amplitude 1.8 Hz; second peak at >50 Hz

OF05 2.80 36 425 780 Peak amplitude 1.5 Hz; second peak at 13 Hz ≈ 4 m,

RC118 2.50 59 605 1265 Second peak at >30 Hz

OF06 2.80 53 605 1120 Noise in the last 2.5 min removed

Emperor

NNA5709 1.19 95 475 1010 Good peak with a second at 16.3 Hz

RC905 1.41 72 415 860 Good peak

RC128 1.84 68 510 1030 Strong peak at 1.88, with a second peak at 25.2 Hz

Table 7. Modelled shear-wave velocities (Vs) at drillhole sites that have depth to basement (cover thickness) information located

in the southeast and northwest part of the PNC Baseline and Emperor traverse (for corresponding H/V spectra see

Appendices 2 and 3). Shear-wave velocity (Vs1) is for Cenozoic sedimentary rocks and Vs2 is for underlying shallow

basement

Site IDPeak

frequency (Hz)Vs1 (m/s)

Cover thickness (m)

by method

Selected cover

thickness (m)

Height

(m asl)

Depth of

basement (m asl)

Vs1 p-l

PNC4 2.20 500 57 55 55 460 405

PNC3 2.03 500 62 60 60 457 398

RC119 2.12 420 48 57 48 447 399(a)

PNC5 2.20 500 57 55 55 445 389

PNC6 – – – – – 444 –

OF05 2.90 420 36 43 36 433 397(a)

PNC7 – – – – – 440 –

PNC8 2.80 500 45 44 44 438 394

RC118 2.50 590 59 49 59 443 384(a)

PNC9 3.07 500 41 41 41 435 394

PNC10 2.70 500 46 46 46 429 384

PNC11 2.24 500 56 54 54 422 367

OF06 2.80 590 53 44 53 416 363(a)

PNC12A(b) 1.56 500 80 76 76 414 337

PNC12B – – – – – 408 –

PNC12 1.78 1238 174 – 174 407 233

PNC13 1.16 1238 267 – 267 396 129

PNC14 1.26 1238 246 – 246 403 157

PNC15 1.25 1238 248 – 248 395 147

PNC16 1.22 1238 254 – 254 378 124

PNC17 1.06 1238 292 – 292 364 72

PNC18 1.03 1238 300 – 300 352 52

PNC18A(c) 1.08 1238 287 – 287 352 66

PNC19 0.88 1238 352 – 352 344 –8

PNC20 0.88 1238 352 – 352 334 –18

PNC21 0.75 1238 413 – 413 332 –80

PNC22 0.69 1238 449 – 449 330 –118

PNC23 0.66 1238 469 – 469 332 –137

PNC24 0.56 1238 553 – 553 335 –218

PNC25 0.51 1238 607 – 607 346 –261

Table 8. Summary of resonance frequencies (fz) for sites along the PNC Baseline and Emperor traverse sorted from

northwest to southeast (see Fig. 9). Cover thickness estimates are calculated for the shear-wave velocity (Vs1)

and power-law (p-l) equation of Figure 3. Depths of basement above sea level (asl) are derived from the selected

cover thicknesses and surface heights of each site. Rows highlighted grey indicate sites with known depth to

basement (Table 7).

Scheib

24

Site IDPeak

frequency (Hz)Vs1 (m/s)

Cover thickness (m)

by method

Selected cover

thickness (m)

Height

(m asl)

Depth of

basement (m asl)

Vs1 p-l

PNC26 0.47 1238 659 – 659 352 –307

PNC27 0.50 1238 619 – 619 350 –269

PNC28 0.44 1238 703 – 703 345 –358

PNC29 0.47 1238 659 – 659 343 –315

PNC30 0.50 1238 619 – 619 334 –285

PNC31 0.44 1238 703 – 703 338 –365

OF09 0.44 1238 703 – 703 340 –364

PNC1 0.47 1238 659 – 659 342 –316

RC339 0.51 1238 607 – 607 336 –271

NNA5503 0.51 1238 607 – 607 338 –269

RC70 0.44 1238 703 – 703 335 –368

NNA5509 0.49 1238 632 – 632 335 –297

RC340 0.54 1238 573 – 573 335 –238

RC10 0.51 1238 607 – 607 341 –266

AC999 0.49 1238 632 – 632 345 –286

NNA5501 0.45 1238 688 – 688 346 –342

NNA5502 0.50 1238 619 – 619 347 –272

AC1000 0.49 1238 632 – 632 341 –290

PNC2 0.46 1238 673 – 673 338 –335

OF10A 0.52 1238 595 – 595 347 –248

RC1034 0.50 1238 619 – 619 341 –278

RC1169 0.53 1238 553 – 584 338 –246

RC1302(d) 0.56 1238 553 – 553 338 –214

RC1302 0.53 1238 584 – 584 338 –246

SSP4 0.56 1238 553 – 553 344 –208

SSP3 0.63 1238 491 – 491 345 –146

SSP2 0.75 1238 413 – 413 341 –72

SSP1 0.84 1238 368 – 368 339 –29

CD1252(d) 1.18 560 119 99 99 338 239

CD1252 1.13 560 124 103 103 338 214

RC1460 0.93 500 134 124 124 348 213

AC1036 1.19 560 118 98 118 349 231(a)

RC1296 0.90 500 139 127 127 349 222

OF11 1.67 500 75 72 72 344 273

AC1033 1.56 500 80 76 76 365 289

RC151 1.85 500 68 65 65 359 294

RC11A 1.55 495 76 77 76 354 278(a)

PNC32 2.70 500 46 46 46 352 306

PNC33 – – – – – 342 –

PNC34 – – – – – 345 –

EMP1 0.81 1238 382 – 382 325 –57

EMP2 0.84 1238 368 – 368 364 –4

EMP3 0.91 1238 340 – 340 378 37

EMP4 0.94 1238 329 – 329 330 0

NNA5709(b) 1.19 470 95 98 95 339 244(a)

RC905 1.41 420 72 84 72 403 331(a)

EMP5 – – – – – 369 369

RC128 1.84 510 68 65 68 364 296(a)

EMP6 – – – – – 352 –

EMP7 2.49 500 50 49 49 396 347

EMP8 1.30 500 96 90 90 395 304

NOTES: (a) true depth to basement derived from drillhole log; (b) site location coincides with the inferred eastern extent of shallow basement; (c) acquisition at the same

location as PNC18, but with engine running; (d) acquisition from 2012

Table 8. (continued)


25

Secondly, in the southeastern part of the basin, there is a pronounced thinning of the cover rocks over a short distance (e.g. thickness of 553 m at RC1302 to 118 m at AC1036, over a distance of less than 1.5 km). This change coincides with a fault zone (Fig. 9) marking the margin between the Northern Foreland of the Albany–Fraser Orogen and Yilgarn Craton.

Based on the passive seismic-derived estimates of cover thickness, the base of the Gunbarrel Basin appears to be strongly undulating (Fig. 15), with cover thicknesses varying from 570 to 700 m over approximately 17 km.

Figure 14. Plot of the resonance frequencies and

corresponding depth-to-basement (cover

thickness) information for nine control sites along

the PNC Baseline and Emperor traverse. See Table

8 for data

This deepest part of the Gunbarrel Basin is associated with the sharpest and highest peak in the H/V spectra, indicating a strong impedance contrast possibly related to the contact between the basement rocks and glacigenic sedimentary rocks of the Paterson Formation. This is consistent with results reported from Prominent Hill in central South Australia (Smith et al., 2013), where the highest peaks and largest velocity contrasts were recorded at the unconformity between Cretaceous and Permo-Carboniferous sedimentary rocks and basement.

It should be noted that the cover thickness and depth to basement estimates rely on a constant shear-wave velocity of 1238 m/s, and do not take into account any changes in shear-wave velocity with depth. Also, this average shear-wave velocity is based on laboratory measurements carried out at surface pressure and temperatures and on dry core material. Thus, the true average shear-wave velocity is likely to be higher, and depth to basement is likely to have been underestimated. Using a shear-wave velocity of 1600 m/s (i.e. close to values for Permo-Carboniferous diamictite; Table 1) would result in a thickness ranging from 740– 900 m at the deepest part of the Gunbarrel Basin.

The Eucla Basin

The Eucla Basin is a succession of carbonate and siliciclastic sedimentary rocks that extends some 800 km from Balladonia in Western Australia to the South Australian border (Fig. 2) and beyond. The Eucla Basin is suitable as a passive seismic test area for several reasons: 1) GSWA has completed a deep drilling programme in the area to determine the thickness of Eucla Basin sedimentary rocks and the nature of the underlying basement; 2) the Trans-Australian Railway (Transline) access road provides relatively easy access to the area;

Figure 15. Distribution of depths to basement in metres above sea level (m asl) based on cover thickness estimates for 89 sites

located along the PNC Baseline and Emperor traverse, including a geological interpretation. Note: the Emperor

traverse runs parallel to, and is offset by approximately 5 km to the southeast from, the PNC Baseline traverse. Data

for Kakarook Corner are not included. See Table 8 for data

Scheib

26

3) a reflection seismic survey along the Transline access road was carried out at the same time as passive seismic acquisition; 4) a passive seismic program would augment the sparse information on the thickness of cover throughout the basin.

Passive seismic measurements were made at a total of 42 sites (Figs 16 and 17). Of these, 35 sites extend between Forrest and Gunnadorrah Station (Fig. 16; Table 9). The spacing between these sites ranges from c. 20 km along the Transline access road to c. 10 km for sites between FOR004 and FOR010 near Forrest, and for sites west of Kybo Station and north of Haig. Of these 35 acquisition sites, seven are at drill sites, where drillhole logs and depth to basement data are available. A further seven acquisitions were made along the Transline access road at Zanthus (Fig. 17) approximately 240 km west of Haig. These sites are spaced more closely (every 500 m) along a four km section with the aim of detecting the thickness of the basin at its western margin.

Geology

The sedimentary rock succession overlying the basement in the study area is thought to comprise two broad units: the Cretaceous sedimentary rocks of the Bight Basin Group, comprising the Madura and Loongana Formations, which are overlain by younger Cenozoic sedimentary rocks of the Eucla Group (Lowry, 1970). The Loongana Formation is a lenticular, conglomeratic, felspathic sandstone, deposited on an irregular basement surface of gneiss and granitic rocks. The sandstone is overlain conformably by glauconitic, carbonaceous, pyritic sandstone, siltstone, claystone and shale of the Madura Formation. This formation contains Berriasian–Aptian, Albian–Cenomanian, and Senonian (Upper Cretaceous, possibly Santonian–Maastrichtian) palynomorph assemblages that indicate marine deposition for all but the base of the succession (Lowry, 1970). About 300 m of Cretaceous rocks were deposited in the Madura area, followed by 180 m of Eocene and 165 m of lower Miocene beds (Lowry, 1970). About 60–120 m of Miocene strata have since been eroded. The Eocene succession comprises limestone of the Wilson’s Bluff Formation, which is a soft, poorly lithified cool-water carbonate, and the Oligocene to early Miocene Abrakurrie Limestone, which has the same origin as the Wilson’s Bluff Formation, but is better lithified (Lowry, 1990; Miller, 2012). These Eocene strata are overlain by the Nullarbor Limestone, the youngest formation in a series of middle Cenozoic, carbonate-rich sedimentary rocks, which were deposited during cool to subtropical neritic conditions and partially filled the broad, shallow Eucla Basin. This early to middle Miocene unit is one of the most extensive Cenozoic carbonate platform deposits described to date (Miller, 2012). The whole Cenozoic succession is also referred to as the Eucla Group, in which the Nullarbor Limestone is the most lithified, due to the development of low-Mg calcite cement, but with extensive secondary porosity (Miller, 2012). It also differs from the two older formations in that it is subtropical and rich in coralline algae (rhodoliths and articulated types), large and small benthic foraminifera, and molluscs (O’Connell, 2011).

Figure 18 shows the stratigraphic correlation between the seven drillholes available for this study. Drillholes HDDH01, HDDH02 and LNGD01 lack depth information for the Nullarbor Limestone because the various carbonate sedimentary rocks were not differentiated in the logs. From the remaining drillholes, the thickness of the Nullarbor Limestone ranges from 35–52 m. The depth of the underlying Wilson’s Bluff Formation is more variable, ranging from 87–194 m below the surface. The Abrakurrie Limestone has not been identified in any of the drillcores, whereas the log for FOR010 (Fig. 18) indicates possibly 20 m of Hampton Sandstone beneath the Wilson’s Bluff Formation. This Hampton Sandstone is a lenticular sandstone unit that accumulated on top of Cretaceous strata when deposition recommenced in the Middle Eocene (Lowry, 1970). Beneath the Wilson’s Bluff (and Hampton Sandstone), the Cretaceous Madura Formation has been recognized in all seven drillholes to maximum depths between 214 and 394 m. The older Loongana Formation appears in five of the seven drillholes, and is between 27 and 49 m thick. The Loongana Formation as observed in drillcores is largely unconsolidated and poorly lithified. Drillhole FOR010 is the only one that possibly contains a Paleozoic sandstone unit, which may relate to rocks of the Officer Basin. However, this needs to be examined and verified by detailed core logging. Hocking (1994) postulated that Phanerozoic sedimentary rocks of the Gunbarrel or Officer Basins extend beneath the Eucla Basin, but this is still unresolved due to an almost total lack of subsurface data. However, the important information for this study is that each of the drillholes intercepted basement, providing a complete stratigraphic succession. The depth to basement ranges from 265– 421 m, with the deeper contacts found at either end of the study area.

Basement beneath the Eucla Basin is divided by the Mundrabilla Shear Zone into two major tectonic units: the Madura Province to the west and the Forrest Province to the east (Fig. 16). In the Madura Province, exploration drilling at Loongana intersected ultramafic, metagabbroic and metagranitic rocks. In drillhole LNGD01, unconformity-bounded basement, described as layered mafic–ultramafic cumulates, was intersected at 265 m (Bunting and McIntyre, 2003). A fine-grained, equigranular, unfoliated biotite microtonalite from the same drillhole has been dated at 1408 Ma (GSWA 178071, Nelson, 2005; Spaggiari et al., 2012). Basement rocks intercepted in HDDH01 and HDDH02 near Haig comprise strongly weathered, orange-cream, medium- to coarse-grained, foliated, intermediate intrusive rock, and a weakly weathered, dark green-grey, coarse-grained gabbro, respectively. At HDDH02, the basement contact is a sharp unconformity beneath a <1 m-thick layer of conglomerate, possibly the Loongana Formation (Tillick, 2011). At HDDH01, the contact between the sedimentary rocks and the basement is also marked by Loongana Formation conglomerate, although here the unit is thicker, comprising rounded quartz-rich and mafic to intermediate plutonic rock fragments, derived from the underlying basement. Information on the basement geology from drillholes (Fig. 18) is not currently available in the Forrest Province, but may include granites of the Esperance Supersuite (Spaggiari et al., 2012).


27

Figure 16. Distribution of passive seismic acquisition sites in the Eucla Basin trial area. Black triangles indicate drillhole locations

of the 2013 GSWA drilling program and green diamonds indicate existing deep drillholes (drilled by Teck and Helix

Resources). See Table 3 for site coordinates. Base map is derived from the 1:500 000 bedrock geology of WA (Tyler

and Hocking, 1998) and State 1:500 000 regolith map (Marnham et al., 2002)


Passive seismic data were recorded at 42 sites in the Eucla Basin, including seven near Zanthus (Figs 16 and 17). The H/V spectral ratios for each site are shown in Appendices 4 and 5, and a representative selection of H/V spectra recorded between Forrest and Haig are shown in Figure 19a–d. All H/V spectra exhibit a pronounced peak at resonance frequencies below 1 Hz, and at about half of the sites there is a broad, and in some cases high, peak between 1 and 10 Hz (Fig. 19c). Some spectra also include a low-amplitude peak above 30 Hz (e.g. Fig. 19c,d). The resonance frequencies indicated by peaks at these high frequencies are not discussed here, as they are difficult to model and correspond to very shallow impedance contrasts, typical for thin, surficial deposits. Features found in the central part of the H/V spectra are discussed, but the data interpretation emphasizes the lowest frequency and most pronounced peaks (i.e. those between 0.4 and 0.91 Hz; Table 10).

Compared to Mulga Rock, H/V peaks have a lower amplitude, with more consistent/uniform form, and correspond to an average frequency of 0.52 Hz. For

the Transline sites (Fig. 20), points shown at the same longitude have been measured at different latitudes (i.e. north and south as well as along the Transline). Measurements near Forrest have slightly higher values of resonance frequency than those from the Haig area, and there is a gradual decrease in resonance frequency from Forrest towards Loongana (i.e. east to west). There is no obvious spatial variation in resonance frequency in the north–south direction.

The H/V spectra for seven sites near Zanthus, and at the predicted edge of the Eucla Basin (Fig. 17), are shown in Appendix 5, with corresponding site resonance frequencies listed in Table 10. This Table shows that data from sites near Zanthus have peaks at higher resonance frequencies than those in the Haig–Forrest area. Along this short traverse, peaks indicate a gradual east to west decrease in site resonance frequency from 1.66 Hz (ZAN01) to 1.13 Hz (ZAN06) (Fig. 21), but there is a sharp increase in resonance frequency to 2.8 Hz for the westernmost measurements (ZAN07 and ZAN08, taken at the same site). Results for ZAN03 only indicate one peak at a much higher frequency of 18.8 Hz. This is possibly related to a shallower impedance contrast.

Scheib

28

Site IDNumber of

acquisitionsZone Longitude (°E) Latitude (°S)

MAD002 3 GDA94 125.83167 30.97565

HDDH02 3 GDA94 126.10360 30.94519

EUC01 1 GDA94 126.09053 30.86644

EUC02 1 GDA94 126.21268 30.80703

EUC03 1 GDA94 126.31192 30.83626

LNGD01 1 GDA94 126.41507 30.81977

EUC04 1 GDA94 127.05566 31.07168

EUC05 1 GDA94 127.05386 30.94441

EUC06 1 GDA94 127.26130 30.92685

EUC07 1 GDA94 127.46955 30.90882

EUC08 1 GDA94 127.68008 30.89011

FOR004 3 GDA94 128.55406 31.28015

EUC09 1 GDA94 128.48593 31.20571

EUC10 1 GDA94 128.41235 31.14900

EUC11 1 GDA94 128.32068 31.09983

EUC12 1 GDA94 128.22664 31.03171

EUC13 1 GDA94 128.16484 30.96123

FOR011 1 GDA94 128.17599 30.61552

EUC14 1 GDA94 128.28372 30.57233

FOR010 1 GDA94 128.36603 30.51862

FOR011A 1 GDA94 128.17587 30.61684

EUC15 1 GDA94 128.18980 30.71935

EUC16 1 GDA94 128.09984 30.85149

EUC17 1 GDA94 127.88786 30.87134

EUC18 1 GDA94 126.78290 30.96726

EUC19 1 GDA94 126.58582 31.02337

EUC20 1 GDA94 126.50782 31.04518

EUC21 1 GDA94 126.42334 31.09703

EUC22 1 GDA94 126.38161 31.17248

MAD007 1 GDA94 126.30485 31.21989

EUC23 1 GDA94 126.22889 31.28603

EUC24 1 GDA94 126.23208 31.17663

EUC25 1 GDA94 126.23488 31.08012

HDDH01 1 GDA94 126.07937 31.05341

EUC26 1 GDA94 125.98067 31.00998

Zone Easting (m) Northing (m)

ZAN01 1 51 558432 6566424

ZAN02 1 51 557440 6566508

ZAN03 1 51 556953 6566530

ZAN04 1 51 556463 6566547

ZAN05 1 51 555942 6566512

ZAN06 1 51 555439 6566470

ZAN07 2 51 554493 6566490

Table 9. Site locations (lat/long) of 35 passive seismic recordings collected in the central

Eucla Basin area during 2013, and seven site locations (eastings/northings)

for passive seismic recordings collected in the Zanthus (western Eucla Basin)

area. Site IDs highlighted grey correspond to deep drillholes with basement

information


29

Figure 17. Distribution of passive seismic acquisition sites along the Transline access road at Zanthus.

See Table 3 for site coordinates. Dotted line near site location ZAN04 marks the estimated

western extent of the Eucla Basin. Base map is derived from the State 1:500 000 regolith

map (Marnham et al., 2002)

Scheib

30

Figure 18. Simplified stratigraphic correlation between seven drillholes from the Haig and Forrest areas, Eucla Basin,

displaying depths (dip corrected) of the major formations and geological units. The horizontal scale is

compressed. See Figure 16 for site locations

Shear-wave velocity

Laboratory-based shear-wave velocities were derived for eight core plug samples from drillhole FOR004 and for four from FOR011 (Table 6; Appendix 6). The core plug samples encompass the whole sequence of sedimentary rocks found in the Eucla Basin excluding the Nullarbor Limestone, which is the youngest in this sequence and which was not cored as part of the drilling program (Fig. 18). The eight samples taken from FOR004 span 236 m, whereas the four samples from FOR011 represent a segment of 85 m. In the latter, drillcore of the limestone above the Madura Formation was not retained and sedimentary rocks of the thinner, underlying Loongana Formation (Fig. 18) were too brittle to analyse. Shear-wave velocities from the experimental analysis are listed in Table 6. Values for carbonate rocks of the Wilson’s Bluff Formation are highest and decrease with depth. Shear-wave velocities determined for the deeper clastic sedimentary rocks of the Madura and Loongana (FOR004 only) Formations are substantially lower than those of the Wilson’s Bluff Formation and less variable, with values ranging from 684 to 951 m/s. The Madura Formation in samples from FOR011 and FOR004 produced similar velocities. However, the results for FOR004 and FOR011 are based on well-consolidated core material, which means that the less consolidated parts of the succession would likely have much lower shear-wave velocities.


Analysis of resonance frequencies from 34 sites between Haig and Forrest (Table 10; excluding those from Zanthus) focused on the lowest frequency peaks in the H/V spectra, generally between 0.44 and 0.63 Hz, because these peaks not only appear in all H/V spectra, but are usually the highest and most pronounced. Peaks observed at lower resonance frequencies are likely to correspond to the impedance contrast at the sedimentary rock – basement interface, whereas higher frequency peaks, observed in some of the H/V spectra, possibly relate to changes in the degree of consolidation within the Cretaceous and Cenozoic sedimentary rocks. Figure 20 shows that, despite the gradual east–west decrease, there is otherwise little variation in lower resonance frequencies across the Eucla Basin. Assuming a common stratigraphy, this may indicate a nearly uniform depth to the impedance contrast.

Similarly to the approach taken at Mulga Rock, passive seismic data were first analysed at drill sites where the depth to basement is known (Fig. 18). Shear-wave velocities for sedimentary rocks and basement (Table 11) were derived from analysis of the respective H/V spectra (Appendix 5) using the available depth data from drillholes and the following assumptions: a simple two-layer model, an average density of 2.1 g/m3, a Poisson ratio of 0.38 for the Cenozoic to Cretaceous sedimentary rock


31

Fig

ure

19. S

ele

cti

on

of

H/V

sp

ectr

al ra

tio

s a

nd

co

rresp

on

din

g t

hre

e-c

om

po

nen

t (F

ou

rier)

plo

ts f

or

fou

r sit

es in

th

e E

ucla

Basin

: a)

sit

e M

AD

002 (

GS

WA

DH

2013);

b)

sit

e F

OR

011

(GS

WA

DH

2013);

c)

sit

e E

UC

02;

d)

sit

e E

UC

04. H

/V t

races f

rom

th

e r

est

of

the s

tud

y a

rea a

re lis

ted

in

Ap

pen

dix

4.

Scheib

32

Table 10. Lowest resonance frequencies (fz) derived from

peaks in H/V spectral ratios for 42 sites in the Eucla

Basin. Sites are ordered from west to east and

assigned to one of three groups based on location:

t = along the Transline, n = north of the Transline and

s = south of the Transline. Resonance frequencies

from drillhole sites with complete depth information

are highlighted in grey (corresponding H/V spectra

in Appendix 4)

Site ID GroupResonance

frequency (Hz)

ZAN07 t 2.84

ZAN08(a) t 2.78

ZAN06 t 1.13

ZAN05 t 1.13

ZAN04 t 1.09

ZAN03 t –

ZAN02 t 1.19

ZAN01 t 1.66

MAD002 t 0.44

EUC26 t 0.48

HDDH01 t 0.44

EUC01 n 0.44

HDDH02 n 0.50

EUC02 n 0.44

EUC23 s 0.37

EUC24 s 0.48

EUC25 s 0.53

MAD007 s 0.51

EUC03 n 0.44

EUC22 s 0.46

LNGD01 n 0.53

EUC21 s 0.44

EUC20 s 0.50

EUC19 t 0.47

EUC18 t 0.56

EUC05 t 0.44

EUC04 s 0.47

EUC06 t 0.41

EUC07 t 0.47

EUC08 t 0.55

EUC17 t 0.58

EUC16 t 0.62

EUC13 s 0.62

FOR011 n 0.53

EUC15 n 0.53

EUC12 s 0.60

EUC14 n 0.50

EUC11 s 0.61

FOR010 n 0.63

EUC10 s 0.59

EUC09 s 0.91

FOR004 s 0.57

succession, and a density of 2.5 g/m3 and a Poisson ratio of 0.35 for the basement layer. The average shear-wave velocity for both the Cenozoic carbonate and Cretaceous clastic sedimentary rocks is 618 m/s, which is consistent with values for sedimentary rocks listed in Table 1. In comparison with shear-wave velocities measured in core samples from FOR004 and FOR011 (Table 6), the value of 626 m/s seems low. However, the lower shear-wave velocity based on the H/V analysis may reflect the unconsolidated nature of the clastic sedimentary rocks. Additionally, the experimental results tend generally to be higher than those derived by the H/V method.

Some independent assessment of shear-wave velocity data is also available from deep reflection seismic line program 12GA-AF3 along the Transline (Spaggiari and Tyler, 2014). As part of the acquisition, a multichannel analysis of surface waves (MASW) was carried out to produce a shear-wave velocity model from low-frequency sweep seismic reflection data (unpublished data). Modelling of three-phase velocity curves indicated a high-velocity unit above a lower velocity unit in the subsurface. These possibly correspond to the indurated Nullarbor Limestone and Wilson’s Bluff Formation, jointly representing the high-velocity unit, overlying poorly consolidated Cretaceous sedimentary rocks of the Madura and Loongana Formations. Depending upon which phase velocity curve is used, the MASW model suggests that shear-wave velocities in the top 80 m could be either 800–1000 m/s or 1200–2000 m/s. A more precise model is not possible because the spacing of the geophones was too wide, having been set up to investigate to a much greater depth of several kilometres. Despite the spacing, the 800–1000 m/s velocity range, combined with the qualitative observation from passive seismic data that the shear-wave velocity is lower below 80 m (i.e. for the clastic sedimentary rocks), suggests that the average shear-wave velocity of 626 m/s derived from the passive seismic data may be an appropriate estimate for the entire stratigraphic column.

Cover thickness estimates based on a shear-wave velocity of 626 m/s are listed in Table 12, which includes the corresponding depths to basement above sea level. However, the disadvantage of using an average value for shear-wave velocity is that sites with the highest and lowest resonance frequencies under- or overestimate the values for thickness. The regression model approach (e.g. Ibs-van Seht and Wohlenberg, 1999; Parolai et al., 2002) offers an alternative as the power-law equations reflect the influence of thickness to some degree. Figure 22a shows a strong relationship between the resonance frequency and depth to basement for data from the six drillholes available in the study area (Table 11). Based on this close relationship, a power-law equation was used to convert the resonance frequencies of the remaining 28 sites in the area into cover thickness estimates (Table 12). This power-law model produces cover thickness estimates ranging from 113 m (EUC09) to 519 m (EUC23), with an average of 311 m. Although the average thickness of 308 m based on a shear-wave velocity of 626 m/s is almost identical, the narrower range of thicknesses in that case (171 m in EUC09 to 420 m in EUC23) is the consequence of using a constant shear-wave velocity, which does not account for

NOTE: (a) same site as ZAN07


33

Figure 20. Resonance frequencies of peaks (amplitude >2) corresponding to low frequencies (<1 Hz) shown in H/V spectral

ratios (Appendix 5; Fig. 17) for measurements taken in the Eucla Basin in 2013. Arrows indicate locations along

the Transline

Figure 21. Distribution of the resonance frequencies from

peaks (amplitude >2) in H/V spectral ratios

(Appendix 5) for acquisitions taken at Zanthus at

the western edge of the Eucla Basin. All sites are

on the Transline

changes in lithology (e.g. Fig. 22b). This further supports the application of the power-law regression model, which takes this depth-relation into account. The stratigraphic successions in the Eucla Basin is similar to the sediment-to-basement relationships discussed by Ibs-von Seht and Wohlenberg (1999) and Parolai et al. (2002) for two sedimentary successions in Germany, and both studies produced good results using power-law relations.

Passive seismic results from drillhole FOR004 have not been included in the above discussion, mainly because the data from three measurements taken at and near this location (Appendix 4) are highly variable, with none of the H/V spectra indicating a clear peak with amplitudes >2. Those weak peaks indicate resonance frequencies that range between 0.53 and 0.63 and by using the known depth to basement of 420 m (Fig. 18), the calculated shear-wave velocities would be 820–980 m/s. These shear-wave velocities are different to those from the remaining drillhole sites (Table 11) and possibly reflect the fact that the three measurements were not taken at the same location and therefore represent localized variation in lithology, or reflect a different rheology at FOR004, which is located much further south than other drillhole sites. This should be taken into consideration when modelling other sites away from the main traverse, such as EUC09 and EUC10.

The cover thickness estimates were converted into actual depths to basement (Table 12), which are displayed in cross-section (Fig. 23a,b) along with available drillhole data. Figure 23a shows the passive seismic estimates of depth to basement only for sites along the Transline, and

the dotted line connecting the data points equates to a traverse length of 220 km, along which there is only weak undulation of the basement under Phanerozoic cover. In the eastern half, the cover becomes gradually thinner towards Forrest, decreasing by 200 m over a distance of

Scheib

34

Site ID fz (Hz)

Depth to

basement

(m)(a)

Vs1

(m/s)

Vs2

(m/s)

Thickness (m) by methodFeatures in H/V spectra

(Appendix 4)Vs1(b) p-l

p-l

(1999)

p-l

(2002)

HHDH01 0.44 421 740 1660 356 387 300 386 Strong peak at 0.44 Hz

HDDH02(c) 0.50 295 560 1280 313 312 251 316 Strong peak at 0.5 Hz; very broad

peak between 2.5 and 12 Hz

FOR011 0.53 281 630 1300 295 283 232 289 Good peak at 0.53 Hz; second

broader peak between 2.5 and 4 Hz

LNGD01 0.53 265 560 1215 295 283 232 289 Strong peak at 0.53 Hz; good double

peak at 16.5 and 27 Hz

FOR010 0.63 245(d) 615 1190 248 211 182 221 Subtle peak at 0.63 Hz; first two

windows excluded

MAD002(c) 0.44 383 650 1530 356 387 300 386 Strong peak at 0.44 Hz; first seven

and last three windows excluded;

very broad peak between 2.8 Hz and

4.4 Hz

NOTES: (a) dip corrected; (b) based on the average Vs1 = 626 m/s; (c) average of three acquisitions; (d) depth to Officer Basin and base of the Loongana Fm.

Table 11. Modelled shear-wave velocities for the sedimentary rocks (Vs1) and underlying basement (Vs2) based on the recorded

resonance frequencies and known depth to basement values at six drillhole sites in the Eucla Basin. Table also

includes estimates of cover thickness based on the average shear-wave velocity (Vs1) of 626 m/s and the power-law

regression model of Figure 22a. Cover thickness estimates based models from Ibs-von Seht and Wohlenberg (1999)

and Parolai et al. (2002) — indicated by p-l (1999) and p-l (2002), respectively — are given for comparison.

Figure 22. Regression plots for passive seismic and depth data for six drillhole sites along the Transline: a) depth to basement

(cover thickess) and measured resonance frequency; b) depth to basement and modelled shear-wave velocity. See

Table 11 for data

about 100 km. The deepest inferred basement contact is approximately 250 m below sea level and located about 20 km west of the Mundrabilla Shear Zone (Fig. 23a). However, the wide spacing of passive seismic stations means that no conclusion can be drawn about a causal relationship between the fault and the change in depth. Estimates of the depth to basement for sites located north and south of the Transline (Fig. 16) are shown in Figure 23b, together with the sites along the Transline.

In the Haig area, depth estimates for sites north and south of the Transline are generally similar to or shallower than those on the Transline, except for one site where the estimated cover thickness is 519 m (EUC23) and more than 150 m deeper than the remaining sites. For most sites east of Forrest, both north and south of the Transline, the basement depth is greater than those along the Transline, which may suggest that the basement at Forrest is a localized high.


35

Site ID Group fz (Hz)Height

(m asl)Cover thickness (m) Depth of basement (m asl)

Vs1

626 m/s

p-l Actual(a) Vs1

626 m/s

p-l Actual(a)

MAD002(b) t 0.44 171.0 356 387 383 –185 –216 –212

EUC26 t 0.48 167.1 326 334 –159 –167

HDDH01 t 0.44 173.7 356 387 421 –182 –214 –247

EUC19 t 0.47 173.0 333 347 –160 –174

EUC18 t 0.56 183.2 279 258 –96 –74

EUC05 t 0.44 183.9 356 387 –172 –204

EUC06 t 0.41 180.7 382 437 –202 –256

EUC07 t 0.47 190.4 333 347 –143 –156

EUC08 t 0.55 181.8 285 266 –103 –84

EUC17 t 0.58 169.4 270 243 –100 –73

EUC16 t 0.62 162.4 252 217 –90 –54

HDDH02 n 0.50 184.4 313 312 295 –129 –128 –111

EUC01 n 0.44 186.2 356 387 –170 –201

EUC03 n 0.44 187.0 356 387 –169 –200

LNGD01 n 0.53 188.2 295 283 265 –107 –95 –77

EUC02 n 0.44 187.8 356 387 –168 –200

EUC15 n 0.53 166.2 295 283 –129 –117

FOR011 n 0.53 178.4 295 283 281 –117 –104 –103

EUC14 n 0.50 178.5 313 312 –135 –134

FOR010 n 0.63 182.4 248 211 245 –66 –29 –63

EUC23 s 0.37 157.8 423 519 –265 –362

FOR004(b) s 0.57 133.9 275 250 383 –141 –116 –249

MAD07 s 0.51 154.2 307 302 –153 –148

EUC09 s 0.91 142.9 172 113 –29 –30

EUC24 s 0.48 163.5 326 334 –163 –171

EUC22 s 0.46 153.9 340 359 –186 –205

EUC10 s 0.59 147.6 265 236 –118 –88

EUC11 s 0.61 156.2 257 223 –100 –67

EUC21 s 0.44 156.0 356 387 –200 –231

EUC25 s 0.53 162.2 295 283 –133 –121

EUC04 s 0.47 175.0 333 347 –158 –172

EUC20 s 0.50 154.7 313 312 –158 –157

EUC12 s 0.60 163.8 261 229 –97 –65

EUC13 s 0.62 160.6 252 217 –92 –56

Table 12. Cover thickness estimates for 35 sites in the Eucla Basin calculated from the average shear-wave velocities (Vs1) of

626 m/s and from power-law (p-l) equation of Figure 22a. The depths of basement above sea level (asl) are derived

from the cover thickness and surface height of the site.

NOTES: (a) from drillhole log and dip corrected; (b) average of three acquisitions

Scheib

36

Figure 23. Depth of basement in metres above sea level (m asl) based on cover thickness estimates for sites along the

Transline: a) sites on the Transline only; b) including sites north and south of the Transline. See Table 12 for data


37

At this scale and coverage, the data do not clearly indicate a depth increase or decrease in one particular direction. However, the thickness estimates do suggest that the basement surface is weakly undulating in both east–west and north–south directions. However, in contrast to the thickness model for Mulga Rock, the undulating basement of the Eucla Basin is characterized by very little variation, with minor fluctuations between 100 and 250 m below sea level over a distance of more than 200 km,

Extending this to other parts of the Eucla Basin must take into account the effect of differing rheology of cover rocks on the passive seismic data. The degree of cementation of Cretaceous sedimentary rocks directly overlying basement is highly variable. This will certainly have an effect on shear-wave velocities and add a degree of uncertainty to the two-layer model. This may be the case in the eastern part of the study area, in particular for sites FOR004 and EUC09, for which the drillhole data suggest a much deeper contact and the applied regression model may not be suitable (dotted outline in Fig. 23b).

Passive seismic data measured near Zanthus provide some information on cover thickness near the edge of the Eucla Basin (Fig. 24a,b). There are no drillhole data for this area, but this sand-dominated area is similar to that at Mulga Rock. Cover thickness estimates have been derived by H/V analysis using three different shear-wave velocities including 250 m/s (to represent sandy soft soils, Table 1), 500 m/s (the average for the northwest PNC Baseline) and 626 m/s (the average shear-wave velocity for the Eucla Basin), and the two power-law functions used at Mulga Rock and in the Eucla Basin (Figs 14 and 22). The results of these five different approaches (Fig. 24a) produce thicknesses of the sedimentary rocks ranging from 55 m (using a shear-wave velocity of 250 m/s) to greater

Figure 24. Comparisons of cover thickness and depth estimates for seven sites along a 4-km section of the Transline at

Zanthus: a) thickness of cover; b) derived depths of basement in metres above sea level (m asl). The estimates are

based on three different shear-wave velocities and the power-law (p-l) models developed for Mulga Rock and the

Eucla Basin.

than 140 m (using a shear-wave velocity of 626 m/s) at its deepest point. Figure 24b shows the same data but expressed as basement depths above sea level. The shape of the curve connecting the data points closely mirrors that of the surface and this further suggests that the edge of the Eucla Basin may coincide with the most westerly site (ZAN07/08). The results from the three average shear-wave velocities vary substantially, whereas depths determined by the two power-law functions appear to be more similar, which could suggest they provide better estimates. However, the example from Zanthus clearly highlights the necessity of drillhole control when using passive seismic.

The Boorabbin sand resource

survey – an application

The Tromino system was employed as part of a strategic sand resource survey in an area within the Youanmi Terrane of the Yilgarn Craton north of Boorabbin, approximately 90 km west of Kalgoorlie. This strategic sand resource was protected in 2007 by a Section 19 exclusion under the Mining Act 1978 to allow for possible future quarantining of the area as a medium- to very long term source of construction sand for the greater Eastern Goldfields region (Normore, 2014). The passive seismic system was deployed to provide estimates of sand thickness because no drillhole information (apart from hand-auger records) was available within the survey area, although limited information is available from nine drillholes just outside the area (Fig. 25). Logs from those drillholes were used as controls on interpretation of the passive seismic data. The study area is entirely covered by eolian sand, which overlies variably weathered Archean granitic rocks of the Youanmi Terrane.

Scheib

38

Figure 25. Location of passive seismic acquisition sites across the Boorabbin sand resource survey (modified after

Normore, 2014). Control sites with drillhole data are also indicated.


39


The passive seismic survey acquired 35 recordings, including the nine control sites. The 26 sites within the survey area are distributed along four main traverses (Fig. 25). Figure 26 presents H/V spectral ratios and corresponding Fourier plots for the nine control sites. The data are based on an acquisition duration of 14 minutes at 128 Hz. The H/V spectra display overall very weak responses, with the exception of spectra from sites CPR002 and 4618 (Figure 26b and d, respectively). These spectra have peaks with amplitudes >2. Figure 26b shows two strong peaks at resonance frequencies of 9.2 and 3.7 Hz, and site 4618 has one broad and low peak at 2.95 Hz. Most of the other H/V spectra have small peaks with H/V ratios <2 at frequencies >30 Hz. Some of the spectra also seem to have very subtle and broad features between 2 and 5 Hz, but without producing a peak with an amplitude of >2. These possibly correspond to a second, much weaker impedance contrast similar to that shown by CPR002 (Fig. 26b). Because the aim of the passive seismic survey is to estimate the thickness of the sand cover, Table 13 lists resonance frequencies for only the first peak in the H/V spectra. In cases where the peak in the H/V spectra is below an amplitude of 2, the Fourier plot was assessed to see if it showed a lens-shape feature. This lens-shaped feature, however, often relates to only one of the

Site ID Control

Sand

thickness

(m)

fz (Hz)Estimated

Vs (m/s)Comments

109 CPR001 9 4.1 160 Very broad peak (<2) at 4.1 Hz; weak peak in E–W component at 46 Hz;

silcrete layer between 1 and 9 m; 9–17 m clay saprolite

110 CPR002 9 9.2 300 Strong peaks at 9.2 and 3.69 Hz, second peak indicates impedance at

49 m depth; silcrete layer between 2 and 6 m

111 45/40 1 46.6 190 Poor to weak signal, no response, only peak at 46.6 Hz, shallow E–W

peak at 17.5 Hz and N–S at 27 Hz; 0–1 sand, 1–10 m weathered rock

112 4618 4 47.8 – Strong peak at 47.8 Hz, likely corresponds to an impedance at 1 m (and

Vs = 190 m/s). Peak at 2.95 Hz may indicate basement at 35 m. Spike

at 13.23 Hz, could be indication of clay–sand interface; 0–4 sand, 4–9 m

soft rock and 9–40 m sand and sandy clay

129 RAB045 1 48.5 200 Very broad feature from 27–52 Hz; highest at 37 and 48.5 Hz;

former would result in Vs = 150 m/s at 1 m; 0–1 m sand, 1–18 m

undifferentiated saprolite

130 RAB044 3 16.3 210 Very broad feature from 27 to >64 Hz might indicate a shallow

impedance at 38 Hz; lens-shape feature in Fourier at 16.3 Hz may relate

to the 3 m; 0–3 m sand, 3–37 m upper saprolite, 37–53 m lower saprolite

and basement at 53 m

131 RAB043 5 36.0 – Highest peak at 36.2 Hz indicates a shallow impedance at 1.4 m using

Vs = 200 m/s; 0–5 m sand, 5–38 m upper saprolite, 38–63 m lower

saprolite and basement at 63 m

132 RAB042 – 49.2 – With Vs = 200 m/s impedance at 1 m; potential impedance at 17.5 Hz

based on E–W component only; low broad peak at 3.0 Hz may be base

of saprolite; 1–3 m of sandy gravel at surface

133 RAB041 – 55.4 – Highest peak at 55.4 Hz indicates an impedance at <1 m; broad peak

(<2) at 3.0 Hz may indicate base of saprolite; 1–3 m of sandy gravel at

surface

Table 13. Known sand thickness and corresponding resonance frequencies (fz) based on the first peak in H/V spectra for

control sites in the Eucla Basin (Fig. 25). Estimated values for the shear-wave velocities (Vs) are derived from H/V

analysis using given sand thicknesses.

horizontal components, thus resonance frequencies should be viewed with caution.

Figure 27 displays H/V spectra and Fourier plots for a selection of the 26 sites measured along the four traverses. Similarly to results from the control sites, the H/V spectra are in many cases featureless and provided very few strong peaks. However, most of the H/V spectra display a small peak or feature at frequencies >20 Hz (Fig. 27b,d). In some cases a second peak appears at lower frequencies between 2 and 5 Hz (Fig. 27a,d), very similar to the second peak at control site CPR002, and this may correspond to a second, deeper impedance contrast. Following assessment of data for all 26 sites, Table 14 lists the resonance frequencies based on the first peak or peak-like feature in the H/V spectra. Resonance frequencies range from 16.7 to 51.3 Hz.

Results for site 48 are based on four acquisitions recorded at exactly the same location, but under varying settings and parameters in an attempt to improve the signal. Changes included a different gain setting and a wider frequency range, but despite the changed settings, the four resulting H/V spectra shown in Figure 28 are indistinguishable over the 14 minute acquisition interval. Three three weak features are indicated, including two subtle peaks at 48.2 and 4.1 Hz.

Scheib

40

Fig

ure

26. H

/V s

pectr

al

rati

os a

nd

co

rresp

on

din

g t

hre

e-c

om

po

nen

t (F

ou

rier)

plo

ts f

or

co

ntr

ol

sit

es i

n t

he B

oo

rab

bin

are

a:

a)

CP

R0

01 (

sit

e I

D 1

09);

b)

CP

R0

02 (

110);

c)

4540 (

111);

d)

4618 (

112)


41

Fig

ure

26. H

/V sp

ectr

al

rati

os an

d co

rresp

on

din

g th

ree-c

om

po

nen

t (F

ou

rier)

p

lots

fo

r co

ntr

ol

sit

es in

th

e B

oo

rab

bin

are

a:

e)

RA

B045 (1

29);

f)

R

AB

044 (1

30);

g

) R

AB

043 (1

31);

h)

RA

B042 (

132)

Scheib

42

Figure 26. H/V spectral ratios and corresponding three-component (Fourier) plots for

control sites in the Boorabbin area: j) RAB041 (133)

Site ID Traverse fz (Hz) Comments

45 A 40.0 Small, subtle peak (<2) at around 40 Hz; second very broad peak (>2) at 6 Hz; possible small V peak at 13.2 Hz

47 A 48.6 Small, single peak (>2) at 48.6 Hz; second, very strong, single peak (>5) at 4 Hz; small V peak at 13.2 Hz

48(a) A 48.2 In all H/V plots, one small, single peak (<2) at 48.2 Hz; possible second broad feature between 2 and 5 Hz;

small single V peak at 13.2 Hz

49 A 40.0 Very broad peak (>2) at 20–44 Hz with strongest peak at 40 Hz; small V peak at 13.2 Hz

113 A 27.4 Good broad, single peak (>2) at 27.4 Hz

114 A 47.0 Broad peak (>2) between 40 and 56 Hz with strongest feature at 47 Hz; in Fourier plot, all three components

are peaking at approximately 8 Hz

115 A 38.0 No peak in H/V; gradual fz increase from 20 Hz, peak in E–W component at 38 Hz; strong V peak at 8.7 Hz

116 A 48.5 Overall broad feature from fz >20 Hz; one strong single peak (>2) at 48.5 Hz

117 A 48.3 Good small, single peaks (>2) at 48.3 and 6.8 Hz; V peak at 13.2 Hz

71 B 34.3 Broad feature with peak (>2) at 34.3 Hz; strong single V peak at 13.2 Hz

72 B 16.7 Strong, single peak (>2) at 16.7 Hz followed by a strong V peak at 13.2 Hz

118 B 51.3 Broad peak (>2) at 51.2 Hz with an E–W peak at 32 Hz; second smaller H/V peak (<2) at 9.6 Hz just after a

strong V peak at 13.2 Hz

119 B 49.0 Single, small peak (>2) at 48.5 Hz; second strong peak (>3) at 7.5 Hz; minor V peak at 13.1 Hz

120 B 26.5 Large broad feature with peaks in the Fourier plot between 25 and 38 Hz; best peak (<2) at 26.5 Hz; second

small single peak at 5.6 Hz; strong V peak at 13.2 Hz

121 B 35.0 Broad shallow feature between 25 and 47.5 Hz; second small peak (<2) at 6.7 Hz

122 C 25.0 Broad shallow feature with peak (>2) at 25 Hz, mainly based on E–W component; second single peak at 4.8 Hz

123 C 26.0 Broad shallow feature with peak (>2) at 26 Hz, mainly based on E–W component

124 C 29.1 Broad feature between 14 and 50 Hz with peak (>2) at 29.1 Hz; second single peak at 3.7 Hz

125 C 20.0 Very broad feature between 14 and 50 Hz, strongest feature in E–W at around 20 Hz

97 D 46.0 Broad peak (>2) from 31 to 48Hz

101 D 22.4 Good broad peak (>2) at 22.4 Hz

104 D 48.8 Small, single peak (>2) at 48.8 Hz; second small peak (<2) at 3.7 Hz (mainly in N–S)

107 D 40.0 Broad peak (<2) from 30 to 50 Hz

126 D 31.7 Small single peak (>2) at 31.7 Hz (mainly in E–W); second small peak (<2) at 7.3 Hz

127 D 48.6 Very broad feature >20 Hz with one small peak (>2) at 48.6; peaks in N–S at 55 Hz and in E–W at 27 Hz

128(b) D 48.8 Broad feature >30 Hz with main peak at 48.8 Hz

NOTES: (a) based on four acquisitions; (b) based on two acquisitions

Table 14. Resonance frequencies (fz) based on the first peak in the H/V spectral ratios of 26 sites in the Boorabbin study area

(Fig. 25). Comments include brief description of the H/V spectrum, and further peaks and features


43

Fig

ure

27.

S

ele

cti

on

of

H/V

sp

ectr

al ra

tio

s a

nd

co

rresp

on

din

g t

hre

e-c

om

po

nen

t (F

ou

rier)

plo

ts f

or

test

sit

es in

th

e B

oo

rab

bin

are

a:

a)

sit

e 4

7;

b)

sit

e 1

13;

c)

sit

e 7

2;

d)

sit

e 1

24

Scheib

44

Figure 28. H/V spectral ratio plots for four 14 minute recordings at site 48 using following

settings: a) high gain and 128 Hz; b) gain x 8 and 128 Hz; c) high gain and

512 Hz; d) high gain and 128 Hz


The H/V analysis of the passive seismic data from the Boorabbin area follows the same approach as discussed for the two trial areas. For this area, the H/V analysis is based on a simple, two-layer subsurface model, comprising a sand layer of unknown thickness overlying weathered granitic basement. The H/V analysis of data from the control sites help to derive an estimate for the shear-wave velocity of the sand cover in conjunction with drillhole information. This estimate can then be used for the remaining sites within the study area to provide sand thickness estimates.

Control sites

The derived resonance frequencies are highly variable

and range from 4.1 to 49.2 Hz (Table 13). The same can be said for the corresponding sand thickness, data for which was collated from drill logs, but seemed of variable quality. However, this depth information was the only source for sand thickness and therefore was used in modelling a synthetic curve to the H/V spectra (like that shown in Fig. 4). In addition to the sand thickness information, a density of approximately 1.8 g/cm3 and a Poisson ratio of 0.4 were used to model a sand layer. The synthetic model provides shear-wave velocities between 160 and 300 m/s for the sand layer (Table 13). Shear-wave velocities for sites 4618 and RAB043 were excluded as the resonance frequencies of 47.8 and 36 Hz are too high to be consistent with the known sand thicknesses at each site of 4 and 5 m, respectively. These frequencies are more likely an indication of impedance


45

contrast within 1 m of the surface. Similarly, the lack of sand thickness information for sites RAB042 and RAB041 did not allow a shear-wave velocity to be calculated, and the high frequencies detected at these two sites also indicate a shallow impedance contrast. Excluding data from these four control sites, the remaining results indicate an average shear-wave velocity for the surface sands of 212 m/s. This estimate is consistent with values listed in Table 1 for sandy materials or soft soils, and suggests that the shear-wave velocity of 212 m/s is an acceptable value and can be applied in this study.

As described for Mulga Rock and the Eucla Basin, an alternative to using an average shear-wave velocity for the whole study area, is the application of a regression model that allows the resonance frequency to be converted directly into a thickness estimate. Figure 29a shows the relationship between the measured resonance frequencies and known sand thickness at the control sites. A fitted power-law regression model has a reasonably good fit indicated by a r2 = 0.56. The fit and regression model however, improves substantially to an r2 = 0.92, when data points for 4618 and RAB043 are excluded (Fig. 29b). Comparative estimates of the sand thickness for the control sites based on the average shear-wave velocity of 212 m/s and on power-law models 1 and 2 (Table 15) show that results derived from using the average shear-wave velocity and model 2 are very similar and (with the exclusion of sites 4618 and RAB043) reflect the drillhole information well. However, estimates derived from model 1 appear to be too high.

Sand thickness estimates for sites within the survey areaRefined parameters based on data from the control sites were used to calculate sand thicknesses from the

Figure 29. Two power-law regression models based on recorded site resonance frequencies (y-axis) and sand thickness

from drill logs (x-axis) for control sites CPR001, CPR002, 4618, RAB045, RAB044 and RAB043 in the Boorabbin area

(Table 1). Data for site RAB043 are excluded from plot (b).

resonance frequencies at the 26 sites within the survey area using the three approaches: the average shear-wave velocity of 212 m/s, regression model 1, and regression model 2 (Fig. 29a,b). Table 16 lists the resulting sand thickness estimates for the 26 sites within the sand survey area. As with results from the control sites, estimates derived from the average shear-wave velocity of 212 m/s and from model 2 are very similar, indicating average sand thicknesses of 3.17 m and 3.37 m, respectively. Estimates from model 1 are almost twice as high and may be an overestimate. Notably, however, some thickness estimates using model 1 seems to be a better match to sand thicknesses determined by hand augering.

Resonance frequencies for sites along traverses A and D are largely consistent and at or above 40 Hz, indicating a sand cover of 1.5 m (model 2 and average shear-wave velocity) to 2.8 m (model 1). Sand thickness estimates of sites from traverse B are most variable in all three approaches (Table 16) as a result of very different resonance frequencies measured between sites. Resonance frequencies for the four sites of traverse C are the lowest at about 25 Hz, suggesting the thickest sand units in the study area are approximately 2.2 m (model 2 and average shear-wave velocity) to 4.5 m (model 1) thick.

The H/V spectral ratios were largely featureless, possibly related to weak impedance contrasts at depths, which made interpreting the data somewhat difficult. As a result sand thickness estimates from all three approaches should be viewed with caution. Nevertheless, the estimates indicate a shallow sand cover of a few metres throughout the survey area and, in conjunction with other mapping techniques, have provided information on an extensive, highly prospective deposit of construction-grade sand for the Eastern Goldfields region (Normore, 2014).

Scheib

46

Site ID Control siteKnown

depth (m)fz (Hz)

Estimated depth (m)

Average Vs Regression

model 1

Regression

model 2

109 CPR001 9 4.0 13.25 18.99 12.86

110 CPR002 9 9.2 5.76 9.79 5.89

111 4540 1 46.6 1.14 2.70 1.29

112 4618 4 47.8 1.11 2.64 1.26

129 RAB 045 1 48.5 1.09 2.61 1.24

130 RAB 044 3 16.3 3.25 6.22 3.45

131 RAB 043 5 36.0 1.47 3.31 1.64

132 RAB 042 – 49.2 1.08 2.58 1.22

133 RAB 041 – 55.4 0.96 2.35 1.10

Table 15. Estimates of sand thicknesses for the nine control sites in the Boorabbin study area,

based on the conversion of the resonance frequency (fz) using an average shear-

wave velocity (Vs) of 212 m/s and regression models from Figure 29.

Site ID Traverse

Sand

thickness(a)

(m)

fz (Hz)

Estimated sand thickness

Vs = 212

m/s

Regression

model 1

Regression

model 2

45 A 0.4 40.0 1.33 3.04 1.49

47 A >2 48.6 1.09 2.61 1.24

48(b) A >2 48.2 1.10 2.63 1.25

49 A >2 40.0 1.33 3.04 1.49

113 A 27.4 1.93 4.11 2.12

114 A 47.0 1.13 2.68 1.28

115 A 38.0 1.39 3.17 1.56

116 A 48.5 1.09 2.61 1.24

117 A 48.3 1.10 2.62 1.25

71 B >2 34.3 1.55 3.44 1.72

72 B >2 16.7 3.17 6.10 3.37

118 B 51.3 1.03 2.50 1.18

119 B 49.0 1.08 2.59 1.23

120 B 26.5 2.00 4.22 2.19

121 B 35.0 1.51 3.39 1.69

122 C 25.0 2.12 4.42 2.31

123 C >2 26.0 2.04 4.29 2.23

124 C 29.1 1.82 3.92 2.00

125 C 20.0 2.65 5.28 2.85

97 D >2 46.0 1.15 2.72 1.30

101 D 1.17 22.4 2.37 4.83 2.56

104 D >2 48.8 1.09 2.60 1.23

107 D >2 40.0 1.33 3.04 1.49

126 D 31.7 1.67 3.66 1.85

127 D 0.3 48.0 1.10 2.63 1.25

128(c) D 48.8 1.09 2.60 1.23

Min 1.03 2.50 1.18

Max 3.17 6.10 3.37

Mean 1.55 3.41 1.72

NOTES: (a) based on hand-augering; (b) based on four acquisitions; (c) based on two acquisitions

Table 16. Estimates of sand thicknesses for the 26 test sites in the Boorabbin study area

based on the conversion of the resonance frequency (fz) using an average shear-

wave velocity (Vs) of 212 m/s and the regression models from Figure 29. Estimates

in italics are derived from resonance frequencies that relate to broad features or

weak peaks in the H/V spectrum.


47

ConclusionsThe acquisition and interpretation of passive seismic data from the three trial areas — Mulga Rock, Eucla Basin, and the Boorabbin area — provide realistic estimates of the depth to basement, and therefore information on the thickness of Phanerozoic sedimentary rock and sand cover. The success of the trial surveys shows that this method can be used to measure the thickness of thin (5 m) sandplain deposits to successions of sedimentary rocks exceeding 700 m. The acquisition process is rapid and the data modelling approach is uncomplicated.

The approach of using a regression model to convert measured resonance frequencies directly into estimates of the depth to the impedance contrast (e.g. Ibs-von Seht and Wohlenberg, 1999; Parolai, 2002) has been shown to be successful. However, this approach can only be used where there is drillhole control that allows comparison of resonance frequencies with depth information and the stratigraphy is simple. Overall, it is essential that any passive seismic survey has some degree of depth control, ideally from drillholes, which allows ground truthing of passive seismic data and provides depth information that can be used to derive shear-wave velocity estimates by modelling of H/V spectral ratios. The paucity of drillhole data typical of greenfields areas will introduce problems, and in these cases, regression models established from other, geologically similar areas could be used as proxies. Of the three studies discussed here, the use of passive seismic to estimate the thickness of shallow surficial sand was the least successful, in that few peaks in H/V spectra were detected. This probably reflects the limited velocity contrast between sand and weathered Archean granitic basement, resulting in only a weak impedance contrast across the sand–granite boundary. Despite this, the modelled results are broadly consistent with sand thicknesses recorded from control sites adjacent to the area, and should be useful in future studies of sand resource potential (cf. Normore, 2014).

AcknowledgmentsThe author would like to thank Josef Holzschuh from Geoscience Australia, Canberra, for useful discussions and carrying out multichannel analysis of surface waves of reflectance seismic data acquired in the Haig area. The contributions of Claudio Della Piane and colleagues at CSIRO, Perth, are also acknowledged for carrying out rheological analyses of the core samples and provision of shear-wave velocity data for this study.

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49

Appendix 1

Horizontal to vertical (H/V) spectral ratios for sites at Kakarook Corner at Mulga Rock

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50

Appendix 2

Horizontal to vertical (H/V) spectral ratios for sites along the PNC Baseline at Mulga Rock

(sites are in order southeast to northwest)


51

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52


53

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54


55

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56


57

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58

Appendix 3

Horizontal to vertical (H/V) spectral ratios for sites along the Emperor traverse at Mulga Rock

(sites are in order southeast to northwest)


59

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Appendix 4

Horizontal to vertical (H/V) spectral ratios for sites in the Eucla Basin. The first 14 graphs correspond

to drillhole locations, followed by infill measurements (site IDs EUC listed in numerical order)


61

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62


63

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Appendix 5

Horizontal to vertical (H/V) spectral ratios for sites along the Transline at Zanthus

at the western edge of the Eucla Basin


65

Appendix 6

Photographs of drillcore (85 mm diameter) of sedimentary rocks recovered from holes

Empress 1/1A (pictures 1–6), FOR004 (pictures 7–14) and FOR011 (pictures 15–18). Red tape or

crosses indicate location where shear-wave velocity measurements were taken. For formation

names and lithologies, see Table 6 using the corresponding GSWA number.

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66


67

Appendix 7

Selection of waveforms of elastic pulse transmitted through core samples 212457, 212458 and

212459 from site FOR004

THE APPLICATION OF PASSIVE SEISMIC TO ESTIM

ATE COVER THICKNESS IN GREENFIELDS AREAS OF W

ESTERN AUSTRALIA — M

ETHOD, DATA INTERPRETATION AND RECOM

MENDATIONS

RE

CO

RD

2014/9

Sch

eib

This Record is published in digital format (PDF) and is available as a free

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<www.dmp.wa.gov.au/GSWApublications>.

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