an analysis of aeromagnetic data to reveal palaeochannels...

AN ANALYSIS OF AEROMAGNETIC DATA TO REVEAL PALAEOCHANNELS IN THE LOWER MACQUARIE VALLEY, CENTRAL WEST NSW

Adrian Fisher and Peter Milligan

This report was produced by Geoscience Australia for the Bureau of Rural Sciences (BRS) project: Salinity mapping – Geophysics. The report was submitted to BRS on 30/6/2006.

Contents

List of Figures iii

List of Tables iii

Acknowledgements iv

Executive Summary v

1. Introduction 1 1.1 Study site – lower Macquarie valley 2

2. Compilation of geophysical datasets 6

3. Post-processing and image processing 7 3.1 Standard enhanced images 7 3.2 Derivative processing and reduction to the pole processing 9

3.21 Vertical derivatives 9 3.22 Horizontal derivatives 10 3.23 Reduction to the pole 12

3.3 Multiscale edge detection 12 3.4 Euler deconvolution depth estimates 12 3.5 Processing of point-located data 14

4. Interpretation of the magnetic data 14 4.1 Palaeochannels 16 4.2 Basement geology 18

5. Conclusions 21

References 22

Appendix: Data and images 24 Plus attached DVD

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List of Figures Figure 1. Location of the main rivers and towns of the lower Macquarie valley, central west NSW. 1 Figure 2. Simplified surface geology of the lower Macquarie valley, central west NSW. Geological data was sourced from the NSW Geological Survey. 3 Figure 3. Digital elevation of the lower Macquarie valley, central west NSW. Data was compiled from geophysical surveys at 50 m grid cell size. 4 Figure 4. Isopachs of the Cainozoic as interpreted from borehole logs by BRS. The surface geology is also shown according to Fig2. 5 Figure 5. Examples of enhanced images. 8 Figure 6. Examples of vertical derivative of TMI images. 10 Figure 7. Examples of horizontal derivative of TMI images. 11 Figure 8. Example of gradient maxima polylines of the TMI-RTP for an upward continuation of 50 m (green). 13 Figure 9. Some examples of preliminary Euler processing. 13 Figure 10. TMI, VD1 and AGC data in profile form for the ties of project 735, and in particular Tie D570400, which crosses the east-west section of the mapped palaeochannel. 15 Figure 11. Drainage pattern magnetic anomalies visible in the first vertical derivative image of the central west NSW survey area. 16 Figure 12. Structural zones of the basement geology, lower Macquarie valley, central west NSW. 19 Figure 13. Stratotectonic map of the basement geology, lower Macquarie valley, central west NSW. 20

List of Tables Table 1. Timescale and stratigraphic reference. 6 Table 2. Individual surveys used in compiling the central west NSW geophysical data. 7

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Acknowledgements Many people at Geoscience Australia assisted with the research presented in this report. The datasets upon which the report is based were compiled by Ross Franklin and Phillip Wynne. Technical input during the post-processing, image enhancement and interpretation stages was received from Murray Richardson, Brian Minty, Richard Lane and Ross Brodie. Dave Gibson gave valuable advice on previous research into dendritic drainage pattern magnetic anomalies. The Bureau of Rural Sciences provided the borehole information, including interpreted isopachs of the Cainozoic cover. The Shuttle Radar Topography Mission (SRTM) data products result from a collaborative mission by the National Aeronautics and Space Administration, the National Imagery and Mapping Agency, the German space agency and Italian space agency, to generate a near-global digital elevation model of the Earth using radar interferometry. It has been provided to Geoscience Australia for research purposes only.

Disclaimer GA has tried to make the information in this product as accurate as possible. However, it does not guarantee that the information is totally accurate or complete. Therefore, you should not rely solely on this information when making a commercial decision.

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Executive Summary This report outlines the work conducted by Geoscience Australia for two components of the Bureau of Rural Sciences (BRS) project: Salinity mapping – Geophysics. The first component was the compilation of existing geophysical data for the central west NSW survey area. The second component was to post-process and analyse magnetic data for the area to reveal any palaeochannels. 1. Compilation of geophysical data Four geophysical datasets were compiled for the central west NSW survey area from several existing individual surveys: magnetic, 4-channel gamma-ray, elevation and gravity. Each grid was then image processed to best reveal the information content, following standard image processing techniques. All datasets and images are listed in the appendix and stored on the attached DVD. 2. Analysis of magnetic data to reveal palaeochannels The magnetic data was post-processed following several standard geophysical methods. This included creating vertical and horizontal derivatives, reduction to the pole processing, multiscale edge detection, Euler deconvolution depth estimation, and the processing of point located data. The grided data produced from the post-processing and the enhanced images made from the data are stored in the appendix. The palaeochannels of interest to BRS in the southern part of the survey area, west of Narromine, are not directly detectable in the magnetics data. Some dendritic pattern magnetic anomalies are visible to the west of the area; however, these are clearly correlated with surface or near-surface drainage. This result is consistent with many previous studies that have only observed dendritic pattern magnetic anomalies in shallow cover, close to outcropping source rock. The magnetic data is strongly influenced by the basement geology, which appears to significantly influence the palaeo-drainage. The north-south trending structural boundaries of the Lachlan Fold Belt rocks seem to control the palaeo-drainage direction; in particular, the boundary between the Girilambone structural zone and the Parkes structural zone acts as a barrier that forces the palaeo-channel northwards.

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Palaeochannels in the lower Macquarie valley

1. Introduction This report outlines a small component of the larger Salinity mapping – Geophysics Project being conducted by Geoscience Australia for the Bureau of Rural Sciences (BRS), which started in May 2006. The main aim of the larger project is to examine the subsurface of the lower Macquarie River valley in central west NSW in order to characterise the regions groundwater and map the underground salt deposits. This is currently being conducted through examining existing and new boreholes, and if it proves feasible, the acquisition of a large airborne electromagnetic (AEM) survey.

Figure 1. Location of the main rivers and towns of the lower Macquarie valley, central west NSW.

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The southern section of the proposed survey area is known to contain buried palaeochannels that act as groundwater flow paths, and which are tapped as a source of water by local farmers. As an important component of the regions groundwater system, the location of these palaeochannels needs to be established. This report gives the details of an attempt to delineate the location of these buried palaeochannels through processing and analysing existing geophysical data for the region. The main data used was airborne magnetic, which has been shown to reveal buried palaeochannels in previous studies, due to the presence of magnetic minerals within the gravels of the channel sediments (Chan et al., 2003a; 2003b; Ford, 1996; Gibson and Chan, 1999; Gibson and Wilford, 2002; Mackay et al., 2000; Wildman and Compston, 2000).

1.1 Study site – lower Macquarie valley The study site includes the lower reaches of the Castlereagh, Bogan and Macquarie Rivers, which flow northwards across the flat Darling riverine plain into the Barwon River, which in turn flows west into the Darling (Fig.1). The meandering upper reaches of the Macquarie River change at Narromine into a complex system of anabranches and distributary channels which flow into the Bogan and Barwon Rivers (Thoms et al., 2004). Before reaching the Barwon however, the Macquarie River flows through the Macquarie Marshes, a large expanse of riverine wetlands that are one of 64 Australian sites listed on the Ramsar List of Wetlands of International Importance (see www.ramsar.org). Like many regions of inland Australia, the surface of the region is flat and has a thick cover (up to 160m) of Cainozoic sediments that obscures the basement geology. Underlying the Cainozoic fluvial sediments of the riverine plain are sedimentary rocks of the Mesozoic Surat Basin, which in turn are underlain by the metamorphic and igneous rocks of the Palaeozoic Lachlan Fold Belt (Table 1). The surface expression of this layered subsurface is controlled by the shape and extent of the Surat Basin, and the extent of the Darling riverine plain (Fig.2). The Surat Basin forms a saucer that dips up at the southern and eastern edges, exposing the Mesozoic sedimentary sequence. Palaeozoic Lachlan Fold Belt rocks are found to the south and west of the Mesozoic rocks. The boundary between the two rock groups marks the present southern limit of the Surat Basin, which may have extended further to the south. The boundary between the flat Cainozoic sediment cover and the outcropping Palaeozoic-Mesozoic rocks is essentially topographic, with the outcropping rocks rising above the plain (Fig.3). The buried palaeochannels of interest to this study are located within the unconsolidated Cainozoic sediments. In the southern part of the study site, they are known to occur as bedrock incised valleys that were subsequently in-filled, a phenomenon that has been observed in several sub-catchments of the Murray-Darling Basin, such as the Lachlan (Chan, 1999) and the Namoi (Young et al., 2002). The basal gravels usually found within such channels act as zones of groundwater flow. Interpretation of borehole logs by BRS has revealed the depth to the base of the Cainozoic across the proposed AEM flight zone (Fig.4). The Cainozoic isopachs reveal two buried palaeovalleys to the south west of Narromine. If the basal gravels present in these buried valleys contain magnetic minerals, they may show up on the magnetic imagery.

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Figure 2. Simplified surface geology of the lower Macquarie valley, central west NSW. Geological data was sourced from the NSW Geological Survey.

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Figure 3. Digital elevation of the lower Macquarie valley, central west NSW (image number 5).

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The following report first outlines the methods used in compiling, post processing and enhancing the geophysical data for the central west survey area. The next section discusses interpretations of these datasets, with emphasis on the magnetic data. The interpretations are based on relating the data to the areas geology, in particular the presence of possible buried palaeochannels and the influence of the basement rock on palaeo-drainage.

Figure 4. Isopachs of the Cainozoic as interpreted from borehole logs by BRS. The surface geology is also shown according to Fig2.

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Marra Creek Formation

Holocene Bugwah Formation

Carrabear Formation Quaternary

Pleistocene

Trangie Formation

Pliocene

Miocene Tertiary gravels

Oligocene Eocene

Cai

nozo

ic

Tertiary

Palaeocene Late

Griman Creek Formation Surat Siltstone

Coreena Member

Rowling Downs Group Wallumbilla

Formation Doncaster Member Drilldool Beds

Cretaceous Early

Keelindi Beds Late Pilliga Sandstone Middle Purlawaugh Formation

Mes

ozoi

c

Jurassic Early Garrawilla Volcanics

Late Hervey Group Sandstone Middle Devonian Early Mt Foster Monzonite and Milmiland Granite Late

Quambone-Young and Gilgandra Structural Zones

Middle Silurian Early

Mt Foster – Tumut Structural Zone

Late Parkes Structural Zone Ordovician Early

Pala

eozo

ic

Cambrian Late

Girilambone Group

Table 1. Timescale and stratigraphic reference, simplified from Watkins and Meakin (1996), Vine et al. (1967), and Reiser (1970).

2. Compilation of geophysical datasets Four geophysical datasets were compiled for the central west NSW survey area: magnetic, 4-channel gamma-ray, elevation and gravity. They were generated by merging grid data of several individual surveys (Table 2.) using the Gridmerge program in Intrepid. They were compiled with a grid cell size of 50 m, and they have a datum of GDA94 and are in the MGA55 coordinate system. The datatype is IEEE4ByteReal (i.e. single precision floating point binary). An extended area was first prepared, and then a subset made to an area slightly larger than that required by the bounding box specified by BRS. The datasets are stored in the appendix as ERMapper raster datasets.

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Survey number Survey area Year

P453 Gilgandra 1979 P454 Nyngan 1979 P569 Dubbo 1991 NSW_P734 Area D Surat Basin 1994-5 NSW_P735 Area E North Parkes 1994-6 NSW_P736 Area F Brewarrina 1994-7 NSW_P741 Area J Cobar - Nymagee Pt 1 1998 NSW_P742 Area J Cobar - Nymagee Pt 2 1999 NSW_P753 Area L Gilgandra 1999

Table 2. Individual surveys used in compiling the central west NSW geophysical data. Several artefacts of the merging process are visible in the grids, found along adjoining edges of individual surveys. This problem is particularly apparent in the elevation data along a north-south line through the Macquarie Marshes area. In an effort to present BRS with the best available data, a second elevation grid was compiled from Shuttle Radar Topography Mission (SRTM) dataset at a grid cell size of 90m. This dataset is more consistent across the whole survey area, and should be used when viewing the area as a whole, while the dataset compiled from merged geophysical surveys may be more useful when examining smaller subsets of the area.

3. Post-processing and image processing The grid data listed in the previous section were post-processed following various procedures using Intrepid, producing further binary grids. All the grids were then image processed using ERMapper to best reveal the information content, with the output either a single-band greyscale byte image, or a three-band colour byte image. Further details on the enhancement and presentation of geophysical data are available in Milligan and Gunn (1997) and Milligan et al. (1992). Twenty two of the most relevant images are described in the following text. All images are stored in the appendix in the Geotiff format, in the GDA94/MGA55 coordinate system, and can easily be imported into GIS software such as ArcGIS.

3.1 Standard enhanced images Each of the subset area grids listed in section 2 were image processed to produce initial standard colour images. These are listed below: 1. tmi_enhanced.tif This colour image (Fig.5) was derived in a two-stage process by combining in the hue-intensity-saturation (HIS) colour space a histogram-equalised colour image of the data using the rain-gomp natural rainbow look-up table with a grey-scale gradient image using a "sun-angle illumination" from the north-east. The advantage of this image is that the colour reveals long wavelength information in the mid-ranges of the data while the illumination reveals small wavelength details. A possible disadvantage is that the illumination "biases" the view of structures to a direction perpendicular to the illumination direction.

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Figure 5. Examples of enhanced images. (A) Dose rate (total count) with a north-east gradient (image number 3). (B) Potassium, thorium, uranium (red, green, blue) colour composite (image number 4). (C) TMI with a "sun-angle" illumination from the north-west (image number 1). (D) Gravity with north-east gradient (image number 6).

B A

D C

2. tmi_rain1_he.tif This colour image uses the rainbow1 look-up table of ERMapper, which ranges from white (high) to magenta (low), and has no gradient enhancement. 3. dose_col_neg.tif This colour image combines histogram equalisation of the dose data (derived from total-count of gamma-ray spectrometry) in colour with a transparency derived from a north-east gradient greyscale image (Fig.5). 4. kthu_he.tif The three main channels of gamma-ray data are combined into a single three-band file, with data of each channel histogram equalised. The output is a colour red-green-blue (RGB) image, with potassium (K) assigned to red, thorium (Th) assigned to green and uranium (U) assigned to blue. The resulting image shows subtle variations in the ratios of the three radioelements as changes in colour (Fig.5).

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5. dem_col_neg.tif This colour image combines histogram equalisation of the digital elevation model data in colour with a transparency derived from a north-east gradient grey-scale image. The main long wavelength variations in height show in the colour, and the subtle short-wavelength variations show in the gradient (Fig.3). 6. grav_pseud_shade.tif Colour is derived from the pseudocolour look-up table of ERMapper, and the gradient enhancement is from the real-time sun-angle illumination available in the software. By using a very steep altitude with the illumination, the short wavelength features in the data are imaged well (Fig.5).

3.2 Derivative processing and reduction to the pole processing Potential field data can be enhanced by special filtering processes that take advantage of the properties of the potential fields. Several such processes have been applied to the magnetic data, and these are described below. Multiple filtering processes can also be applied in one operation to data. Upward continuation is essentially a low pass filter, removing short wavelength content. It emulates the process in which the magnetic data would have been acquired at a higher acquisition level. Reduction to the pole removes the asymmetry effects in anomaly responses caused by the non-vertical inclination of the Earth's magnetic field. Both upward continuation and reduction to the pole have been applied along with other processing in some of the procedures outlined below.

3.21 Vertical derivatives These are essentially high-pass filters, enhancing the short-wavelength components of the data with no change in phase. For example, the first vertical derivative produces gradient data that would be obtained if the acquisition had occurred at two different vertical levels and the data subtracted from each other. Any order of derivative can be computed, with the higher the order the greater the short-wavelength enhancement. Ultimately, noise will be the limiting factor in how high an order can be computed. Vertical derivatives can also be of fractional order, as the exponent involved is a real number. Images produced of vertical derivative processing are: 7. tmi_vd1_rain1_he.tif Image of the first vertical derivative, histogram equalised and using the rainbow1 look-up table. 8. tmi_vd1_grey_he.tif Greyscale histogram equalised image of the first vertical derivative (Fig.6). 9. tmi_vd2_rain1_he.tif Image of the second vertical derivative, histogram equalised and using the rainbow1 look-up table. 10. tmi_vd2_grey_he.tif Greyscale histogram equalised image of the second vertical derivative (Fig.6). 11. tmi_vd3_grey_he.tif Greyscale histogram equalised image of the third vertical derivative.

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Figure 6. Examples of vertical derivative of TMI images. (A) VD1 TMI (image number 8) and (B) VD2 TMI (image number 10). The vertical derivative filter enhances short wavelengths without a phase change, generally emphasizing the signal from near-surface sources.

B A

12. tmi_vd3_grey_med_he.tif Greyscale image of the third vertical derivative, with the application of a non-linear median filter to remove some of the short-wavelength "speckle" noise.

3.22 Horizontal derivatives These are similar to gradient filters operating in a specific horizontal direction. There are three specific products generated here, the north-south horizontal derivative, the east-west horizontal derivative and the total horizontal derivative (THD). The latter combines the two directional derivatives to produce an unbiased dataset with data at each grid point representing the maximum magnitude of the gradient. For example, this would be for elevation data the gradient magnitude at each point in the direction in which water would run downhill. Images produced for the horizontal derivatives are: 13.tmi_rtp_xd_grey_he.tif Greyscale image of the north-south gradient, histogram equalised (Fig.7). 14. tmi_rtp_yd_grey_he.tif Greyscale image of the east-west gradient, histogram equalised (Fig.7). 15. tmi_rtp_xd_uc_200_grey_he.tif Greyscale image of the north-south gradient, histogram equalised, for an upward continuation of the data of 200 m. 16. tmi_rtp_yd_uc_200_grey_he.tif Greyscale image of the east-west gradient, histogram equalised, for an upward continuation of the data of 200 m. 17. tmi_rtp_xd_vd1_grey_he.tif Greyscale image of the north-south gradient of the first vertical derivative, histogram equalised.

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18. tmi_rtp_thd_uc_200_rain1_he.tif Colour image of the total horizontal derivative, histogram equalised, using the rainbow1 look-up table. Data have been upward continued 200 m. 19. tmi_rtp_thd_uc_200_grey_he.tif Greyscale image of the THD, histogram equalised, with data upward continued 200 m. 20. tmi_rtp_thd_vd1_grey_he.tif Greyscale image of the THD, of the 1st vertical derivative histogram equalised, at survey altitude.

Figure 7. Examples of horizontal derivative of TMI images. (A) north-south horizontal derivative of TMI (image number 15). (B) east-west horizontal derivative of TMI (image number 14). (C) colour image of the total horizontal derivative of TMI. (D) greyscale image of the total horizontal derivative of TMI.

B A

D C

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3.23 Reduction to the pole This is a potential field process that involves a phase change of the data. The inclination of the Earth's magnetic field over the survey area is -62.51 degrees, and the declination is 10.74 degrees. This means that the response of any inductive magnetisation in vertical geological bodies will have an asymmetry, with a low to the south and a high to the north. Reduction to the pole changes the phase of the data so that it represents the response of the Earth to a vertical inducing field. This makes interpretation of anomalies simpler. If there is an asymmetry over a body, it is then due either to the dip of the body, or to a significant remanent component. Reduction to the pole has already been taken into account in the horizontal derivative images described above. 21. tmi_rtp_enhanced.tif This image has been derived from RTP data, and the image processing is the same as for the first of the standard enhanced images described above.

3.3 Multiscale edge detection Multiscale edges and "worms" are products some interpreters find useful to map and quantify anomalies in potential field data. They are produced by upward continuing the data to many levels and then finding the maxima points in the total horizontal derivatives for each of the levels. These maxima points form "strings" or "worms" in space as they plot the maximum gradients in the data, which are often closely associated with the edges of geological bodies. Small scale features are highlighted by such strings at the lower levels, while with successive upward continuation, short wavelength features are gradually filtered out of the data, and the mapping is of longer wavelength features. Longer wavelengths are often associated with deeper source bodies, but great care must be exercised in using this assumption. Long wavelengths can, for example, be generated by near-surface tabular bodies, and the inherent non-uniqueness of potential field data should always be borne in mind. An example of multi-scale edge polylines is shown in Figure 8. Two ArcGIS shapfiles of the polylines are also included in the appendix. These were produced for a subset of the data by vectorizing the maxima points into polylines. The shapefiles are listed below, where the number prefix refers to the upward continuation level:

0_polyline.shp 50_polyline.shp

3.4 Euler deconvolution depth estimates Euler deconvolution is one of the many methods by which rapid depth estimates to magnetic sources may be made from both profile and grid data. Preliminary Euler processing of the TMI-RTP grid data has been undertaken to test if depth solutions obtained are likely to reveal information regarding the palaeochannels (Fig.9). With a grid cell size of 50 m, this is pushing such an estimation method to the limits to accurately resolve shallow depths. 22. tmi_rtp_Euler_7_sub1.tif An image of Euler solutions for a kernel size of 7, white high, magenta low.

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Figure 8. Example of gradient maxima polylines of the TMI-RTP for an upward continuation of 50 m (green). The red contours represent depth of Cainozoic cover, outlining the palaeochannel located by drilling.

B

C

Figure 9. Some examples of preliminary Euler processing. (A) Example Euler depth solutions, gridded and plotted as an image, with shallow solutions represented by red to deep solutions represented by magenta. The black contours represent depth of Cainozoic cover. (B) Euler solutions coloured by depth. (C) Cross-section, as indicated in (B).

A

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3.5 Processing of point-located data Data covering the palaeochannels identified by BRS are mainly archived in Geoscience Australia survey project 735. These data were originally acquired by NSWDMR under the Discovery 2000 initiative in 1995, Area E, Northern Parkes. The survey acquisition height is 60 m above ground level, and the flight-lines are spaced 250 m apart. Kevron Geophysics was the geophysical contractor. Significant fine detail in magnetic data can be lost in the gridding process, as the cell size is always a compromise between the closely spaced samples along a flight-line (7 m in this case) and the more widely spaced sampling between flight lines (250 m in this case). Thus, analysis of profile data has the potential to provide more information from sources very close to the ground surface. The profile data of project 735 has been processed to produce both vertical derivative and automatic gain control data (AGCD), which can then be examined in stack-profile form, or gridded to produce images (Fig.10). There is no significant evidence of the channels in the data of these profiles.

4. Interpretation of the magnetic data The magnetic data, post-processed and presented in the previous section, relate to the distribution of magnetic minerals within the sub-surface of the survey area. Different rocks have different magnetic responses, allowing the data to be interpreted in terms of sub-surface geology. For example, basaltic lavas usually contain relatively large concentrations of the mineral magnetite, so they show-up as anomalies of high magnetic intensity. Sedimentary rocks and unconsolidated sediments may contain detrital magnetite grains from an igneous source, as well as the magnetic mineral maghemite. Maghemite can form in situ within sediments and rocks from the mineral goethite, due to the extremely high biological and chemical reactivity of iron (Evans and Heller, 2003). Bush fires, lightening strikes and biological activity are often attributed as possible causes (Gibson and Wilford, 2002; Gibson and Chan, 1999; Schwertmann and Fletcher, 1984). Qualitative interpretation of magnetic data relies on recognising the spatial patterns present (Brodie, 2002). Dendritic drainage patterns that are indicative of fluvial channel deposits containing magnetic minerals have been observed in many areas of NSW, such as north of Cobar (Ford, 1996), the Girilambone region (Chan et al., 2003a; 2003b), the Lachlan Valley near Forbes (Gibson and Chan, 1999) and the region south-east of West Wyalong (Mackay et al., 2000). They have also been observed in the Honeysuckle Creek area of Victoria (Gibson and Wilford, 2002) and the Yandal greenstone belt of Western Australia (Wildman and Compston, 2000). They are usually interpreted as channel deposits which contain detrital magnetic pisoliths as part of the granule to small pebble sized component of the sediment. The magnetic pisoliths are most probably maghemite sourced from lag, soils and ferruginised zones of weathered rocks.

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A

5000 0 5000 10000 15000

m etres

GDA94 / M ap Gri d o f Aus tralia zone 55

Scale 1:250000

Bureau of Rural SciencesTotal Magnetic Intensity Profiles of Ties

VD1Peter R. Milligan, Geoscience Australia

6400

000

6410

000

6420

000

6430

000

6440

000

6450

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6460

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64000006410000

64200006430000

64400006450000

6460000

530000 540000 550000 560000 570000 580000 590000 600000 610000 620000 630000 640000

530000 540000 550000 560000 570000 580000 590000 600000 610000 620000 630000 640000

B

agc2

agc

vd1

FINALMAG

Tie line D570400

databas e: E:\prm \3\brs \p735\geos oft s ub1\p735 s ub1 gdb l ine/group: D570400 2006/06/22

28000 29000 30000 31000 32000 33000 34000 3500027178 35969

0.004.00

agc2

0 .00

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agc

-2 .00

0.00

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4800.00

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C D

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databas e: E:\prm \3\brs \p735\geos oft s ub1\p735 s ub1 gdb l ine /group: D570400 2006/06/22

31600 31800 32000 32200 32400 32600 32800 3300031476 33249-2 .00

0.002.00

4.00

ag

c2

-2 .00

0.00

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agc

-0 .40

-0 .20

0.00

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5200.00

5600.00

6000.00

FINA

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G

Figure 10. TMI, VD1 and AGC data in profile form for the ties of project 735, and in particular Tie D570400, which crosses the east-west section of the mapped palaeochannel. (A) First vertical derivative of TMI profiles for tie lines (black), with Cainozoic depth contours (blue). (B) Profile plot of data for tie line D570400, with TMI (red), VD1(green), AGC1 (blue) and AGC2 (purple). AGC1 uses a window size of 50, and AGC2 uses a window size of 200. (C) A close-up of (A) zoomed so the centre of the palaeochannel is at the centre of the profiles. (D) A close-up of (B).

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The following interpretation of the magnetic data for the central west NSW survey area is divided into two sections. Firstly, the search for dendritic drainage pattern magnetic anomalies in the imagery is discussed. Secondly, an interpretation of basement geology and how it may influence buried palaeo-drainage is presented.

4.1 Palaeochannels As dendritic drainage pattern magnetic anomalies are usually of high frequency and low amplitude, they are often best revealed in vertical derivative images. The vertical derivative image of the central west survey area reveals many dendritic anomalies in the south-west corner (Fig.11). A comparison of the dendritic patterns with the elevation and surface geology data indicates that they are associated with surface or near-surface drainage. In particular, they are draining the Palaeozoic Girilambone Group rocks that outcrop along the west of the survey area, and which are elevated above the riverine plain. The palaeochannels as identified by BRS do not have an obvious expression in the potential field data. If they provide a magnetic signature at all, it must be very subtle, and probably only provided either by the slight reduction in amplitude of anomalies produced by magnetisation at the top of the basement, as the basement deepens into the channels, or by reduction or enhancement of intra-channel magnetic mineral content. The channels themselves may possibly provide a signature of a longer wavelength.

Figure 11. Drainage pattern magnetic anomalies visible in the first vertical derivative image of the central west NSW survey area.

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From an inspection of a TMI image such as image 8, there is considerable magnetic signal at a range of wavelengths from rocks of the basement, and also superimposed short-wavelength anomalies from intra-cover sources. Separating the effects of a range of sources at different close levels is very difficult, if not impossible, unless there is either a clear separation of wavelengths, or a particular spatial signature that can be associated with a particular source set. The lack of visible drainage lines in the magnetic data across the riverine plain is consistent with previous research. Most of the previous studies observed that the dendritic magnetic anomalies were only visible in the upper part of the catchment close to outcrop, and they became broader and more diffuse with distance downstream. It is not known what causes this lack of magnetic signature in the downstream component of buried palaeochannels, however, there are many possible reasons, as discussed by Gibson and Wilford (2002):

The channel sediments may become progressively finer downstream due to a decrease in gradient and stream energy, limiting the magnetic minerals to the upstream gravel dominated sediments.

It is possible that the maghemite pisoliths were deposited in the downstream sections

of the channel but that they undergo dissolution/transformation at a certain depth, losing their magnetic properties.

The magnetic pisoliths may become diluted by non-magnetic gravels in the

downstream section. This may occur through coarse sediments being deposited as point bar sequences across whole floodplains with a laterally migrating channel. Alternatively, it may occur simply through downstream sections having increased catchment areas that lack a maghemite source.

The data may not be able to discern small differences in magnetic response at

increased depths because of the thick non-magnetic overburden, and/or the attenuation of the signal with increased distance to the airborne sensor.

Despite the lack of obvious drainage pattern magnetic anomalies in the area of interest, several complex geophysical post-processing methods were trialled in an effort to reveal any possible subtle palaeochannel signal. These methods were multiscale edge detection (section 3.3), Euler deconvolution depth estimation (section 3.4) and the processing of point located data (section 3.5). Features highlighted by the multiscale edge detection appear to relate to the magnetic signature of the basement rock, and do not reveal any subtle features relating to the palaeochannels (Fig.8). The structure of the basement does appear to relate to the broad structure of the palaeo-valleys, and this is discussed in the following section. Conclusions that can be made from the results of the Euler deconvolution depth estimation are mixed. In image form (Fig.9), they show depths that may relate to the mapped palaeochannels in the south, where the channels trend east-west. Where the channel trends northerly, it follows a linear set of deeper Euler solutions that trend further to the north. This result is strongly influenced by the magnetic signature of the basement rock, which will be discussed in the next section.

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The processing of point located data can reveal more subtle magnetic features than the gridded data. Unfortunately the TMI, VD1 and AGC data in profile form for Tie D570400, which crosses the east-west section of the mapped palaeochannel (Fig.10), do not reveal any evidence for the palaeochannels.

4.2 Basement geology The magnetic and gravity data for the central west NSW survey area show a very strong response from the basement, in spite of a thick overlying sequence of Cainozoic and Mesozoic cover throughout most of the area. Interpretations of the basement geology (Fig.12 & 13) have been compiled by the NSW Geological Survey (Scheibner, 1993; 1997) and are very useful in determining the likely influence the basement rocks have on palaeo-drainage. The broad structures of the region (Fig.12) are characterised as north-south trending zones of the Lachlan Fold Belt (LFB), a Palaeozoic active plate margin orogenic belt within the Tasman Fold Belt System (Scheibner, 1993). The stratotectonic map reveals further detail in the basement, including the subdivision of some structural zones and the presence of several igneous intrusive bodies (Fig.13). The buried palaeochannels found to the west of Narromine lie directly above the Parkes structural zone of the LFB. This zone is characterised by several stratotectonic elements including shallow water marine metasediments (including mafic and felsic volcanics) and continental/subaerial metasediments (Scheibner, 1997). It is likely that the north-south boundaries between these units form structural barriers to the palaeo-drainage, as is indicated by the rectangular drainage pattern in the Cainozoic isopachs (Fig.4). The palaeo-valleys visible in the Cainozoic isopachs merge and become less distinct, presumably trending west and then north (Fig.4). Although this change in direction is not strongly shown in the borehole data, it is supported by the presence of the boundary between the Parkes structural zone and the Girilambone structural zone to the west, which is visible in the basement geology. The Girilambone structural zone is characterised by deep water marine metasediments, which are commonly fine grained quartzose rocks deposited by turbidity currents (Scheibner, 1997; Watkins and Meakin, 1996). The Girilambone Group rocks outcrop above the riverine plain about 10 km to the west of the subsurface structural boundary, indicating that they rise from the basement and could act as a north-south trending barrier to the palaeochannel. It is likely that the channel would be diverted north as the LFB rocks also outcrop to the south. It is important to note that unlike the modern Bogan River, which roughly follows the eastern edge of outcropping Girilambone Group rocks, the buried palaeochannel may not continue to follow the buried structural boundary as it trends north. The basement structure will have less influence on drainage in the north of the survey area, as the basement is covered by a thickening sequence of Mesozoic sedimentary rocks. The palaeochannels are Cainozoic, and lie above or are incised into the Mesozoic sequence.

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Figure 12. Structural zones of the basement geology, lower Macquarie valley, central west NSW. Geological data was sourced from the NSW Geological Survey (Scheibner, 1993).

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Figure 13. Stratotectonic map of the basement geology, lower Macquarie valley, central west NSW. Geological data was sourced from the NSW Geological Survey (Scheibner, 1997).

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5. Conclusions The aeromagnetic data for the central west NSW survey area show a variety of features mainly related to the geology of the basement. Some short wavelength dendritic pattern anomalies are visible in the west of the area, which are clearly correlated with surface or near-surface drainage. The palaeochannels, mapped by BRS using drill-hole depths to basement, have an almost negligible signature in the magnetic data. There are textural hints of the palaeochannel positions, and some correlation with Euler depth solutions. However, it must be concluded that these deeply buried palaeochannels provide so little magnetic susceptibility contrast with the surrounding geology, compared with the variations of the basement geology, that the present airborne TMI mapping does not resolve them to any useful extent. This result is consistent with many previous studies that have only observed dendritic pattern magnetic anomalies in shallow cover, close to outcropping source rock. The theories used to explain why the buried drainage lines are indistinct under deeper cover could be tested by future potential field modelling. Importantly, the magnetic and gravity data reveals aspects of the basement geology, which appears to significantly influence the palaeo-drainage. The palaeochannels change direction several times, and each change can be attributed to a geological structure acting as a north-south trending barrier. Also apparent is the subsurface extent of the Mt Foster-Harris intrusive monzonite (Fig.13), which may influence the buried palaeodrainage. Possible future work could include conducting Euler deconvolution depth estimation on the magnetic data for this area to determine the three dimensional nature of this large intrusive body.

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References Brodie R (2002) Airborne and ground magnetics. In: Papp E (ed.) Geophysical and remote

sensing methods for regolith exploration. CRCLEME Open File Report 144, p33-45. Chan RA Greene RSB, Hicks M, Le Gleuher M, McQueen KG, Scott KM and Tate SE

(2003b) Regolith architecture and geochemistry of the Byrock area, Girilambone region, north-western NSW. CRC LEME Open File Report 159.

Chan RA, Greene RSB, de Souza Kovacs N, Maly BER, McQueen KG and Scott KM

(2003a) Regolith, geomorphology, geochemistry and mineralization of the Sussex-Coolabah area in the Cobar-Girilambone region, north-western Lachlan Foldbelt, NSW. CRC LEME Open File Report 148.

Chan RA, Greene RSB, Hicks M, Maly BER, McQueen KG and Scott KM (2003b) Regolith

architecture and geochemistry of the Hermidale area of the Girilambone region, north-western Lachlan Fold Belt, NSW. CRC LEME Open File Report 149.

Evans, ME and Heller, F (2003) Environmental Magnetism: Principles and Applications of

Enviromagnetism. Academic Press, Elsevier Science, USA, p293. Ford, AJH (1996) Re-interpreting the north eastern margin of the Cobar Basin using drainage

channel morphology. In: Cook, WG, Ford, AH, McDermott, JJ, Standish, PN, Stegman, CL & Stegman, TM (eds.) The Cobar Mineral Field – A 1996 Perspective. Australasian Institute of Mining and Metallurgy, Melbourne, 113-123.

Gibson DL and Wilford J (2002) Aspects of regolith and landscape of the Strathbogie-

Caniambo-Dookie area: the need for interpretation of detailed geophysical datasets in the light of regional data and models. In: Pillips, GN and Ely, K (eds.) Victoria Undercover: Benalla 2002 Conference proceedings and field guide. CSIRO publishing, p235-247.

Gibson, DL and Chan, RA (1999) Aspects of palaeodrainage of the north Lachlan Fold Belt

Region. In: Taylor, G and Pain, C (eds.) New approaches to an old continent. CRC LEME, Perth, p23-37.

Lawrie, KC, Chan, RA, Gibson, DL and de Souza Kovacs, N (1999) Alluvial gold potential

in buried palaeochannels in the Wyalong District, LFB, NSW. AGSO Research Newsletter, May 1999, 1-3.

Mackey, T, Lawrie, K, Wilkes, P, Munday, T, de Souza Kovacs, N, Chan, R, Gibson, D,

Chartres, C and Evans, R (2000) Palaeochannels near West Wyalong, New South Wales: a case study in delineation and modelling using aeromagnetics. Exploration Geophysics, 31, 1-7.

Milligan, PR and Gunn, PJ (1997) Enhancement and presentation of airborne geophysical

data. AGSO Journal of Geology and Geophysics, 17, 63-75.

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Milligan, PR, Morse, MP and Rajagopalan, S (1992) Pixel map preparation using the HSV

colour model. Exploration Geophysics, 23, 219-224. Reiser, RF (1970) Stratigraphic nomenclature of the upper part of the Rolling Downs group

in the Surat area. Queensland Government Mining Journal, 1970, p301-303. Scheibner E (1993) Structural framework of New South Wales. Geological Survey of New

South Wales, Quarterly Notes, 93, 1-36. Scheibner E (1996) Geology of New South Wales - synthesis Volume 1: Structural

framework. Geological Survey of New South Wales Memoir, 13(1). Scheibner E (1997) Stratotectonic map of New South Wales, Scale 1:1 000 000. Geological

Survey of New South Wales, Sydney. Thoms, M, Hill, S, Spry, M, Chen, XY, Mount, T and Sheldon, F (2004) The geomorphology

of the Barwon-Darling Basin. In: Breckwoldt, R, Boden, R and Andrew, J (eds.) The Darling. Murray Darling Basin Commission, Canberra, ACT, Australia, p68-103.

Vine, RR, Day, RW, Milligan, EN, Casey, DJ, Galloway, MC and Exon, NF (1967) Revision

of the nomenclature of the Rolling Downs group in the Eromanga and Surat Basins. Queensland Government Mining Journal, 1967, p144-145.

Watkins, JJ and Meakin, NS (1996) Nyngan and Walgett 1:250 000 Geological Sheets

SH/55-15 & SH/55-11: Explanatory Notes. Geological Survey of New South Wales, Sydney, p112.

Wildman, JE and Compston, D (2000) Magnetic expression of palaeodrainage systems in the

Yandal greenstone belt: implications for exploration. Yandal Greenstone Belt, AIG Bulletin 32, 135-144.

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Appendix: Data and images All data and images stored on the attached DVD are in the GDA94 datum and MGA Zone 55 coordinate system, except where noted. Folder

File type File name Description

rad ERMapper raster dataset dose_merge.ers total dose count data ERMapper raster dataset kthu_real.ers potassium-thorium-uranium data combined in a RGB colour space ERMapper raster dataset k_merge.ers potassium dose count data ERMapper raster dataset th_merge.ers thorium dose count data ERMapper raster dataset u_merge.ers uranium dose count data Geotiff image dose_col_neg.tif colour histogram equalised image of the total dose count with a gradient enhancement

Geotiff image dose_col_neg_subsam.tif subsampled image for display Geotiff image kthu_he.tif colour histogram equalised image with potassium-red, thorium-green and uranium-blue

Geotiff image kthu_he_subsam.tif subsampled image for display Geotiff image k_rain_he_neg_trans.tif colour histogram equalised image of potassium count with a gradient enhancement

Geotiff image k_rain_he_neg_trans_subsam.tif subsampled image for display Geotiff image th_rain_he_neg_trans.tif colour histogram equalised image of thorium count with a gradient enhancement Geotiff image th_rain_he_neg_trans_subsam.tif subsampled image for display Geotiff image u_rain_he_neg_trans.tif colour histogram equalised image of uranium count with a gradient enhancement

Geotiff image

u_rain_he_neg_trans_subsam.tif

subsampled image for display

rtp_thd ERMapper raster dataset tmi_rtp_thd.ers total horizontal derivative of the RTP TMI data at survey altitude ERMapper raster dataset tmi_rtp_thd_uc_50.ers total horizontal derivative of the RTP TMI data upward continued 50m ERMapper raster dataset tmi_rtp_thd_uc_200.ers total horizontal derivative of the RTP TMI data upward continued 200m ERMapper raster dataset

tmi_rtp_thd_vd1.ers total horizontal derivative of the 1st vertical derivative of the RTP TMI data at survey altitude

Geotiff image tmi_rtp_thd_grey.tif greyscale image of the total horizontal derivative of the RTP TMI data Geotiff image tmi_rtp_thd_grey_subsam.tif subsampled image for display

Geotiff image tmi_rtp_thd_col.tif colour image of the total horizontal derivative of the RTP TMI data Geotiff image tmi_rtp_thd_col.subsam.tif subsampled image for display

Geotiff image tmi_rtp_thd_uc_200_rain1_he.tif colour histogram equalised image of the total horizontal derivative of the RTP TMI data Geotiff image tmi_rtp_thd_uc_200_grey_he.tif greyscale histogram equalised image of the total horizontal derivative of the RTP TMI data

Geotiff image tmi_rtp_thd_vd1_grey_he.tif greyscale histogram equalised image of the total horizontal derivative of the 1st vertical derivative of the RTP TMI data

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Folder


dem ERMapper raster dataset dem_merge.ers merged airborne geophysics elevation data ERMapper raster dataset srtm_subset.ers subset of the national srtm elevation data Geotiff image dem_col_neg.tif colour elevation image with a north-east gradient enhancement using airborne geophysics data

Geotiff image dem_col_neg_subsam.tif subsampled image for display Geotiff image srtm_col_neg.tif colour elevation image with a north-east gradient enhancement using srtm data

Geotiff image

srtm_col_neg_subsam.tif


grav ERMapper raster dataset grav_merge.ers gravity data ERMapper raster dataset grav_merge_vd1.ers gravity 1st vertical derivative data ERMapper raster dataset

grav_merge_vd2.ers gravity 2nd vertical derivative data

Geotiff image grav_pseud_shade.tif colour gravity image with a gradient enhancement Geotiff image grav_pseud_shade_subsam.tif


Geotiff image grav_grey_he.tif greyscale histogram equalised gravity image Geotiff image grav_vd1_grey_he.tif

greyscale histogram equalised gravity 1st vertical derivative image

Geotiff image grav_rain1_he.tif colour histogram equalised gravity image image

NSW_grav_stn_plot.tif

unregistered map of gravity station locations in the central west NSW survey area

tmi ERMapper raster dataset tmi_merge.ers total magnetic intensity data ERMapper raster dataset tmi_merge_vd1.ers total magnetic intensity 1st vertical derivative data ERMapper raster dataset tmi_merge_vd2.ers total magnetic intensity 2nd vertical derivative data ERMapper raster dataset tmi_merge_vd3.ers total magnetic intensity 3rd vertical derivative data Geotiff image tmi_enhanced.tif colour histogram equalised TMI image with a gradient enhancement

Geotiff image tmi_enhanced_subset.tif subsampled image for display Geotiff image tmi_rain1_he.tif colour histogram equalised TMI image Geotiff image tmi_vd1_rain1_he.tif colour histogram equalised TMI 1st vertical derivative image Geotiff image tmi_vd1_grey_he.tif greyscale histogram equalised TMI 1st vertical derivative image

Geotiff image tmi_vd1_grey_he_subsam.tif subsampled image for display Geotiff image tmi_vd2_rain1_he.tif colour histogram equalised TMI 2nd vertical derivative image Geotiff image tmi_vd2_grey_he.tif greyscale histogram equalised TMI 2nd vertical derivative image

Geotiff image tmi_vd2_grey_he_subsam.tif subsampled image for display Geotiff image tmi_vd3_grey_he.tif greyscale histogram equalised TMI 3rd vertical derivative image

Geotiff image

tmi_vd3_grey_med_he.tif

greyscale histogram equalised filtered TMI 3rd vertical derivative image

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Folder


rtp_xd ERMapper raster dataset tmi_rtp_xd.ers north-south horizontal derivative of the RTP TMI data at survey altitude ERMapper raster dataset tmi_rtp_xd_uc_50.ers north-south horizontal derivative of the RTP TMI data upward continued 50m ERMapper raster dataset tmi_rtp_xd_uc_200.ers north-south horizontal derivative of the RTP TMI data upward continued 200m ERMapper raster dataset tmi_rtp_xd_vd1.ers north-south horizontal derivative of the 1st vertical derivative of the RTP TMI data Geotiff image tmi_rtp_xd_grey_he.tif greyscale histogram equalised image of north-south horizontal derivative of the RTP TMI

Geotiff image tmi_rtp_xd_uc_200_grey_he.tif greyscale histogram equalised image of the north-south horizontal derivative of the RTP TMI data upward continued 200m

Geotiff image tmi_rtp_xd_uc_200_grey_he_subsam.tif

subsampled image for display Geotiff image tmi_rtp_xd_vd1_grey_he.tif greyscale histogram equalised image of the north-south horizontal derivative of the 1st

vertical derivative of the RTP TMI data Geotiff image tmi_rtp_xd_vd1_col_he.tif colour histogram equalised image of the north-south horizontal derivative of the 1st vertical

derivative of the RTP TMI data

rtp_yd ERMapper raster dataset tmi_rtp_yd.ers east-west horizontal derivative of the RTP TMI data at survey altitude ERMapper raster dataset tmi_rtp_yd_uc_50.ers east-west horizontal derivative of the RTP TMI data upward continued 50m ERMapper raster dataset tmi_rtp_yd_uc_200.ers east-west horizontal derivative of the RTP TMI data upward continued 200m ERMapper raster dataset tmi_rtp_yd_vd1.ers east-west horizontal derivative of the 1st vertical derivative of the RTP TMI data Geotiff image tmi_rtp_yd_grey_he.tif greyscale histogram equalised image of the east-west horizontal derivative of the RTP TMI Geotiff image tmi_rtp_yd_uc_200_grey_he.tif greyscale histogram equalised image of the east-west horizontal derivative of the RTP TMI

data upward continued 200m Geotiff image

tmi_rtp_yd_uc_200_grey_he_subsam.tif


rtp_uc ERMapper raster dataset tmi_rtp_enhanced.ers reduced to the pole TMI data with a gradient enhancement ERMapper raster dataset tmi_rtp.ers reduced to the pole TMI data ERMapper raster dataset tmi_rtp_uc_50.ers reduced to the pole TMI data upward continued 50m ERMapper raster dataset tmi_rtp_uc_200.ers reduced to the pole TMI data upward continued 200m ERMapper raster dataset tmi_rtp_vd1.ers reduced to the pole 1st vertical derivative TMI data Geotiff image tmi_rtp_enhanced.tif colour reduced to the pole TMI image with a gradient enhancement

Geotiff image tmi_rtp_enhanced_subsam.tif


worms

ArcGIS shapefiles

0_polyline.shp multiscale edge polylines of the reduced to the pole TMI data 50_polyline.shp

multiscale edge polylines of the reduced to the pole TMI data upward continued 50m

euler

Geotiff image

tmi_rtp_Euler_7_sub1.tif

colour image of Euler solutions for a kernel size of 7 for a subset of the RTP TMI data

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an analysis of aeromagnetic data to reveal palaeochannels...

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