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P O S I V A O Y

FI -27160 OLKILUOTO, F INLAND

Tel +358-2-8372 31

Fax +358-2-8372 3709

Chr i s topher Juh l i n

Ca l i n Cosma

August 2007

Work ing Repor t 2007 -65

A 3D Surface Seismic Pilot Studyat Olkiluoto, Finland:

Acquisition and Processing Report

August 2007

Working Reports contain information on work in progress

or pending completion.

The conclusions and viewpoints presented in the report

are those of author(s) and do not necessarily

coincide with those of Posiva.

Chr i s topher Juh l i n

Uppsa la Un ive rs i t y

Ca l in Cosma

V ib romet r i c Oy

Work ing Report 2007 -65

A 3D Surface Seismic Pilot Studyat Olkiluoto, Finland:

Acquisition and Processing Report

Base maps: ©National Land Survey, permission 41/MYY/07

ABSTRACT

Posiva carries out R&D tasks related to spent nuclear fuel disposal in Finland. Works are currently performed in the Olkiluoto Island, in western Finland. Multidisciplinary engineering and geological investigations have been carried out since 1987. Site and rock characterization continued underground after 2004, in the ONKALO underground research facility. Posiva co-operated with Andra, France (Agence Nationale des Dechets Radioactifs) to purchase the design, acquisition and processing work needed to demonstrate the capability of 3D seismic techniques to image geological and structural features in crystalline rock.

A 3D seismic pilot survey was carried out at the Olkiluoto site during seven days in June 2006. Data acquisition and processing were carried out as a Finnish-Swedish joint effort of Vibrometric and Uppsala University.

The seismic survey covered 600 x 650 m area to the west of ONKALO underground premises. The single-template fixed array consisted of 270 active geophones on nine NW-SE lines. Geophone line interval was 60 m and station interval 24 m (30 channels on line). Sources (469 in total) were placed on seven NE-SW lines perpendicular to the receiver lines. The shot line interval was 100 m and the shot spacing 10 m (71 shots per line). The source was a time distributed Vibsist impact source (2000-2500 J/ impact), using a hydraulic rock breaker. The receivers (28 Hz geophones) were connected to a Sercel 408 UL seismograph. The coverage and signal quality were good. The array size was small compared to imaging depth, compared with conventional 3D works. The fold (CDP bin size 12 x 12 m) was highly variable (5…>100), lower at the edges of area.

Reflections are already seen in the processed receiver and shot gathers. The processing comprised time stacking (impact decoding), first arrival picking, resampling, refraction and residual static corrections, spectral equalisation, band-pass filtering, NMO and DMO, stacking. Static corrections, DMO and filtering were the most important steps. Additional processing consisted of deconvolution, 3D migration with a Finite Difference code, and depth conversion.

The signal penetration in the area was good. After processing, several strong subhorizontal reflection zones, at 200-300 ms (600-900 m), then 450-550 ms (1350-1650 m) and finally 900-1000 ms (2700-3000 m) were well imaged. The zones are evidently due to high velocity and/or density contrast. Zones appear to be undulating and discontinuous, and sometimes offset. Diffracted energy is more visible in the unmigrated sections. Dipping reflectors at 20-30 degrees are met in several sets in the upper 200 ms, and at 400 ms. These reflections strike in an almost W-E direction, dipping to the south. Following the reflectors through volume is difficult, either due to geology, to the geometry of survey, or to locally poorer data. The seismic signatures of the subhorizontal and dipping reflectors are different, indicating a possibly different geological character.

Dips steeper than 40 degrees have not directly been imaged. Near vertical structures (faulting) may be indicated by offsets of the sub-horizontal reflections at 200-300 ms. Some of these are rather clear, indicating a strike near parallel to the cross-line direction.

The pilot survey proved that the technique is viable and usable also for similar works in the future. Some design improvements were suggested. The survey area has been small

compared to depth of investigation. A larger survey area and a smaller bin would be desirable. Clearing the lines for acquisition and other preparations set probably some constraints for further design like line density and number of active channels. Minimum fold 30 should be provided, for adequate imaging. No sustained interpretation attempt has been made to date.

Keywords: Seismic, 3D, reflection, acquisition, processing, migration, nuclear waste, bedrock

Maanpinnan seisminen 3D tutkimus Olkiluodossa: Mittaukset ja prosessointi.

TIIVISTELMÄ

Posiva huolehtii käytetyn ydinpolttoaineen loppusijoituksen tutkimus- ja kehitys-tehtävistä Suomessa. Työt keskittyvät Olkiluodon saarelle länsi-Suomessa. Monialainen tekninen ja geologinen tutkimus on ollut käynnissä 1987, ja jatkunut maan alla 2004 alkaen, ONKALO –tutkimustiloista käsin. Posiva on tilannut yhteistyössä ranskalaisen Andran (Agence Nationale des Dechets Radioactifs) kanssa seismiset tutkimustyöt selvittääkseen seismisten 3D tekniikoiden käyttökelpoisuuden geologisten ja raken-teellisten piirteiden kuvaamiseksi kiteisessä kallioperässä. Seisminen 3D pilottitutkimus tehtiin Olkiluodon alueella seitsemän päivän aikana kesäkuussa 2006. Aineiston tuotanto maastossa ja prosessointi tehtiin suomalais-ruotsalaisena Vibrometricin ja Uppsalan yliopiston välisenä yhteistyönä.

Seisminen tutkimus kattoi 600 x 650 m alueen länteen maanalaisesta ONKALO-tutkimustilasta. Yhdestä templatesta koostunut kiinteä rekisteröintijärjestely koostui 270 aktiivisesta geofonista, jotka oli sijoitettu yhdeksälle luode-kaakko –suuntaiselle linjalle. Geofonilinjojen väli maastossa oli 60 m ja geofoniväli linjalla 24 m (30 kana-vaa linjalla). Lähteet (469 yhteensä) olivat seitsemällä, geofonilinjoja vastaan kohti-suoralla koillis-lounais –suuntaisella linjalla. Lähdelinjojen väli oli 100 m ja lähdeväli 10 m (71 lähdettä linjalla). Lähteenä käytettiin ajan suhteen hajautettua Vibsist –iskusarjalähdettä (2000-2500 J/ isku), joka toteutettiin hydraulisella murtovasaralla. Vastaanottimet (28 Hz geofonit) oli kytketty Sercel 408 UL seismografiin. Tutkimuksen kattavuus ja signaalin laatu olivat hyvät. Mittausjärjestelmän koko oli pieni suhteessa tutkimussyvyyteen, verrattuna tavanomaisiin 3D-tutkimuksiin. Pinoamisen kertaluku (CDP laskenta-alue 12 x 12 m) vaihteli voimakkaasti (5...> 100), ja oli matalin alueen reunoilla.

Heijastuksia voidaan nähdä jo prosessoidussa vastaanotin- ja lähetinkoosteissa. Pro-sessointi sisälsi aikapinoamisen (iskujen dekoodauksen), ensisaapujan poiminnan, uudelleennäytteistyksen, refraktio- ja residuaalistatic-korjaukset, spektritasoituksen, kaistanpäästösuodatuksen, NMO:n ja DMO:n, sekä pinoamisen. Static-korjaukset, DMO ja suodatukset olivat tärkeimmät prosessointivaiheet. Täydentävä prosessointi sisälsi dekonvoluution, 3D migraation käyttäen Finite Difference ohjelmaa, ja syvyys-muunnoksen.

Signaalin läpäisy kohteessa oli hyvä. Prosessoinnin jälkeen havaittiin selvästi useita voimakkaita lähes vaaka-asentoisia heijastusvyöhykkeitä mm. 200-300 ms (600-900 m), 450-550 ms (1350-1650 m) ja syvimmillään 900-1000 ms (2700-3000 m) kohdilla. Nämä heijastusvyöhykkeet aiheutuvat selvästi joko korkeasta tiheyden tai nopeuden kontrastista, tai molemmista. Vyöhykkeet vaikuttavat kaareutuvilta ja epäjatkuvilta, ja katkeilevat paikoin. Diffraktoitunutta energiaa nähdään selvemmin migratoimattomissa tuloskuvissa. Loivasti 20-30º kaltevia jaksoja tavataan useita ylimmän 200 ms aikana, ja 400 ms kohdalla. Näiden heijastajien kulku on melkein itä-läntinen, ja kaade etelään. Heijastajien seuraaminen tutkitun tilavuuden läpi on vaikeaa, johtuen joko geologisesta piirteestä, tutkimusgeometriasta, tai paikallisesti heikompilaatuisesta datasta. Vaaka-asentoisten ja loivasti kaltevien heijastajien seismiset piirteet ovat erilaisia, mahdollisesti geologisista syistä johtuen.

Kaadetta 40º jyrkempiä heijastajia ei ole havaittu. Lähes pystyjä piirteitä (siirros-tumista) voitaisiin havainnoida vaaka-asentoisten piirteiden katkeamista noin 200-300 ms ajan kohdalla. Jotkut näistä piirteistä ovat melko selviä, osoittaen lähetinlinjojen suuntaista kulkua (koillinen-lounas). Pilottitutkimus osoitti että tekniikka on toimiva ja käyttökelpoinen myös vastaavissa töissä tulevaisuudessa. Joitakin parannuksia ehdo-tettiin toteutukseen. Alueen koko oli pieni suhteessa tutkimussyvyyteen. Suurempi alue ja pienempi laskenta-ala (bin) olisi suotavaa. Linjojen avaus ja muut valmistelut asettavat todennäköisesti rajoituksia tulevaisuuden mittausjärjestelyille, kuten linja-tiheydelle ja aktiivisten kanavien määrälle. Vähintään n. 30 kertainen pinoaminen olisi tarpeen riittävän tarkkaa kuvantamista varten. Vakavampaa aineiston tulkintayritystä ei vielä ole tehty.

Avainsanat: seismiikka, 3d, heijastus, mittaukset, prosessointi, migraatio, ydinjäte, loppusijoitus

1

TABLE OF CONTENTS

ABSTRACT

TIIVISTELMÄ

1 INTRODUCTION................................................................................................ 3

2 DATA ACQUISITION.......................................................................................... 5

3 DATA PROCESSING....................................................................................... 13

4 STACKED AND MIGRATED VOLUMES ......................................................... 23 4.1 DMO ............................................................................................................ 23 4.2 Sub-horizontal reflection zones ................................................................... 23 4.3 Dipping reflections ....................................................................................... 23 4.4 Near-vertical structures................................................................................ 23

5 DISCUSSION ................................................................................................... 37 5.1 Acquisition ................................................................................................... 37 5.2 Processing ................................................................................................... 38 5.3 Interpretation................................................................................................ 38

6 CONCLUSIONS AND RECOMMENDATIONS ................................................ 43

ACKNOWLEDGEMENTS.......................................................................................... 45

REFERENCES .......................................................................................................... 47

3

1 INTRODUCTION

Reflection seismics have served as a useful tool for imaging and mapping of fracture zones in crystalline rock along 2D lines in nuclear waste disposal studies (e.g. Mair and Green, 1981; Juhlin and Stephens, 2006). This also applies to mineral exploration where the targets have been the general structural setting of the ore deposits or the ores themselves. Although the primary targets on many of these lines can be approximated as 2D structures it has been evident that there exist numerous out-of-the-plane reflections in the acquired data. This is to be expected in crystalline rock which has been subjected to several phases of deformation and/or metamorphism. Therefore, in order to obtain as an accurate image of subsurface structure as possible, 3D seismic data are required.

A major concern in the localization of a nuclear waste site is the presence of hydraulically conductive sub-horizontal fracture zones since these may have a major impact on water circulation patterns once waste has been deployed underground. Sub-horizontal to gently dipping fracture zones are difficult to detect using geological mapping methods since they generally do not intersect the surface in the area being mapped. In addition, drilling and hydraulic testing often give data which are difficult to interpret resulting in that borehole to borehole correlations may be poor. Reflection seismic surveys provide a tool for integrating surface studies with borehole results in locating sub-horizontal to gently dipping fracture zones, some which may be hydraulically conductive. Reflection seismics can also constrain the geometry at depth of more steeply dipping known fracture zones which intersect the surface.

In order to assess the potential of 3D seismics for imaging the upper km of crust and to gain some structural information, a 3D seismic pilot study was carried out in the Olkiluoto area in south-western Finland (Figure 1) during June 2006 as a joint Finnish-Swedish project. A subsurface area of about 650 m x 600 m was covered by the survey using a fixed receiver array and a mechanical VIBSIST source (Park et al., 1996; Cosma and Enescu, 2001; Juhlin et al., 2002). The survey should be considered a pilot study since the size of the subsurface area is small compared to the depth of investigation (1500 m) and the highly variable fold within the survey. Fold has proven to be a key parameter in obtaining high quality images of the subsurface (e.g. Juhlin et al., 2001).

In this report we summarize the acquisition parameters and progress, present results from the processing, and give some preliminary comments on the seismic images.

The reflection seismic method used here images the bedrock from about 100 m depth down to depths of several km. Zones or changes in the elastic properties of the bedrock, i.e. lithological changes or fracture zones, greater than about a meter in thickness may be imaged. Surveys from the Oskarshamn area in Sweden show that structures dipping up to 75º can be imaged with 2D reflection seismics (Juhlin et al., 2004). Similar results may be expected from 3D data.

4

Figure 1. Location of study area.

5

2 DATA ACQUISITION

The acquisition crew arrived in the field on 5 June, 2006. Deployment of the recording system and source testing was done on 6-7 June. Data acquisition began on 8 June and ended on 16 June with breaks in the acquisition on 10-11 June. Table 1 shows the acquisition parameters used in the survey and Table 2 shows a summary of the acquisition progress. In early morning (00:30-03:00) on 16 June, mine blasts were recorded by the fixed geophone array.

Geophones were placed in the field within 0.5 m of the planned locations with geophone line numbering running from NE to SW as R01…R09 and the geophone numbering from SE to NW 001…030 (Figure 2). Sources were located in the field within 0.5 m in the in-line direction and within 10 m perpendicular to the source line of the planned location. Source lines were oriented N32° and numbered S01 (SE) … S07 (NW) with source numbers on lines ranging from 001 to 071, number 001 is always in the NE (Figure 3). This numbering system allows for possible later extensions of up to 99 lines and up to 999 geophone or source positions on a line. Actual source and receiver positions for the pilot survey are shown in Figure 4. The total number of planned sources was 496 and the numbe in the pror actually usedcessing was 469.

For full 3D surveys a template scheme is normally used with the aim of having the same acquisition geometry for each template (see e.g. Birkhaüser and Graf, 2000; Spitzer et al., 2001; Juhlin et al., 2006 for high resolution 3D seismic examples). The Olkiluoto 3D pilot survey may be considered as a single template. If the survey is to be expanded, this template would then be shifted by its half-width in both the in-line and cross-line directions to record 3D data over a larger area. However, the distribution of offsets and azimuths would remain the same for the full 3D survey as in the individual template. Figures 5 and 6 show the actual distribution of offsets and azimuths for the pilot survey, respectively. There is a good distribution of all azimuths for offsets between 100 m and 600 m (Figure 7). However, if a larger survey is to be carried out, a study needs to be done on the optimum template design.

6

Table 1. Acquisition parameters for the 3D pilot survey at Olkiluoto 2006.

Parameter Value

Receiver line spacing / number 60 m / 9

Receiver station spacing / channels 24 m / 30

Source line spacing / number 100 m / 7

Source point spacing / number per line 10 m / 71

CDP bin size 12 m x 12 m

Nominal fold Highly variable

Geophones 28 Hz single

Sampling rate 1 ms

Record length 30 s raw

Source VIBSIST: 2000-2500 J/impact at 200-800 impacts/minute on a steel/aluminium plate of 800 mm x 800 mm mounted on a 13 ton tractor, source signal sent via radio to recording system

Instrument SERCEL 408UL

Table 2. Acquisition progress 3D pilot survey at Olkiluoto 2006.

Date Number of source points Hours

8 June 2006 49 9.75

9 June 2006 79 10.25

12 June 2006 98 11.25

13 June 2006 54 5

14 June 2006 60 8.25

15 June 2006 94 8.45

16 June 2006 43 7.5

Total 477 60.45

7

Figure 2. Planned location and numbering system of receiver points for the 3D pilot survey.

8

Figure 3. Planned location and numbering system of source points for the 3D pilot survey.

9

Figure 4. Location of source points (blue) and receiver points (red) for the 3D survey. Green dots indicate location of receiver gathers shown in later figures.

10

Figure 5. Actual distribution of offsets for the pilot survey shown in Figure 4.

Figure 6. Actual distribution of azimuths for the pilot survey shown in Figure 4.

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Figure 7. Azimuth versus offset for the pilot survey shown in Figure 4.

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3 DATA PROCESSING

Decoded VIBSIST shot gathers were delivered by Vibrometric Oy to Uppsala University on 1 July 2006. These decoded shot gathers served as input to the Claritas 3D seismic processing software (www.gns.cri.nz). This software is geared to seismic processing and not interpretation. Therefore, any interpretations suggested here should be considered preliminary.

Source and receiver point coordinates were delivered by Vibrometric and checked for errors. Based on mapping files provided by Vibrometric, the geometry of the sources and receivers was added to the shot gathers and the data were binned into 12 m x 12 m CDP bins. The CDP bins were oriented to lie parallel with the midpoint distribution in the cross-line direction (Figure 8). Fold is highly variable (Figure 9) due to that only one template has been used in the survey. Fold on the first and last 5 or so in-lines and cross-lines is generally too low to allow a reliable image to be obtained at these locations.

Processing steps were kept relatively simple (Table 3) to allow the processing to be carried out quickly and minimize the potential for introducing artefacts into the processed volumes. Statics, both refraction and residual, and choice of temporal filter had the greatest influence on the processed stack. The stack was relatively insensitive to the velocity used. DMO helped significantly in imaging dipping reflections.

Receiver refraction statics are greatest in the south-east corner of the survey, in the area of the peat bog (Figure 10).

For 3D migration, a 45 degree finite difference code was applied using the DMO velocity function. The same DMO velocity function was used for depth conversion of the migrated volume.

Three receiver points have been selected (Figure 4) to illustrate the effects of some of the processing steps on the data. Receiver point 10030 lies in the north-western corner of the survey area where the cover is relatively thin, receiver point 50015 is from the central part of the survey area where the cover may be somewhat thicker, and receiver point 90005 is from the bog area in the south-eastern corner of the survey area. Low frequencies dominate on raw source gathers, masking any potential reflections (Figure 11). Note also the poorer data quality for the receiver point in the peat bog area (RP 90005). After refraction statics the receiver gathers become considerably more coherent (Figure 12). Spectral whitening and bandpass filtering remove much of the low frequency noise (Figure 13), but the air blast remains and amplitudes are still quite variable from trace to trace. Application of an air blast attenuation filter and trace balancing reduces this variability (Figure 14). However, the direct S-wave arrival remains quite strong and it was found that application of automatic gain control (AGC) before stack (Figure 15) improved the stacked volumes.

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Table 3. Processing steps applied to the full 3D data set.

Step Parameters

1 Read decoded VIBSIST data

2 Bulk static shift: 100 ms

3 Apply geometry

4 Pick first breaks: All offsets

5 Resample: 1 ms

6 Spherical divergence correction: t1.0

7 Refraction statics: Datum 0 m, Replacement velocity 5400 m/s, v0 1000 m/s

8 Spectral Equalization: 50-75-300-400 Hz

9 Bandpass filter:

0-100 ms: 80-120-300-450 Hz

150-300 ms: 65-100-270.400 Hz

400-600 ms: 50-80-240-360 Hz

800-1200 ms: 50-75-225-360 Hz

10 Airblast attenuation: 345 m/s, 71 ms window

11 Trace balance: 0-1000 ms

12 Spectral equalization 20-40-90-120 Hz

13 Velocity analysis: every 10th CDP in the in-line and cross-line direction

14 AGC: 50 ms window

15 Residual statics

16

Normal moveout correction: 50% stretch mute

DMO (ms, v)

50-5300, 150 -5600, 250-5800, 500-6000

17 Stack


19 FX-Decon: In-line and cross-line directions


21 Migration: 3D FD using DMO velocities

22 Depth conversion with DMO velocities

15

Figure 8. Midpoints (black dots) for all sources and receivers. Shown in purple is the orientation of the 3D binning grid.

16

Cross-lines

Figure 9. Fold for the pilot 3D survey. Fold is for 12 m x 12 m square CDP bins. The system of in-lines (running WNW-ESE) and cross-lines (running NNE-SSW) is also shown.

17

Figure 10. Receiver refraction statics for the pilot 3D survey.

18

Figure 11. Three selected receiver gathers processed up to step 6 in Table 3.

19


20


21


22


23

4 STACKED AND MIGRATED VOLUMES

Display of 3D data is somewhat problematic. It is best to look at results on a workstation where the stacked and migrated volumes may be looked at interactively. For the purposes of this report representative slices have been extracted from the volume and displayed.

4.1 DMO

As mentioned in the previous section, DMO was an important processing step for imaging dipping reflections in the 3D volume. Figure 16 shows an in-line slice without and with DMO. It is clear that the slice with DMO provides a better image, especially of dipping reflections. Since the area covered by the survey is small, most of these dipping reflections migrate out of the volume, leaving mostly sub-horizontal reflections remaining within the volume after migration (Figure 17).

4.2 Sub-horizontal reflection zones

Inspection of a cross-line slice shows that the sub-horizontal reflections observed on the in-line are truly sub-horizontal and not from out-of-the-plane of the in-line (Figure 18). There are three zones of sub-horizontal reflectivity, the first at 200-300 ms (~600-900 m), the second at 450-550 ms (~1350-1650 m), and the third at 900-1000 ms (~2700-3000 m). Even though this last zone of sub-horizontal reflectivity is probably below the depth of interest for the repository, it does show, in general, that signal penetration was good in the area. The uppermost zone of sub-horizontal reflectivity lies at close to repository depths and it is of interest to look closer at it. Inspection of individual reflections from within the zone on migrated slices shows that they appear to be undulating and discontinuous, sometimes appearing to be offset (Figures 19 and 20). The curved pattern of some of the reflections on a time slice through the unmigrated volume at 205 ms (Figure 21) indicates that diffracted energy is present. Figures 22 and 23 show depth converted versions of Figures 19 and 20. Significant contrast in velocity and/or density would be expected to be encountered at 600 m to 800 m depth.

4.3 Dipping reflections

There are several sets of dipping reflections in the upper 200 ms (Figure 18). These reflections strike in an almost W-E direction as shown on a time slice through the volume at 150 ms (Figure 24) and dip to the south. They are somewhat difficult to follow southwards through the volume (Figure 21). Whether this is due to geology, the geometry of the survey, or poorer data towards the south is not clear. Dipping reflections with a similar orientation are also observed at about 400 ms (Figure 25).

4.4 Near-vertical structures

The apparent offsets in the sub-horizontal reflections at 200-300 ms suggest the presence of near vertical faulting. The lowermost reflection in this interval is relatively distinct (Figures 19 and 20) and traveltimes to it were picked using an automatic picking routine. These traveltimes are plotted in Figure 26. There is a marked increase

24

in traveltimes close to cross-line 1040, suggesting the presence of a steeply dipping fault in this area. A time slice through the migrated volume at 220 ms also indicates the undulations near the top of this reflective zone also strike parallel to the cross-line direction (Figure 27).

25

Figure 16. In-line 1015 processed up to step 19 in Table 3 without DMO (left) and with DMO (right). See Figure 9 for location of in-line.

26

Figure 17. In-line 1015 processed up to step 19 in Table 3 with DMO (left) and then the corresponding migrated slice (right). See Figure 9 for location of in-line.

27

Figure 18. In-line 1030 and cross-line 1030 processed up to step 19 in Table 3 with DMO. See Figure 9 for location of in-line and cross-line.

28

Figure 19. Migrated slices of in-line 1030 and cross-line 1030 down to 0.6 s processed up to step 21 in Table 3 with DMO. See Figure 9 for location of in-line and cross-line. Arrow points to the reflection that traveltimes were picked for.

29

Figure 20. Migrated slices of in-line 1015 and cross-line 1015 down to 0.6 s processed up to step 21 in Table 3 with DMO. See Figure 9 for location of in-line and cross-line. Arrow points to the reflection that traveltimes were picked for.

30

Figure 21. Time slice at 205 ms from the pilot 3D survey processed up to step 19 in Table 3 with DMO. The system of in-lines (running WNW-ESE) and cross-lines (running NNE-SSW) is also shown.

31

Figure 22. Migrated depth converted slices of in-line 1030 and cross-line 1030 down to 0.6 s processed up to step 22 in Table 3 with DMO. See Figure 9 for location of in-line and cross-line.

32

Figure 23. Migrated depth converted slices of in-line 1015 and cross-line 1015 down to 0.6 s processed up to step 22 in Table 3 with DMO. See Figure 9 for location of in-line and cross-line.

33


34


35

Figure 26. Traveltimes to the reflection in the migrated indicated by the arrows in Figures 19 and 20. The system of in-lines (running WNW-ESE) and cross-lines (running NNE-SSW) is also shown.

36


37

5 DISCUSSION

5.1 Acquisition

It cannot be overemphasized that the size of the survey area was small compared to the depth of investigation. In 2D surveys, profiles should generally be 3 times as long as the desired depth of investigation to allow unbiased imaging on the central third of the profile. Therefore, an area about 9 times the size of the present area would be required to properly image to 600-700 m depth. The 12 m x 12 m bin size is also somewhat large. There are indications in the stacked volume that some steeply dipping events are aliased. A 6 m x 6 m bin size would be better and on par with the 5 m CDP spacing on SKB profiles.

Given that cutting of tracks for the source is a high cost item it is of interest to design a template that minimizes the number of source tracks. One way of keeping up fold, but limiting source lines is to increase the number of receiver lines. The template outlined in Table 4 is a compromise between higher lateral resolution and a limited number of source lines. It would require that about 800 channels are available in order for work to proceed smoothly. Assuming that 120 source points can be completed per day the time to cover the entire area would be 40 production days. Taking into account delays due to weather, equipment problems and switching of swaths, the total time to complete such a survey should be less than 2 months.

Table 4. Possible acquisition parameters for future 3D seismic surveys at Olkiluoto.

Parameter Value

Receiver line spacing / number 72 m / 10

Receiver station spacing / channels 18 m / 48

Total number of receivers per template 480

Source line spacing / number 72 m / 6

Source point spacing / number per line 36 m / 40

Total number of sources per template 240

CDP bin size 9 m x 9 m

Nominal fold 30

Template size for full fold 432 m x 720 m

Number of templates required to cover a 2 km x 2 km area

20

38

5.2 Processing

As with 2D data from similar targets, the most important processing steps were refraction statics, choice of filter and DMO. DMO was especially important in imaging the dipping reflections in the uppermost 200 ms. In contrast to 2D data, it is not possible to apply NMO with an apparent velocity appropriate for a specific dip when CDP bins contain multi-azimuth data. Therefore, even though velocity analysis was applied prior to DMO, it was not possible to image the dipping reflections clearly in the upper 200 ms without DMO. Velocity analysis after DMO showed that a single velocity function for the entire data set gave the best results. It is this velocity function that was used for migration and depth conversion, so the depth volume should be used with caution.

5.3 Interpretation

No serious attempt has been made to interpret the data at this stage. However it is worthwhile to point out that the south dipping reflections, dipping at about 25-30 degrees in the upper 200 ms, and the sub-horizontal ones appear to have a different seismic signature. The south dipping ones are fairly continuous, distinct and linear in nature (Figures 28 and 29), whereas the sub-horizontal ones are more undulating, discontinuous and possibly show offsets (Figure 30). This difference may be of importance when correlating with borehole data and generating geological models for the area.

39

Figure 28. X, Y and T slices through the stacked 3D volume at 150 ms processed up to step 19 in Table 3.

40

Figure 29. X, Y and T slices through the stacked 3D volume at 170 ms processed up to step 19 in Table 3.

41

Figure 30. X, Y and T slices through the migrated 3D volume at 225 ms processed up to step 21 in Table 3.

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6 CONCLUSIONS AND RECOMMENDATIONS

The 3D pilot seismic survey has shown that it is possible to acquire high-resolution seismic data in the Olkiluoto area. Reflectors from about 100 m down to at least 3 km are imaged. Two groups of reflections are observed, (1) sub-horizontal zones of reflectivity at 200-300 ms, 450-550 ms and 900-1000 ms, and (2) more distinct, but weaker reflections corresponding to structures striking nearly in the W-E direction and dipping to 25-30 degrees to the south. It is a reflection from this latter group that can be traced to about 100 m from the surface in the northern part of the survey. The former group contains stronger and more undulating reflections, some of which may be offset, suggesting faulting. The small area covered by the survey makes structural interpretation difficult since most of the dipping reflections migrate out of the volume.

For a larger scale 3D survey a template system needs to be used where the template is moved over the area to be covered by the survey. Work should be carried out to design an optimum template. An important component of any template design is the required fold. Based on the current data set, an estimate of the required fold for imaging the upper 1000 m should be made. Given that reflections can be observed already in processed source and receiver gathers, a fold of 30 is probably sufficient, however, this needs to be confirmed. It is important to have the template design completed at an early stage since it will steer the cutting of trails in the forest, the required rentals for extra channels, and the data storage and processing capacities of the systems used.

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ACKNOWLEDGEMENTS

We thank Posiva, Andra and Pöyry for their support in this work. The work has been overviewed by Jussi Mattila of Posiva, Yannick Leutsch of Andra and Turo Ahokas of Pöyry.

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REFERENCES

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