rock fabric determination

8/9/2019 Rock fabric determination

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ASEG 15 th Geophysical Conference and Exhibition, A ugust 2001, Brisbane. Extended Abstracts

Measurement of Rock Fabric in Shallow Refraction Seismology

Derecke Palmer

University of New South Wales, Australia [email protected]

INTRODUCTION

Shallow 3D refraction methods are not very common.However, geological structures and the corresponding depthsto and wavespeeds within bedrock, can show as muchvariation in the cross-line direction as in the in-line direction.Furthermore, azimuthal anisotropy of wavespeeds is common,especially with complex 3D structures.

Anisotropy can be caused by lamination, foliation or by thepreferred orientation of joints and cracks within the refractor,and it is another important parameter for assessing rock strength for rippability and foundation design. However, themost important near-surface application may be in thedetermination of fracture porosity in crystalline rocks for thedevelopment of groundwater supplies for domestic andirrigation purposes, in studies of contaminant transportespecially of radioactive wastes (Barker, 1991), the stability of rock slopes and seepage from dams, the construction of underground rock cavities for storing water, gas, etc, and theconstruction of tunnels.

The relationship between anisotropy and crack parameters hasbeen the subject of considerable research in the past (Crampinet al, 1980; Thomsen, 1995). Nevertheless, there are noestablished approaches for the routine mapping of theseparameters with shallow geotechnical or environmentaltargets, although radial surveys to measure azimuthalanisotropy (Bamford and Nunn, 1979; Leslie and Lawton,1999) represent the first steps in that direction.

DATA PROCESSING WITH THE GRM

This study describes the results of a 3D shallow seismicrefraction survey recorded some time ago across a shear zoneat Mt Bulga in southeastern Australia. The data are processedwith a traditional approach using the generalized reciprocalmethod (GRM) (Palmer, 1980; Palmer1986), rather than withtomographic inversion for the following reasons.

Efficient tomographic imaging generally requires largevolumes of data, but nevertheless it is still not able to reliablyresolve large variations in depths to and wavespeeds withinrefractors. In this study, the volume of data is relatively low,while the wavespeeds in the refractor range from less than2000 m/s in the shear zone to more than 5000 m/s in theadjacent rocks. Model studies and case histories (Palmer,1980; Palmer, 1991) demonstrate that the GRM can resolvelarge variations in the depths to and wavespeeds withinrefractors using considerably smaller data volumes than is thecase with most tomography programs.

Azimuthal anisotropy is rarely accommodated with mosttomography programs. In addition, the traveltime differencesdue to anisotropy are quite small, and are often within theaccepted range for the residuals of inversion.

In this study, the amplitudes of the refracted head waves areused to map anisotropy. Previous studies have shown that thehead coefficient, the parameter which controls the amplitudeof the refracted signal, is approximately proportional to theratio of the specific acoustic impedances of the overburdenand the refractor (Palmer, 2000a). However, the head waveamplitudes are generally dominated by the rapid variation dueto geometric spreading. Another study (Palmer, 2000b),demonstrates that the effects of geometrical spreading anddipping interfaces can be minimised with either themultiplication of the amplitudes of the forward and reversetraces, or by the convolution of those traces. In this study, theratios of the amplitude products for pairs of shot points withvarying azimuths are used as a qualitative measure of azimuthal anisotropy.

SURVEY DETAILS

SUMMARY

A three dimensional (3D) seismic refraction survey wascarried out across a shear zone. The data were processedwith the generalized reciprocal method (GRM) ratherthan with tomographic inversion because of the relativelysmall volume of data, the occurrence of large variationsin depth to and wavespeeds within the main refractor andthe presence of azimuthal anisotropy.

The amplitude products of the refracted signals areapproximately proportional to the square of the ratio of the specific acoustic impedances between the upper layerand the refractor. The ratios of these amplitude productsfor different azimuths of shot pairs for a given set of geophones provide a convenient and detailed measure of apparent azimuthal anisotropy or rock fabric.

Qualitative measures of azimuthal anisotropy areobtained from the wavespeeds and the time-depthscomputed from the traveltime data with the GRMalgorithms and from the amplitudes. These threemethods give similar consistent results, with the directionof the greater wavespeed being approximately parallel to

the direction of the dominant geological strike.Furthermore, all three methods show that the direction of the greater wavespeed is approximately orthogonal to thedirection of the dominant geological strike in one regionadjacent to the shear zone.

Key words: Anisotropy, refraction, GRM, fabric,seismic.


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Measurement of rock fabric in shallow refraction seismology Palmer

ASEG 15 th Geophysical Conference and Exhibition, August 2001, Brisbane. Extended Abstracts

The data used in this study were recorded with a 48 traceseismic system in approximately the same location as a 2D setof data described previously (Palmer, 2000b).

Figure 1. Plan of in-line and cross-line geophones and shotpoints. Shots 1 to 15 are shown as bold symbols and wererecorded with in-lines 17 and 21. Shots 16 to 42 are shownas open symbols and were recorded with cross-lines 45 to69.

The data were recorded with two sets of geophonearrangements which are described in more detail in Palmer(2001). The results obtained with the two parallel in-lines 17and 21 constitute the major part of this study. These lines are20 m apart, with each consisting of 24 geophones at a 5 mspacing. The results obtained with the seven cross-lines arecovered in more detail in Palmer (20001).

Figure 1 is a plan of the two geophone arrangements and shotpoint locations.

ANALYSIS OF THE IN-LINE TRAVELTIMEDATA

The traveltimes were hand picked from the field monitors, andstandard corrections for the uphole time and the system delayin the analogue components were applied. The previous 2Dstudy (Palmer, 2000b), showed that a three layer model wasapplicable. It consists of a thin surface layer of friable soilwith a wavespeed of about 400 m/s, a thicker layer of

weathered material with a wavespeed of approximately 700m/s, and a main refractor with an irregular interface withwavespeeds between approximately 2000 m/s and 5000 m/s.

The traveltime data for in-line 21 for thirteen shots are shownin Figure 2. The graphs for the shot points which are offset by60 m from the geophone spreads in the in-line direction andwhich are located along cross-lines 33 and 81, namely shots 1to 4 and 8 to 11, all show arrivals which originate from themain refractor. The graphs in the forward and reversedirections appear to be essentially parallel, but in factgradually converge.

Figure 2. Traveltime data recorded on in-line 21 with in-line, adjacent and oblique shot points. In general, the

graphs gradually converge in each direction of recording.

Figure 3 shows the time-depths computed with equation 1using a 5 m XY value, for four shot pairs. They are shots 2and 9 which are collinear with the detectors, shots 3 and 10which are collinear with the adjacent parallel line of detectorson in-line 17, shots 1 and 11 which form a northwest-southeast shooting orientation, and shots 4 and 8 which form anortheast-southwest shooting orientation.5.3. The reciprocaltime for the in-line shots 2 and 9 was computed with equation33 of Palmer (1980). The reciprocal times for the other shotpairs could not be derived as conveniently, and they have beenadjusted until the differences in the time-depths betweencross-lines 45 and 49 were minimized. This facilitates the

recognition of the systematic divergence of the time-depths forthe oblique shot pairs from the collinear values.

time-depth = (t forward + t reverse - t reciprocal )/2. (1)

The increase in the time-depths between cross-lines 50 and 60corresponds to the region in the refractor with the lowwavespeed.

The systematic divergence of the time-depths computed withthe oblique shot pairs from cross-line 49 to cross-line 69, canbe employed as a qualitative measure of azimuthal anisotropyin the following way.

The time-depths are related to the depths with equation 2,

zG = t G / DCF (2)

where the DCF, the depth conversion factor relating the time-depth and the depth, is given by:

DCF = V V n / (V n2 - V 2) (3)

It is reasonable to assume that the point of critical refractionbelow each station is much the same whether the energypropagating in the refractor is travelling in the northeast-southwest direction or the northwest-southeast direction. Thisimplies that the depth to the refractor is the same irrespectiveof the direction of measurement. Therefore, any variations inthe time-depth at each station will be related to variations inthe DCF through equation 2.


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Figure 3. Time-depths computed for the in-line, adjacentand oblique shot pairs. The reciprocal times for the oblique

shots have been adjusted so that the time-depths are thesame between cross-lines 45 and 49, in order to emphasizethe systematic divergence from the in-line values.

The time-depths for the oblique shots were then re-adjusted inthe following manner. In-line 21 intersects the line joiningshots 1 and 11 at cross-line 53 and the line joining shots 4 and8 at cross-line 61. The time-depths for the oblique shots atthese points were computed from the in-line depths withequation 3 using the refractor wavespeeds appropriate to eachdirection (Palmer, 2001, Fig. 3). The reciprocal times for theoblique shots were then adjusted until the depths at theintersections were the same as those just computed. Finally,the ratios of the time-depths in the two oblique directions werecomputed and they are presented in Figure 4.

Figure 4. The ratio of the time-depths computed with shots4 and 8 in the northeast-southwest direction and shots 1and 11 in the northwest-southeast direction.

As with the wavespeed analysis function in Palmer (2001),this method of detecting azimuthal anisotropy makes noallowances for the variations in reciprocal time for thedetectors which are not collinear with the shot points. Thereciprocal time subtracted in equation 1 should be increasedfor the detectors offset from the line joining the shot points, inorder to take into account the extra path length in the refractor.The use of a constant reciprocal time should increase thecomputed time-depths at the offset detectors and as a result,

the time-depth profile should appear to be flattened.However, no such flattening is obvious in Figure 3.

Despite these reservations, the results are presented becausethey are consistent with those determined with amplitudes. Inparticular, the region between cross-lines 45 and 49 showsvalues less than one, while the remainder shows values greaterthan one. These results are qualitatively similar to thosederived from amplitude ratios in Figure 5 below and from acomparison of the in-line and cross-line wavespeeds.

These results also demonstrate the benefits of including ananalysis of the residuals as a function of azimuth withtomographic methods. The differences in traveltimes betweenthe offset shots in Figure 2 show little variation about themean and as a result, the time-depths also show the same smallvariations. For example, the variations about a zero meandifference in the time-depths in Figure 3 are less than a fewmilliseconds. Although such variations are within theacceptable ranges of residuals for most tomographicapproaches, nevertheless, there may still be a systematic

correlation with azimuth and therefore an indication of azimuthal anisotropy.

ANALYSIS OF THE IN-LINE AMPLITUDE DATA

The amplitudes of the first arrivals were hand picked from thetrace values. A correction was applied for geometricspreading using a reciprocal of the distance cubed expression,which previous studies had indicated was appropriate for thissite (Palmer, 2000b). The corrected amplitudes for the fourpairs of shots described above, were then multiplied.

Figure 5. Amplitude products corrected for geometricspreading for shots 2 and 9.

Figure 5 shows the amplitude products for shots 2 and 9which are colinear on in-line 21. They show low valuesbetween cross-lines 45 and 49, which correspond with thewavespeed of 5000 m/s, higher values between cross-lines 50and 62, which correspond with the wavespeed of 1850 m/s,and lower values between cross-lines 63 and 69 whichcorrespond with the wavespeed of 3930 m/s. The amplitudesin this last region gradually increase towards cross-line 69,and correspond with the decrease in wavespeed when theregion is further sub-divided into two regions.

These results are consistent with previous studies which

demonstrate that the amplitude product is approximatelyproportional to the square of the ratio of the specific acousticimpedances of the overburden and the refractor (Palmer,


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2000b). Since the wavespeeds in the layers above the mainrefractor exhibit little lateral variation in the in-line direction,the amplitudes are essentially a function of the wavespeedsand densities in the refractor.

Figure 6. An apparent anisotropy factor obtained from thesquare root of the ratio of the corrected amplitudes for thetwo pairs of oblique shots.

The amplitudes of the other shot pairs show the same generalpattern.

Figure 6 shows the square root of the ratio of the amplitudesobtained with shots 4 and 8 in the northeast-southwestdirection to the amplitudes obtained with shots 1 and 11 in thenorthwest-southeast direction. This parameter should reflect arelative anisotropy factor, since it is not possible to provide anabsolute scale, because as yet, there is no method for

compensating for the different energy levels and coupling of each shot. However, an approximate scaling factor wasobtained from the ratio of the wavespeeds in the differentdirections for the region with the low wavespeeds betweencross-lines 50 and 62 in Palmer (2001, Fig. 3).

DISCUSSION AND CONCLUSIONS

This study demonstrates that there are small but consistentvariations in both traveltimes and amplitudes due to azimuthalanisotropy. The variations in traveltimes can be shown withboth the refractor wavespeed analysis function (Palmer, 2001,Fig. 3) and the time-depth function in Figure 3. There aresome reservations with applying these algorithms to the

oblique shot pairs, because they are strictly applicable only tocollinear shot points and receivers. However, the fact thatthey give qualitatively a similar result to those obtained withthe amplitude results gives some measure of reassurance.Furthermore, the results are consistent with he cross-line datadescribed in Palmer (2001).

All of the results show that in general the wavespeeds arehigher in the cross-line direction, that is along the dominant

geological strike, than in the in-line direction. However, theexception is the region between cross-lines 45 and 49 wherethe reverse applies.

This study shows that the GRM can demonstrate anisotropyeven where there are large variations in refractor depths andwavespeeds, and that. amplitudes can also provide a usefulmeasure of relative azimuthal anisotropy. The fact that Figure6 is consistent with the model provides confidence in thevalidity of the relative anisotropy factor. Also, it is clear thatthe amplitude ratios are providing greater detail, although it isnot possible to separate those features of a geological originfrom those related to the limited sampling with only twoparallel recording lines.

REFERENCES

Bamford, D., and Nunn, K. R., 1979, In-situ seismicmeasurements of crack anisotropy in the Carboniferouslimestone of North-west England: Geophysical Prospecting,27, 322-338.

Barker, J. A., 1991, Transport in fractured rock, in Downing,R. A., and Wilkinson, W. B., eds., Applied groundwaterhydrology: Clarendon Press, 199-216.

Crampin, S., McGonigle, R., and Bamford, D., 1980,Estimating crack parameters from observations of P-wavevelocity anisotropy: Geophysics, 45, 345-360.

Leslie, J. M., and Lawton, D. C., 1999, A refraction-seismicfield study to determine the anisotropic parameters of shales:Geophysics, 64, 1247-1252.

Palmer, D., 1980, The generalized reciprocal method of seismic refraction interpretation: Society of ExplorationGeophysicists.

Palmer, D., 1986, Refraction seismics - the lateral resolutionof structure and seismic velocity: Geophysical Press.

Palmer, D., 1991, The resolution of narrow low-velocity zoneswith the generalized reciprocal method: GeophysicalProspecting, 39, 1031-1060.

Palmer, D., 2000a, Imaging refractors with convolution:Geophysics, in press.

Palmer, D., 2000b, Resolving refractor ambiguities with

amplitudes: Geophysics, in press.

Palmer, D., 2001, A simple approach to 3D shallow refractionseismology: in preparation.

Thomsen, L., 1995, Elastic anisotropy due to aligned cracks inporous rock: Geophysical Prospecting, 43, 805-82.

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