Shear Wave Splitting and Fracture Patterns at the Geysers (California) Geothermal Field

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PROCEEDINGS, Twenty-Sixth Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, January 29-31, 2001 SGP-TR-168 SHEAR WAVE SPLITTING AND FRACTURE PATTERNS AT THE GEYSERS (CALIFORNIA) GEOTHERMAL FIELD

D. Erten, M. Elkibbi and J.A. Rial

Wave Propagation Laboratory, Department of Geological Sciences, University of North Carolina at Chapel Hill , NC 27599

ABSTRACT along the ray joining the source and the receiver. In addition, the source time history, me- Shear-wave splitting analyses from recorded dium heterogeneity, a weathered surface layer, microearthquakes at The Geysers (California) the presence or absence of fluids in the cracks, indicate that subsurface predominant fractures and the surface topography tend to obscure the in the field are oriented N10oE to N50oE under effect of crack-induced anisotropy. the NW Geysers seismic array and N40-50oE Ideally, the approaching S-wave is split and N40-50oW under the SE Geysers array. into a fast wave polarized along the predomi- These crack alignments seem consistent with a nant crack direction, and a slow wave, polarized local structure characterized by a pull-apart perpendicular to it and arriving a few tens of block structure limited by two right-lateral milliseconds later. Polarization diagrams are strike-slip faults oriented NW-SE and regionally used to detect the marked switch in polarity of controlled by the San Andreas fault. the two arrivals, which provide the clearest indi- cation of medium anisotropy (Figure 1). It is the INTRODUCTION assumption here that the splitting of shear waves Shear-wave splitting due to patterns of stress- in The Geysers is indeed induced by the pres- aligned cracks in the crust has been widely ob- ence of oriented fractures in an otherwise homo- served in a variety of earthquake settings and geneous medium. Typically, the orientation of controlled-source seismic data. It has also been the fast wave polarization can be measured with recognized that the polarization of the leading errors not exceeding 10%. split shear wave is usually parallel to the local The time delay between the fast and strike of crack systems (or normal to the direction slow shear waves is measured directly after the of the minimum horizontal stress). In addition, seismogram is rotated to orient the fast and slow the time delay between fast and slow shear waves arrivals along the instrument’s horizontal com- is directly related to the number of cracks per unit ponents. This operation cleanly separates the volume (crack density) in the medium (Crampin, two shear wave arrivals allowing direct and ac- 1987; Crampin and Lovell, 1991). Therefore, the curate measurement of the time delay. Occa- interpretation of shear-wave splitting is an im- sionally, rotation of all three components is portant diagnostic tool to determine the direction performed to align the vertical component with and evaluate the bulk density of subsurface frac- the approaching ray. Most delay times recorded tures in hydrocarbon and geothermal reservoirs. at The Geysers range from 20 to 70 ms (see Fig- ure 1). The smallest time interval that can be METHODOLOGY measured is 2.1 ms (480 samples/sec). The shear-wave splitting method has been de- At each receiver polarization data are scribed extensively (e.g., Crampin, 1981; Lou collected in equal area projection diagrams and and Rial, 1997; Lou et al., 1997). Although rose diagrams. Numerous measurements per straightforward, the analysis of shear-wave station (Figures 2, 3, 4, 5) allow for a statisti- splitting for the purpose of crack detection must cally robust determination of the preferred di- be based on a large set of shear-wave seismo- rections. gram data. This is in part because the behavior One important restriction to the shear- of shear waves in cracked media is usually as wave splitting analysis is that the arriving rays complex as the distribution of cracks may be need to be within the shear wave window de- 2

-1 fined by a critical angle ic = sin (b/a), where a · Automatic selection of event from data and b are the P-wave and S-wave surface ve- bases locities, respectively. For angles of incidence · Interactive display of 3-component greater than ic, shear-waves interact strongly seismograms and their power spectra with the free surface, thus distorting the incom- · Interactive filter selection and win- ing waveform (Crampin and Booth, 1985). dowing of the S-wave train · Interactive horizontal and vertical rotation of windowed components · Interactive display of rotated seismograms and determination of time delay · Two- and three-dimensional displays of particle motion · Automatic recording of all measured parameters.

THE GEYSERS The Geysers geothermal field is located northeast of the San Andreas Fault in the northern Coast Range of California (110 km north of San Francisco). There is abundant microseismicity in the field, some of which appears induced by steam production. Two major right-lateral Figure 1. Shear wave window for a selected event. strike-slip faults related to the San Andreas fault Top row shows the two horizontal components and the system, the Mercuryville and Collayami fault horizontal particle motion of the S-wave train. Note that the angle between the two split shear waves is close to zones, appear to form the southwest and north- 90 degrees. The lower row shows how the rotated seis- east boundaries of the steam field (McLaughlin, mograms separate the arrivals into fast and slow. The 1981). (uncorrected) crack orientation is N65E. Time delay is There have been extensive seismic 64.5ms, measured directly from the rotated seismograms. studies at the Geysers geothermal field in recent years. Conventional seismological studies were The critical angle is 35o in a half-space with a first carried out to obtain P- and S-wave veloci- Poisson’s ratio of 0.25. In The Geysers it is ties, seismic Q values within the geothermal possible to increase the angle to 45o due to the reservoir etc (e.g., Zucca et al., 1994, Romero et presence of the near surface low velocity layers. al, 1995, Julian et al, 1993). More recently, Routine measurement of shear wave studies by our UNC-CH group (Erten and Rial, splitting is tedious because it requires direct 1998, Erten and Rial, 1999) and at Duke Uni- inspection of each seismogram by a trained op- versity (Shalev and Lou, 1995) have focused on erator. Our experience with The Geysers data the determination of the crack directions using suggests that automatic picking of polarization the shear-wave splitting method. directions and time delays without human inter- For simplicity we shall subdivide The vention is unreliable. This is because of the Geysers reservoir into two areas: the NW and great variability and diversity of wave patterns SE Geysers. The two areas are different in the that result from the interaction of shear waves number and quality of data available as well as with complex cracked media. Thus, to expedite in the type of instrumentation used. For in- and simplify the measuring process we devel- stance, seismographs in NW Geysers are down- oped computational graphic user interfaces hole instruments, while in SE Geysers all in- (GUIs) based on MatlabTM software (e.g., Figure struments are on the ground surface. 1). These automate the measurement process through a series of user-friendly, interactive SHEAR-WAVE SPLITTING FROM measurement devices and multidimensional NW GEYSERS displays that include the following: The NW Geysers area is a fairly active seismic zone with an average of 17 microearthquakes per day. Most of the earthquake activity is con- 3 centrated in the southeastern part of the Cold- polarization directions, large signal-to-noise water Creek Steam Field (CCSF), coinciding ratio, clear, robust shear-wave splitting and arri- with the production area. The data used for this val angle within the shear-wave window (e.g., study were recorded by a 16-station, digital, Figure 1). Since the seismometers at NW Gey- three component network operated by Lawrence sers are down-hole instruments, it is necessary Berkeley Laboratory during 1988 and part of to determine the true geographic orientations of 1994. All 16 geophones record at 400 sam- the horizontal components. To do this, we com- ples/sec and are buried about 30 meters below pared predicted and observed P-wave arrival the surface. angles for events surrounding the station. An After processing and analyzing over average of 20 events per station was used to 6000 events we have collected a large number of estimate the ‘text-book’ seismograms that show consistent actual horizontal component orientations.

Figure 2. The rose diagrams at each station indicate the preferred polarization. Station names are indicated. An average of 70 events per station were used to generate the rose diagrams. Station S13 (NW) used the least number of events (11) and station S12 (SE) the largest (204).

This resulted in standard deviations within 10o to N500E. Two exceptions are stations 9 and 14 for most station corrections. The geographic where the orientations are NW-SE. In general, orientations of the cracks are obtained after cor- the crack pattern as suggested by the fast shear recting for station component orientation. wave polarizations is consistent with the existence of aligned fractures held open by a re- STRIKE OF LEADING SHEAR-WAVES: The gional northeast-southwest maximum horizontal preferred orientations for the leading shear- compressive stress in the Northwest Geysers waves within the steam field range from N100E (Romero et al., 1995). It is also consistent with a 4 study by Nielson et al. (1991) which measured fracture orientations in cores inside the CCSF and found predominant north-northeast strikes of fractures parallel to the maximum horizontal compressive

Figure 3. Observed fast shear wave polarizations at stations 11 (above) 4 (middle) and 6 (below) in NW Geysers plotted on the epicenters of selected events.

stress. Since most of the earthquake activity is concentrated in the southeastern area of the reservoir, data for rays arriving within the shear- wave window outside of the steam field are limited. Outside of the steam field crack orientations are approximately N350E-N60oE (stations 12,13,15), N25oW (station 14), and N-S (station 16). In addition to the predominant polarizations, we have detected subtle variations in the inferred orientation of the cracks and observed complex seismograms that strongly suggest variation of crack directions with depth. This means that the polarization directions record not only the main directions of fracture near the receivers but some also contain information on deep-seated cracks. Figure 3 shows the observed polarizations at three selected stations (large triangles) within the NW array (small triangles). As can be easily seen, the observed polarizations are essentially independent of event location or azimuth, as they reflect the predominant crack directions in the neighborhood of the station. However discrepancies (cf. station 6) some of which may be related to deep-seated cracks are noticeable. These require a more detailed study (see also Figures 4 and 5).

TIME DELAYS: Comparison of data recorded in 1988 and 1994 indicate that although there has not been a significant change in the polarization directions with time, there has been an apparent increase in delay times. In fact, time delays for the NW area range from 10 to 45 ms for the 1988 data, and from 20 to 60 ms for the 1994 data, consistently, and for all stations. In NW Geysers the shortest resolved time interval is 2.5 ms (400 samples/sec), so that the delay time increment may be real. Normalized to the 5 length of the ray path the time delays consis- the field. In fact, well-documented observations tently increased at all stations in NW Geysers exist of how split shear wave delays detect from around 5 ms/km in 1988 to over 10 ms/km changes in stress produced by earthquakes in in 1994. The increase does not appear to be the seismic zones. For instance, Li (1995) using product of unrelated circumstances, such as in- shear wave splitting, found that the usual aver- strumental improvement, changes in event loca- age time delay of ~10 ms/km of the split shear tion accuracy or in local velocity models. If con- wave dropped to ~7 ms/km right after the firmed by further data analyses, the time delay Northridge, CA earthquake increase could be attributed to changes in mag- of 1994 in the hypocentral region of the event. nitude of local maximum horizontal compres- Similar observations have been reported for the sive stress, the widening of the cracks or their Landers 1992 earthquake, and in the Los Ange- filling with fluid. If so, shear wave splitting les basin (Li et al., 1994; Hauksson, 1994). could be used, for instance, to monitor stress changes produced by fluid migration throughout

Figure 4. Equal area projections of the best polarization measurements in NW Geysers. In NW Geysers best measurements are those for which the estimate error in polarization is 5 degrees or less and the uncertainty in time delay is 7.5 ms or less. The inner circle corresponds to angles of incidence within the shear wave window. The stations shown are down- hole. . 6

SHEAR-WAVE SPLITTING FROM Geysers by a 13-station, 3-component, high fre- SE GEYSERS quency digital (480 samples/s) network operated The SE Geysers area is also an active seismic by Lawrence Berkeley National Laboratory and zone with an average of 20 microearthquakes Lawrence Livermore National Laboratory per day. The data used were recorded at SE (LLNL). All

Figure 5. Equal area projections of the best measurements in SE Geysers. In SE Geysers best measurements are those for which the estimate error in polarization is 5 degrees or less and the uncertainty in time delay is 6.0 ms or less. The inner circle corresponds to angles of incidence within the shear wave window.

northwest (N40-50oW) and a nearly perpen- 13 stations have geophones at the surface. We dicular set along the northeast (N40-50oE) di- have assembled a catalog containing the shear- rection (Figure 2). wave splitting parameters (first-motion direction Since the earthquake activity is higher and time delay) and event locations after proc- in the western part of the area, there are less essing and analyzing over 700 events from high quality events for stations 1, 2, 3 and 5. March, 1999. Here too, the high quality of re- Thus, it was necessary to use lower quality cordings produced a special set of ‘textbook’ events for analyses for these stations. Stations 7 events, those that showed large signal-to-noise and 9 were non-operational during this period. ratio, and clear, robust shear-wave arrivals. TIME DELAYS: Time delays range from 15 to STRIKE OF LEADING SHEAR-WAVES 50 ms for SE Geysers. Only data from 1999 There are two main sets of preferred orientations have been processed so far in SE Geysers. No for the leading shear-waves, a first set striking 7 systematic variation in time delay or polariza- stations. These may indicate that multiple direction direction has yet been observed. tions of aligned fractures exist either beneath the station or along significant fractions of the source-receiver path. CONCLUSIONS AND DISCUSSION A possible contributor to changes in polarization includes violation of the assumption In previous studies of The Geysers and Coso that all cracks are vertical. However, it can be geothermal reservoirs we have shown that shear- shown that polarization of the fast wave is ro- wave splitting is a useful tool in crack detection bust against variations in crack dip. In fact, (e.g., Rial and Lou, 1996; Lou et al.,1995, synthetic seismogram experiments show that the 1997; Lou and Rial, 1997; English and Rial, polarization of the fast split wave is mostly con- 1997; Erten and Rial, 1998; 1999). The method trolled by the crack strike, and only weakly af- provides two important parameters with each fected by their dip. Specifically, synthetic seis- seismogram observation: the preferential direc- mograms computed following Crampin (1981) tion of the cracks (from the polarization of the indicate that vertical cracks show the same po- fast shear wave) and a measure of the crack den- larizations as the crack dip is decreased to 70 sity along the seismic ray between source and and even 50 degrees. receiver (from the delay time of the slow or split Full investigation into the 3D structure shear wave). At The Geysers we have shown and depth of the crack system using full-wave that the leading polarization direction of shear- synthetic seismograms that incorporate the ef- waves range from N10oE to N50oE in NW Gey- fect of rock heterogeneity and anisotropy is cur- sers and N40-50oE and N40-50oW in SE Gey- rently under way. sers (see Figure 2). Time delays give an estimate of the in- Crack alignments in the region seem tensity of cracking that is not much different consistent with a local structure characterized by from that found in the Coso field (Lou and Rial, a pull-apart block structure limited by two right- 1997). The delay information obtained at The lateral strike-slip faults oriented NW-SE. The Geysers will be next transformed into a 3D map inferred crack patterns at NW Geysers are fully of crack density by tomographic inversion. consistent with the existence of aligned fractures held open by a regional northeast-southwest FUTURE WORK maximum horizontal compressive stress in Our work at NW Geysers has so far been mostly Northwest Geysers (Romero et al., 1995). The observational, concentrating on the measure- results are also consistent with studies by Niel- ment of fast wave polarizations and split wave son et al. (1991), who measured fracture orien- time delays. The next phase is mostly analytic: tations in cores inside the CCSF and found pre- we shall make use of synthetic seismograms and dominant north-northeast strikes of fractures seismic wave propagation modeling to invert for parallel to the maximum horizontal compressive a realistic geologic structural model of NW Gey- stress. sers that optimally satisfies the measurements. In SE Geysers we observe two main fast Work in progress includes 1) Use of S-wave polarizations, namely in the northwest synthetic seismogram modeling to confirm and northeast direction. This is consistent with measured polarizations and to determine the an earlier investigation of shear-wave splitting extent of deep crack systems. 2) Determination (Evans et al., 1995) in the region. It is possible of an optimal 3D anisotropic velocity model. 3) that one set of splitting directions (northeast Construction of a 3D tomographic map of the trend) is produced by extensive dilatancy- crack density distribution. induced anisotropy and the other set of splittings by fault motion. Indeed the proximity of the Big Sulphur Creek Fault to stations at the lower ACKNOWLEDGEMENTS southwest side of the SE Geysers area could be the main cause why the particle motion is This research is supported by the Idaho National roughly parallel to the fault. Engineering Laboratory (INEL) of the US De- There are however strong indications of partment of Energy under Contract No. DE- multiple polarization directions at a number of FG07-961D13468. We are grateful to Ernest Majer, Arturo Romero, Ann Kirkpatrick, and 8

John Peterson at Lawrence Berkeley Laboratory available. (LBNL) for making seismic data at the Geysers

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