Converted-wave processing of a 3D-3C refection seismic survey of Soda Lake geothermal field Tyler Kent* and John Louie, University of Nevada, Reno, Jim Echols, Magma Energy (U.S.) Corporation Summary: This 3D-3C seismic survey greatly improves the structural model of the Soda Lake, Nevada geothermal system. Determining a “geothermal indicator” in the seismic signal, and processing of the 3D converted-wave data are unsuccessful to date. Due to a high near-surface Vp/Vs ratio the shear-wave energy is under-sampled with 220 ft receiver spacing and 550 ft (168 m) line spacing. The 2D converted-wave data that we can image shows encouraging similarity to the deep structural features in the P-wave sections, but have little resolution of shallow structures. Higher-density receivers and a better shallow shear-wave model are needed in conjunction with this deep reflection study to effectively image the 3D converted waves. Introduction: The potential for using converted-wave interpretations in geothermal systems can be recognized by the importance of fluid-filled fractures and permeability. The goal of using the fluid sensitivity of shear waves to locate geothermal systems has had multiple setbacks due to the difficulty in processing seismic data in such a laterally heterogeneous area. Soda Lake geothermal field in western Nevada, developed since 2008 by Magma Energy, was chosen for an American Recovery and Reinvestment Act award for the U.S. Dept. of Energy Validation of Innovative Exploration Technologies in the Geothermal Technologies Program. While most geothermal lithologies in primarily volcanic regions are lacking in clean reflections, this site had a four-line seismic survey done by Chevron in the 1970s that imaged coherent reflections. These thick sedimentary sequences, relatively anomalous in Nevada geothermal systems, made this 3D-3C survey possible at Soda Lake. Method: Geologic Setting The Soda Lake geothermal field is located six miles northwest of the town of Fallon in Churchill County, Nevada. It is in the south-central part of the Carson Sink, which is bordered by the ~10,000 year-old Big Soda Lake volcanic explosion crater, and the mafic Quaternary Upsal Hogback volcanic complex. There are multiple operating geothermal fields within 30 mi (50 km) of Soda Lake. Classic of Basin and Range topography, the Carson Sink trends NNE and is covered by Quaternary alluvium, sand

dunes, silt and a large playa surface. The Sink is the surface expression of a deep-seated structural graben. The Walker Lane tectonic belt (Stewart, 1988) borders this area toward the southwest. It has been proposed that the transfer of NW-trending dextral shear in the Walker Lane to WNW extension in the northern Great Basin would allow for the formation of enhanced extension and pull-apart basins that bring about structural controls for geothermal systems (Faulds   and   Henry,   2008). Using a combination of geophysical techniques is the best way to model the local structural controls on this field. Production History There are two current power plants, Soda Lake 1 (5.1 MW gross) and Soda Lake 2 (18 MW gross), although they have never reached maximum output. Twenty-three large diameter wells and six redrills have been completed, with five used for production and five for injection. This low success rate is due to a limited model of the resource, with drilling sites located near the central part of a shallow

thermal anomaly. Vertical Seismic Profile In an effort to calibrate the near-surface seismic data a check-shot survey was performed prior to the reflection

Figure 1: 3D-3C reflection seismic survey over an area of 13 sq mi (34 sq km) with 8,374 source points and 3001 receivers, and 52 paired source lines with a pair separation of 110 ft (33.5 m). Source interval is 110 ft (33.5 m) and the paired source lines are separated by 770 ft (235 m). Receivers are spaced at 220 ft (67 m) on 36 lines separated by 550 ft (168 m). This design provides 55 ft (17 m) bins and high fold in the 2000 ft by 4000 ft (600 by 1200 m) area of interest.

3D-3C reflection seismic survey of Soda Lake

survey. The check-shot survey goes to a depth of 7000 ft (2100 m) and provides the initial direct measurement of time-depth from surface sources. This analysis yields a velocity curve that can be compared to a sonic log. A walkaway study with one leg at 0 ft, 1000 ft, 2000ft, and 3000 ft (300, 600, 900 m) was also performed with a downhole 3C array of 20 geophones at 50 ft (15 m) spacing. In this same well both gamma-ray and sonic logs were taken. Permitting In some areas 3D seismic surveys can be classified as “casual use”. In Nevada “casual use” of public land means vehicles weighing less than 10,000 lbs (4500 kg). Using the vibroseis trucks meant that about 250 miles (400 km) of transects needed to have all human artifacts, archeological and historical sites identified and recorded. The intensive Class III Inventory is 434 pages. To expedite permitting a study that could pass the casual use stipulations would be highly effective. This would require the combination of low-impact seismic sources less than 10,000 lbs (4535 kg) with a cable-free receiver system. Seismic Data The 3D-3C reflection seismic survey was conducted over the geothermal field shown in figure 1 by Dawson Geophysical. The source was three 62,000 lb (28,000 kg) vibroseis trucks doing two sixteen second, 8-72 Hz sweeps per VP. This geometry was originally planned for just single component geophones, but during approval it was upgraded to include three-component recording. However, due to the long permitting process already underway the geometry of the survey could not be changed. Processing: Geokinetics performed the seismic data processing. This survey’s P-wave data were processed first by a field static correction, and then a model-based noise attenuation (MBNA) to eliminate low-velocity surface-wave noise. Two passes of stacking velocity analysis with a 0.5-mile (0.8 km) interval and surface-consistent residual statics were done. A curved-ray 3D Kirchhoff prestack time migration provided velocity analysis. Another curved-ray 3D Kirchhoff prestack time migration with sufficient half-aperture, 75-degree migration dip and including P-wave VTI-anisotropy (if significant) and PSTM residual velocity analysis yielded the final image volume. Special processing of the P-to-S, converted-wave data included azimuthal anisotropy analysis via stacking of common receiver gathers over narrow ranges of azimuth. If there was noticeable azimuthal anisoptropy, components

were rotated to fast and slow shear-wave direction (S1 and S2) and processed until anisopropic effects were removed, then rotated back to the radial direction where the transverse component can be dropped (pre-migration or post migration). In any case, S1 and S2 volumes were processed through C-wave prestack time migration with their vector fidelity preserved. If this processing flow had been successful, the next products would be a fracture orientation map and a relative fracture density map. In cases where no azimuthal anisotropy was present, processing was carried on with the radial component. The point at which processing was unsuccessful was when trying to get asymptotic conversion point (ACP) binning using the available log or estimated Vp/Vs ratio. The ACP binning did not yield coherent reflectors due to unresolved common-conversion-point binning in an area of high Vp/Vs. Discussion of Results/Conclusions There was potential to use the recorded VSP to orient a redrill toward fluid-filled fractures proximal to the well. For this VSP study, however, refraction of energy along shallow horizons caused the 200-2400 ft (60-730 m) depths to be discarded from further efforts. Cased and cemented geothermal wells can have better elastic coupling, which is vital for good signal-to-noise ratios. The cementing unfortunately allowed excessive noise in the check-shot survey. This type of downhole study also must be done in cool wells outside of the thermal anomaly, or immediately following periods of active cooling associated with drilling or injection testing. Most downhole 3C geophones can only handle temperatures to about 265° F (130° C). After processing the P-wave sections the next product was a refraction statics solution of the shallow refractor elevation, which is interpreted as the water table. Figure 2 shows the part of this survey with the greatest structural implications. The seismic volume yielded a detailed map of the fault deformation of a mudstone horizon at a depth of about 800 ft (240 m), and also defined an inverse conical basaltic unit at about 1800 ft (550 m) depth. The structural style of this field is one of nested pull-apart basins with the thickest sections of basalt in the center of the production area. The faults also line up with higher thermal anomalies in wireline data. To date the converted-wave processing has not yielded any results. Thomsen (1999) provides us the basic geometry of the conversion of a down-going P-wave to an up-going shear-wave (figure 3). The location of the conversion point is dependent on the ratio of the P-wave and S-wave velocities (Vp/Vs). The VSP well dipole sonic log yielded a typical ratio between 1.5 and 2.5 below 4000 ft (1200 m) depth, but the near-surface ratio was 5 ± 0.2. With high

3D-3C reflection seismic survey of Soda Lake

near-surface Vp/Vs ratio, the up-going shear-waves are sub-vertical, placing the conversion point almost directly under the receiver. Since the near-offset traces will not contain recognizable converted waves (Aki and Richards, 1980), the C-waves will only appear at the farthest ranges of offset and thus may be beyond the recording patch. The limited size of the recording patch is a result of the need to not spatially alias the P-P reflections. With the cultural resource survey already in progress during the upgrade to 3C, it was not possible to alter the proposal to allow for a larger recoding patch.

Figure 2: Mudstone horizon structure map. Major faults are labeled 1-4. The size of the down-thrown symbol is not related to throw. Not all faults penetrating this horizon are shown. Nested pull-apart basins are inferred from fault offsets and elevation of mudstone surface. Irregular white area inside the survey area indicates no data. Surface elevation is about 4000 ft (1200 m). While a more widely spread shear-wave recording effort could have improved the converted-wave result, it is also evident that creating an accurate shallow shear wave velocity model is paramount in processing data from reservoir depths. Continued tests are needed to determine optimal bin dimensions that collect enough traces to produce an image while not degrading the image from structural dip. In figure 4 there is a 50 ms time delay between S1 and S2. The next challenge will be to image shallow reflectors through a combination of appropriate CMP bin dimensions and muting. Acknowledgments The authors would like to thank Magma Energy (U.S.) Corporation for their cooperation in providing the data and support, Dawson Geophysical Company for conducting the

survey, and Geokinetics for data processing. Magma Energy received support for this project from the American Recovery and Reinvestment Act (ARRA) through the US Dept. of Energy Geothermal Technologies Program. Kent was supported in part by the Great Basin Center for Geothermal Energy through funding from the Department of Energy.

3D-3C reflection seismic survey of Soda Lake

Figure 4: Comparison of P-wave and converted-wave data. Left: Inline 162 is conincident with a line of receivers. Similar structrual features can be identified in both profiles, however shallow data is nearly absent in the P-S C-wave data. In the S1, S2 comparision (R), a time shift of ~50 ms is in the data, attributable to variable basalt depth and azimuthal anisotropy. Better shallow data is needed to “layer strip” the portion of time delay attributable to shallow horizons.

Figure 3: Factors impacting converted-wave survey processing. Left: In this diagram taken from Thomsen (1999), the reflector depth is equal to the source-receiver offset and Vp/Vs = 2 moves the conversion point closer to the receiver creating a more vertical ray path. Right: Receiver line spacing is represented by the five vertical lines above the origin of the graph. With reliable reflections in the upper 2000 ft (600 m) and a high Vp/Vs, the conversion point moves almost under the receiver and thus into the farther offset ranges, that may not be recorded.

References: Aki, K., and E. Richards, 1980, Quantitative Seismology: Freeman Press. Faulds, J. E., and C. D. Henry, 2008, Tectonic influences on the spatial and temporal evolution

of the Walker Lane: An incipient transform fault along the evolving Pacific-North American plate boundary, in Ores and orogenesis: Circum-Pacific tectonics, geologic evolution, and ore deposits: Arizona Geological Society Digest, 22, 437–470.

Hill, D. G., E. B. Laymen, C. M. Swift, and S. H. Yungul, 1979, Soda Lake, Nevada, thermal

anomaly: Geothermal Resource Council. Transactions, 3. Stewart, J. H., 1988, Tectonics of the Walker Lane Belt, western Great Basin: Mesozoic and

Cenozoic deformation in a zone of shear, in The Geotectonic Development of California: Prentice-Hall, 683–713.

Thomsen, L., 1999, Converted-wave reflection seismology over inhomogeneous, anisotropic

media: Geophysics, 64, 678-690.