Development of Active Seimic Vector Wavefield Imaging Technology for Geothermal Applications

This manuscript is the final report for the research project conducted under grant no. DE-FG07-97ID 13573, Development of Active Seismic Vector-Wavefield Imaging Technology for Geothermal Applications, funded by the U.S. Department of Energy, Idaho Operations Office. The report is structured as two parts. The first, and major, portion describes the development and testing of new vector-wavefield seismic sources that can generate shear (S) waves that may be valuable in geothermal exploration and reservoir characterization. The second part describes a 3-D seismic data-processing effort to create images of Rye Patch geothermal reservoir from 3-D sign-bit data recorded over that geothermal prospect. Vector-wavefield illumination of subsurface targets with S-waves is essential for interpreting anisotropic rock systems, particularly systems that are dominated by fractures, as many geothermal reservoirs are. Two new seismic sources were developed and tested in this study that can be used to illuminate geothermal reservoirs with S-waves. The first source was an explosive package that generates a strong, azimuth-oriented, horizontal force vector when deployed in a conventional shot hole. This vector-explosive source has never been available to industry before. The second source was a dipole formed by operating two vertical vibrators in either a force or phase imbalance. Field data are shown that document the strong S-wave modes generated by these sources. Three-dimensional (3-D) seismic technology has had a tremendous economic influence on oil and gas exploration. Thus applications of 3-D seismic techniques may also have an economic impact on geothermal exploration and must be evaluated. One such 3-D seismic evaluation was done as the final phase of this study. Tape copies of a 3-D P-wave seismic survey (not a vector-wavefield survey) recorded in sign-bit format over Rye Patch geothermal field in northwest Nevada were received from Subsurface Exploration Company. These data were reprocessed, and the results of the data-processing research were coordinated with Lawrence Berkeley Laboratory. The sign-bit data recorded at Rye Patch had low signal-to-noise character, and the final migrated data volume had limited interpretation value. Recommendations for improving 3-D seismic data quality in future geothermal surveys are provided.