Surface wave analysis for heterogeneous geological formations in geothermal fields: effect of wave propagation direction

ABSTRACT

We applied advanced surface wave analysis for multichannel and multishot seismic data to estimate S-wave velocity structure with high lateral resolution. Although a horizontally layered structure is typically assumed in surface wave analysis, this assumption might be violated in environments such as geothermal fields because of their heterogeneous geological formation. The lateral variation of phase velocity can be effectively estimated with common midpoint (CMP) cross-correlation (CMPCC) analysis. In this study, we introduced two additional approaches into the CMPCC analysis workflow, to further improve lateral resolution of phase velocity estimates. One approach is window-controlled CMPCC analysis, which applies a spatial window for the CMP gathers while maintaining the accuracy of the phase velocity estimates. In this analysis, we found that it is difficult to improve phase velocity estimates at lower frequencies, as a wider spatial window must be kept to maintain the accuracy of the dispersion curve. Therefore, we introduced another approach in which we consider the direction of surface wave propagation from sources to receivers. Selection of cross-correlations based on source–CMP direction provides a significant improvement in dispersion curve resolution in the presence of lateral velocity variations, for a wide frequency range. We applied direction-controlled CMPCC analysis to seismic data acquired in the Yamagawa geothermal field, Kyushu Island, southwest Japan, and obtained dispersion curves in the heterogeneous geological setting. We then obtained S-wave velocity profiles by applying genetic algorithm inversion to the dispersion curves. The S-wave velocity profiles in the geothermal field resolve shallow and local heterogeneous structures (e.g. volcanic ash, pumice and igneous intrusions) that cannot be identified on reflection seismic profiles.

KEYWORDS:

• Surface waves
• dispersion
• geothermal field
• heterogeneous
• inversion
• exploration methodologies

Acknowledgements

Seismic data used in this study was acquired by Japan, Oil, Gas, and Metals National Corporation (JOGMEC). We are fully grateful for the support provided by JOGMEC and JGI, Inc. This study was supported by the Japan Society for the Promotion of Science (JSPS) through a Grant-in-Aid for Scientific Research on Innovative Areas (no. 15H01143 & 17H05318) and a Grant-in-Aid for Young Scientists B (no. 16K18332). We gratefully acknowledge support of I2CNER, sponsored by the World Premier International Research Center Initiative of the Ministry of Education, Culture, Sports, Science and Technology, Japan. We are grateful for financial assistance provided by the Kyushu University Advanced Graduate Program in Global Strategy for Green Asia.

ORCID

Tatsunori Ikeda http://orcid.org/0000-0002-6151-4043

Takeshi Tsuji http://orcid.org/0000-0003-0951-4596

Additional information

Funding

This study was supported by the Japan Society for the Promotion of Science (JSPS) (no. 15H01143, 16K18332, and 17H05318).