Cross-well seismic attenuation tomography with correction for coupling effects at source and receiver positions in the Yutsubo geothermal field, Japan

Abstract

Seismic attenuation is an important property in the geothermal field because it is related to fluid content and reservoir fractures. However, seismic attenuation analysis is easily affected by various factors that must be carefully treated. We investigated cross-well tomography data acquired in the Yutsubo geothermal field and revealed factors affecting the frequency response of data for attenuation tomography. The amplitude of the first arrival waveforms suggests the presence of effects from source coupling, and properties of the surrounding medium affect the radiation pattern and absolute amplitude values. Also, the amplitude and centroid frequencies of the first arrival waveforms show highly variable values along traces, and analysis using an autocorrelation function suggests that there are also effects due to receiver coupling. In this study, we developed a method to correct a coupling effect by assuming that wavefields inside the first Fresnel zone have the same amplitude and frequency character. Our approach uses neighboring traces inside the first Fresnel zone to estimate correction functions. By applying correction functions, the final attenuation tomography results showed a decrease in standard deviation, which indicates that applying a coupling correction increased the inversion stability. In addition, we investigated the interference of guided waves inside a low-velocity layer using finite-difference modeling. A comparison of synthetic and real data suggests the possibility of the phenomena in our data, which is not widely recognized in the geothermal field. We adopted an approach that excluded data having the potential of interference from those waves. Our attenuation tomography results extracted a high-attenuation zone where open fractures are thought to be distributed.