211 6. The use of passive seismic methods for Geothermal exploration and monitoring For instance, Muñoz-Burbano et al. (2024) have been monitoring seismic velocities using ANSI approaches at the Domo de San Pedro Geothermal field, in Mexico, using a seismic array of 20 broadband stations. The authors have been able to reconstruct velocity relative variations maps sequentially in time and to relate significant velocity changes to fluid injection dynamics and to the associated stress distribution. In addition to this study, Taira et al. (2018) demonstrate that detailed analysis of velocity variations may describe the temporal evolution (5 years) of the Salton Sea geothermal field stress state. Figure 6.17 presents time-lapse measurements of seismic velocity variations in several frequency ranges that were computed as the average of the 9 components of the Green’s tensor. The events marked as DBC, EMC and BS represent sudden drops in the seismic velocity and are related to some local earthquakes. Taira et al. (2018) show that the amplitudes of those drops are too great to be linked solely to the earthquakes for the BS event and suggest that the sudden evolution in the velocity variation time series is linked to an aseismic deformation related to fracture opening. In addition, a long-term upward trend (0.25% in the range 0.5–2 Hz) can be observed in all frequency bands. The authors propose that this long-term trends relate to a progressive poro-elastic contraction linked to geothermal production and the associated evolution of the stress field within the reservoir. Figure 6.17 Modified after Taira et al. (2018). Relative velocity variations time-series computed for several frequency ranges. Dashed black lines indicates sudden drops of seismic velocity, partly related to earthquake events and to deformation processes.
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