209 6. The use of passive seismic methods for Geothermal exploration and monitoring Figure 6.15 Modified after Antayhua-Vera et al. (2022). Water injection rates comparison with b-values for a period of about 4 years at the Tres Virgenes geothermal field (vertical black arrows indicate sudden water injections). Another interesting report describing microseismic monitoring applications on geothermal sites can be found in Cruz-Noé et al. (2018). The authors report key observations following the 30 years duration monitoring of microseismicity of three Mexican geothermal fields, namely Los Azufres, Los Humeros and La Tres Virgenes. Multiple analysis are presented which relate to various interpretation and links to the geothermal field production activity itself, including fault reactivation due to modification of the local stress field, but also induced seismicity related to geothermal water injection and geothermal fluid extraction. They also report on the effects of drilling, hydraulic stimulations, and well testing on the micro seismic activity itself. Beyond the analysis of microseismicity activity characteristics such as statistical distributions or source mechanism, information about geothermal assets evolution in time can also be obtained through the analysis of seismic properties, typically seismic velocity and seismic attenuation. A sound illustration of such analysis can be found in the work by Guo and Thurber (2022). The authors applied a time-lapse tomographic approach based on a double-difference workflow. They analyze a catalog of earthquakes and apply two different tomographic analysis to evaluate both the changes in P-wave velocity (DDV tomography) and P-wave attenuation (DDQ tomography), during a relatively long monitoring period (>5 years). Their work allows them to produce 4D models of velocity and attenuation parameters and to relate the observed variations to injection processes within the geothermal reservoir. An interesting diagram is available in the “Supporting information” document associated with their work (Figure 6.16), which depicts how P-wave attenuation (Qp) and velocity (Vp) are expected to vary depending on the on-going process induced by fluid injection within the geothermal system.
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