208 Geophysics in Geothermal Exploration 6.3 Geothermal monitoring Another crucial aspect of Geothermal energy large-scale deployment is the surveillance of the subsurface environment during operations to ensure performance and conformance. 4D active seismic survey cannot fulfill this objective, again due to their prohibitive deployment cost, in addition to complex logistics for repeated onshore deployment. Yet again, passive seismic methods can be deployed to enhance our capacities for monitoring the evolution of Geothermal assets and their seismic properties, continuously and cost-effectively. Just like for exploration purposes, in this paper we distinguish two “families” of passive seismic methods that can be used for enhanced surveillance of geothermal fields. First, all the methods based on the analysis of impulsive seismic events, independently of their magnitude (from large earthquakes to micro-tremors), which we refer to as Seismological analysis. Second, the methods and analysis tools based on Ambient Noise Seismic Interferometry (ANSI), where incoherent parts of the ambient seismic signal are analyzed and processed to extract coherent seismic wavefields, which can then be analyzed to infer the seismic properties (velocity, attenuation) of the subsurface. 6.3.1 Seismological analysis monitoring Seismological analysis methods have been used to fulfill a variable number of objectives in the geothermal monitoring context. Possibly one of the most known functions of passive seismic monitoring is the detection and analysis of microseismic events to track for potential operation-induced seismicity and ensure that such seismicity remains in the range of expected epicenter locations and events magnitude. This constitutes a typical objective of passive seismic monitoring of Enhanced Geothermal Systems (EGS). This denomination designates geothermal contexts where fluid circulation is artificially stimulated through enhancement of the rock formation permeability by means of hydraulic stimulation technics. Locating and analyzing the microseismic events that are induced by the stimulation operations allows for operators to assess the effectiveness of the process in time, by following the extent of the resulting fracture network, analyze the effect and relationship of the operations with the local stress field, and ultimately avoid the triggering of undesired, large magnitude seismic events. Statistical seismological analysis tools, such as b-value computation, can also be used as a monitoring tool, to fulfill various objectives. The possibility of monitoring water injection processes through b-value analysis within the reservoir has been illustrated by multiple authors, many of which can be found in the review from Pérez and Cuellar (2018). As an illustration for this chapter, we refer to the study of Antayhua-Vera et al. (2022) where the authors studied the spatio-temporal distribution of b-values at the geothermal field of Tres Virgenes in Mexico (Figure 6.15). Their observations report that increases of b-value are generally coincident sudden increases in water injection dynamics.
RkJQdWJsaXNoZXIy NjA3NzQ=