184 Geophysics in Geothermal Exploration Figure 6.1 Ambient seismic signal recording of 250 second duration. The seismic sensor used is a 5 Hz geophone. Coherent seismic events designate impulsive signal such as produced by earthquakes, microseismicity, or other impulsive, high energy seismic sources. The study of how those events propagate within the subsurface (body wave picking, location, focal mechanism, etc.) to infer either seismic properties or characterize their source mechanism is a family of approaches to which we will refer to in the following as Seismological analysis. Incoherent seismic signals designate the large majority of the ambient seismic signal which cannot be directly identified as a single seismic event and isolated to be analyzed as such, but rather the sum of numerous contributions of uncontrolled sources such as ocean waves, atmospheric disturbances, and human activities. Yet even this part of the signal holds valuable information. For instance, Horizontal to Vertical Spectral Ratio (HVSR) analysis is a well-known, robust method to infer the thickness of the sedimentary layer overlying the bedrock, which has seen much use in geotechnical applications. For geothermal characterization and monitoring though, the most relevant family of methods able to take advantage of incoherent seismic signal recordings is the so-called Ambient Noise Seismic Interferometry (ANSI) analysis. This relatively new approach (Shapiro and Campillo, 2004) has been a major development in the field of passive seismic methods, for seismologists have extended their capacity to retrieve Green’s functions, which describe the response of an elastic medium between two points as waves propagate through it. Traditionally, Green’s functions were obtained through earthquake-generated waves, providing data only after significant seismic events. However, ANSI achieves similar insights through the cross-correlation of ambient noise recorded at different seismic stations. By continuously recording these incoherent background vibrations and examining the data between paired stations, scientists can derive Green’s functions, revealing the Earth’s structure without relying on earthquakes. Putting it another way, ANSI tools manage to extract coherent seismic waves components from the incoherent seismic signal, hence making seismic analysis possible. In the rest of the paper, we will refer to this family of methods such as Ambient Noise Seismic Interferometry (ANSI) analysis. Hence, the shift from traditional earthquake seismology to ambient seismic noise seismology relies on the precision of digital technology and the ability to record
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