199 6. The use of passive seismic methods for Geothermal exploration and monitoring the geothermal sites of interest. Examples of typical seismic signals produced in volcanic environments can be found in the work of Wassermann (2012), Zobin (2011) and Inza-Callupe (2014). Subsurface seismic properties analysis – The interest of joint velocity/attenuation analysis Beyond the study of the intrinsic properties of seismic events such as location, focal mechanism, statistical characteristics or waveform, the recording of the events over an array of seismic sensors also opens the way for characterizing directly an estimate of the subsurface seismic properties’ distribution in the area. Using the earthquake as an uncontrolled, yet coherent source of energy, and tracking how the seismic waves propagate within the explored area can help derive 3D models of seismic properties and infer geothermal assets characteristics. The following paragraphs propose some examples of such analysis, distinguishing between attempts to retrieve a seismic velocity model of the area from studies focused upon seismic attenuation analysis. In many seismic analysis, seismic velocity is the “favored” studied parameter and velocity-focused work usually treat seismic attenuation as an “undesirable” effect (Vardy and Pinson, 2018), because strong attenuation contexts tend to decrease the accuracy of velocity estimation. However, attenuation properties are also very relevant to be investigated if one seeks to infer complementary types of information. Indeed, the intrinsic seismic attenuation properties characterizing the anelastic behavior of subsurface rocks are sensitive to different parameters of the rock nature, type and composition than seismic velocity, which makes the joint study of those properties relevant in an exploration context. Velocity models If a sufficient number of seismic events are recorded through a seismic sensor array, it is possible to pick body wave time arrivals and process the travel times to infer a 3D seismic velocity model of the subsurface. Such an approach, sometimes called “Earthquake tomography”, requires specific processing methodologies such as joint inversion of the seismic events hypocenters and of the subsurface velocity model, for in that case the location of the seismic source (the earthquake) is also an unknown parameter. Muksin et al. (2013) performed such an earthquake tomography to characterize the seismic properties of the Tarutung geothermal area, in Indonesia. Their method included simultaneous inversion of micro-earthquake locations and 1D velocity models, followed by a 3D tomographic inversion. The resulting 3D Vp model helped them to accurately delineate the structure of the Tarutung and Sarulla basin, identifying complex zones, and orientation of fault areas, as illustrated in Figure 6.9a). They also derived a Vp/Vs ratio 3D model in which high Vp/Vs values
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