105 2. Surface geophysical methods Passive seismic can be implemented for MASW. Passive seismic tomography or interferometry is a technique used to explore (image and monitor) the subsurface using ambient noise generated by natural or anthropogenic sources (Shapiro et al., 2005; Gouedard et al., 2008; Mordred et al., 2013). Interferometry is based on calculating cross-correlations of the noise signal between pairs of seismic sensors. The noise is dominated by surface waves propagating in the shallow subsurface (Roux et al., 2011; Shapiro and Campillo, 2004). The calculation of cross-correlation between pair of sensors allows the extraction of the surface wave contained in the noise propagating between the sensors. As for MASW, dispersion curves of surface waves are computed and inverted to obtain distribution of S-wave velocity in the subsurface. In practice, several tens of sensors (vertical geophones) are deployed on the ground surface, the listening time can be of several hours or days, the analysis of the dispersion of surface waves is done in the low frequency domain (5–20 Hz). Figure 2.40 shows an example of 3D shear velocity model obtained by passive seismic tomography, implemented for 3D imaging of the subsurface in a tunnel area (Saade et al., 2024). For the study, 199 surface sensors are used, covering the study area with a variable inter-sensor distance averaging about 20 m, and approximately 336 hours of measurements were recorded. Passive seismic interferometry can be used for the monitoring of subsurface fluids – from shallow groundwater to native or storage gas reservoirs (Kremer et al., 2024) Seismic interferometry has been used to investigate velocity variations, and subsequently strain sensitivities, related to a seismic swarm activity that occurred in 2013 along the Alto Tiberina low angle normal fault (Mikhael et al., 2024). Through an optimization procedure based on synthetic modeling to separate the non‐tectonic from the tectonic induced velocity variations, a significant velocity variation in response to small strain perturbations has been unraveled. The deduced strain sensitivity value is comparable to values observed in volcanic settings suggesting the presence of pressurized fluids at depth (Mikhael, 2024). The same approach could be applied in similar contexts where fluids are involved including the monitoring of geothermal systems. In the Eastern Vienna array, a seismic ambient noise survey was conducted for geothermal exploration (Esteve et al., 2024). A reservoir-scale 3-D shear velocity model of the central Vienna basin was obtained by passive seismic interferometry using recordings of ambient seismic noise. 100 seismic nodes were deployed for a duration of 6 weeks during the summer 2023. It has been shown that the location of the Markgrafneuseidl fault is highlighted by a strong velocity contrast in the 2D Love wave group-velocity maps at periods shorter than 3s. The 3D shear-wave velocity model shows a basin shape structure, which is interpreted to be the seismic signature of the Schwechat depression, the main target for geothermal exploration in Vienna (Esteve et al., 2024). Ambient Noise Tomography can support the growth of geothermal sector by providing reliable and affordable exploration methods. This can improve understanding of the subsurface and help reduce drilling uncertainty (Esteve et al., 2024).
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