248 Geophysics in Geothermal Exploration 8.6 Case study: Fracture characterization through azimuthal inversions to prospect the geothermal potential of Geneva basin To develop the exploitation of geothermal resources in the city of Geneva, a prospection phase has been initiated to better characterize the basin with a newly acquired and processed 3D wide-azimuth land seismic. The Geneva basin location between alpine massifs south side and the Jura north side makes its geology complex and subject to variable constraints which enhanced geothermal energy development. The present project consists of an azimuthal anisotropy intensity analysis at different reservoir levels, to be related to more subtle fracture characterization than using conventional seismic attributes. In this project, both AVAZ and VVAZ approaches as described in this chapter are tested and compared. 8.6.1 Processing, conditioning, shift detection CMP gathers are available at different processing stages, allowing the best choice that suits the needs of the study. The gathers with migration and isotropic NMO (Normal Move Out) have been selected; versions with steps such as trim statics and the Radon filter have been discarded as they might alter both the AVAZ and VVAZ responses. To eliminate the surface waves, an outer time-variable mute is applied to the original gathers before stacking. As the anisotropy is mostly contained in the far offset traces, all the available data, regardless of the offset, has been considered. The stack generation tests showed the possibility to get 6 azimuthal stacks (Table 8.1), enhancing therefore the possibility to detect anisotropy and the accuracy of its orientation. Table 8.1 Ranges of the azimuthal stacks. Name Full-stack AZ1 AZ2 AZ3 AZ4 AZ5 AZ6 Range 0–180° 15–45° 45–75° 75–105° 105–135° 135–165° 165–205° The noise-to-signal ratio is enhanced by considering symmetrical azimuthal ranges; the seismic response varies with the direction source-receptor, regardless of the orientation. Remaining noise content can be managed through the model-based inversions (AVAZ) or the probe size during shift detection (VVAZ). In addition, full-stack full-azimuth seismic data has been generated as a reference. The following maps (Figure 8.5) illustrate, as QCs, the correlation map (left) and the RMS map (right), highlighting the area of the survey, covering Geneva city. Part of the survey is offshore (in the Leman Lake), and part of the seismic data is noisier below the city, as visible in both maps.
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