138 Geophysics in Geothermal Exploration Figure 3.18 From left to right Modulus, HN, HE and Z-components after orientation. On the Z-component P-wave first arrivals are sharp to pick. First arrival S-waves are coherent after orientation. The filtered modulus is identical before and after orientation. HN, HE and Z components are displayed in constant gain while modulus has been normalized. The modulus first arrivals are identical before and after orientation (after Kazemi, 2009). Naville et al. (2024a,b) shows an example of a 3C VSP obtained in the deviated section of the high-angle geothermal borehole of Grigny GGR5, targeting intra-Dogger thin, porous beds. Figure 3.19 shows the survey geometry and gives a summary of field parameters. One can notice the wide frequency bandwidth used for the vibrator sweep (5 to 175 Hz). Figure 3.20 shows the PP and PS VSP migrated sections. The reflectors surrounding the top Bathonian are slightly dipping to NE, and affected by several step faults, attenuated by lateral enhancement and migration. On the right side, the PS image converted to P-wave twt scale is restituted with higher definition due to the shorter shear wavelength. Main faults F1 & F2 are drawn on the bottom half of Figure 3.20, underlining lateral interruptions of reflectors. Many additional small faults are present on both PP and PS images (Naville et al., 2024a,b). To assist in a depth prediction of potential low velocity/high porosity target beds beneath the well, an inversion of the VSP PP-up image to acoustic impedance and acoustic velocity was performed. The inverted VSP sections highlight a depth interval of lower relative velocity and impedance at 1600–1612 m, which was revealed porous and productive (Figure 3.21).
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