126 Geophysics in Geothermal Exploration In the open hole part of borehole B2 (Figure 3.7), the presence of a piece of casing generates resonances in the depth interval 88–91 m on the 1 m offset section. The influence of the piece of casing is local on the 3 m offset section, indicating that the length of the piece of casing is a slightly larger than 3 m. On the 1m offset section, it is possible to identify the refracted P-waves, locally the converted refracted S waves, the Stoneley modes and the fluid modes. With a short-offset configuration, the different wave trains can interfere. However, we can notice that a large offset (3 m) better separates the different wave trains over time due to the difference in their propagation velocities. On the 3 m offset section, criss-cross events are visible. The short-offset configuration must be favored to evaluate the borehole cementation. For measurements of wave parameters such as amplitude, frequency content, propagation velocity, a large offset configuration must be favored. Full waveform acoustic data, recorded with the large offset configuration (3 and 3.25 m), in borehole B1 (steel cased hole) in the 30–78 m depth interval and in borehole B2 (slotted PVC cased hole) in the 78–192 m depth interval are merged to obtain composite acoustic sections. Figure 3.8 (right side) shows the composite acoustic section with an offset of 3 m. The acoustic data were processed to obtain a very high-resolution velocity log (Figure 3.8) which was converted in pseudo porosity log using the Raymer equation adapted to carbonate formation (Raymer et al., 1980). The porosity log was then used Figure 3.8 Seismic imaging and full waveform acoustic logging: from left to right: lithological column, acoustic velocity, seismic section converted in porosity, acoustic porosity, Full wave acoustic composite section (0–80 m: borehole B1, 80–190 m: borehole B2 slotted PVC cased hole) and flowmeter. The acoustic section is normalized and displayed with a color scale ranging from 0 to 1 (after Mari et al., 2021).
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