124 Geophysics in Geothermal Exploration Figure 3.5a), while the source, as a weight dropper (Figure 3.5a), was moved and fired in the middle of all pairs of adjacent geophones. Hybrid seismic imaging combining refraction (tomography, Figure 3.5a) and reflection seismic results produced an extended depth reflectivity section starting from the surface up to a depth of 240 m (Figure 3.5b). Time to depth conversion was calculated using the time-depth law given by the VSP recorded in borehole B1 (Mari et al., 2021). The site was also investigated by a near surface 3D seismic survey (Mari and Mendes, 2019: see Figures 2.27 to 2.29, chapter 2). Borehole B1 was drilled to a depth of 80 m and equipped with a cemented steel casing. Borehole B2 was drilled in two drilling phases. In the first phase, B2 was drilled to a depth of 120 m and equipped with cemented steel casing to a depth of 78 m. B2 remains in open hole between 78 and 120 m. In the second phase, B2 was drilled to a depth of 192 m and equipped with a slotted PVC casing in the 78–120 m depth interval. Resonances observed on constant offset acoustic sections reveals that B1 is a poorly cemented case hole (Figure 3.6) and B2 is an uncemented cased hole up to 78 m, B2 being equipped with an uncemented slotted PVC casing from 78 m to 192 m depth. The acoustic tool used for field experiments is a monopole-type flexible tool with a small diameter of 50 mm. It holds a magnetostrictive transmitter (transmission frequencies: 17–22 kHz) and can be equipped with two pairs of piezoelectric receivers offering an acquisition in near offset configuration (receivers at 1 and 1.25 m beneath the source), and in far offset configuration (receivers 3 and 3.25 m beneath the source). Figures 3.6 and 3.7 shows a comparison between acoustic data recorded using the tool in the near (or short) offset configuration (receivers at 1 and 1.25 m from the source) and in the far (or large) offset configuration (receivers at 3 and 3.25 m). Figure 3.5 Seismic imaging: (a) 2D seismic spread – 2D refraction tomography, borehole locations (B1 and B2), view of the seismic source, VSP recorded in B1, (b) 2D hybrid section over depth (after Mari et al., 2021). (a) (b)
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