74 Seismic Imaging 3.2.1 Seismic survey In general, a typical cross-hole seismic profile has sources situated in one borehole and receivers in another, with the source and receiver boreholes being separated by a distance of up to 1 km. For this study, the seismic source was wall-clamped in a vertical borehole and the receivers in a deviated borehole. The distance between the two boreholes increased from 30 m at the surface to 380 m at the reservoir level, located at a depth of 1,850 m. Both boreholes were cased with a 7-inch casing. The total recording time was 40 hours and the whole operation, which involved more than 3,000 shots and the removal and resetting of the tubing, took one week. The principal parameters of this data acquisition were: • Source: S-wave weight-drop, releasing ≈ 2,000 joules/shot, 4 m spacing between 1,314 - 1,916 m in vertical depth; • 8 shots/position; • 400 shot positions; • Receiver: Multilock™ tool with 4 levels and triaxial geophones, 4 m spacing between 1,620 - 1,916 m in logging depth. For promoting the S-wave conversions two conditions were combined: • source directivity pattern diagram with a strong S lobe perpendicular to the borehole; • acquisition geometry designed to explore the wide angles of incidence. Figure 3.9 shows the multicomponent raw data with complex arrivals. Figure 3.9 Raw data. PZ component along the borehole axis. H1 and H2 perpendicular to the borehole axis. Adapted from Becquey et al. (1992).
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