137 3. Borehole geophysical methods Considering the trajectory of the well and the fact that the borehole sensor can rotate from one depth to another, the 3 components (X, Y, Z) of the sensor must be oriented using either hardware orientation device or algorithms based on the analysis of wave polarizations used to define rotation angles for orientation (Naville, 2024). Figures 3.17 and 3.18 are an example of a 3C VSP orientation in a deviated well (Kazemi, 2009). After rotations, the oriented components are defined as follows: Z-component (ZV) is vertical pointing downward, X-component (HN) is horizontal pointing to North and Y-component (HE) is horizontal pointing the East true geographic direction. After orientation, the 3C VSP processing sequence (Hardage, 1985; Mari and Coppens, 2003; Serbutoviez et al., 2003) includes wave separation with apparent velocity filter and polarization filters (Mars et al., 1999) to extract P and S-waves and separate downgoing and upgoing waves, deconvolution of the upgoing wave fields (P and S waves) by a single operator extracted from the downgoing wave fields, normal moveout correction of deconvolved upgoing waves and stack in CMP gather, or prestack migration in time or depth. The most used method is the VSP -CDP stack method proposed by Wyatt and Wyatt (1982). The VSP migrated seismic section is directly comparable to a surface reflection seismic section. The VSP migrated section has a lateral range of investigation of a few tens to a few hundreds of meters. Figure 3.17 From left to right Modulus, X, Y and Z-components before orientation. First arrival S-waves are clear on horizontal components while on the Z-component P-wave first arrivals are sharp to pick. The first arrival S-waves are not consistent before orientation while modulus (X2 + Y2) clearly shows the S-wave first arrivals. X, Y and Z components are displayed with the same constant gain while modulus has been normalized (after Kazemi, 2009).
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