86 Seismic Imaging • band-pass filtered; hydrophone data (40-60-300-450) Hz; geophone data (6-12-150-200) Hz; • no deconvolution; • 3D to 2D amplitude correction, i.e., √t multiplicative amplitude correction to compensate for the transverse (out of plane) spreading. 3.3.2.3 Initial model The starting model for the tomographic inversion was defined by integrating crosshole data with log information from the three holes, and VSP information on P and S-waves in borehole 161. Unfortunately, a shear-wave sonic log was not available, because S-waves, being slower than Stoneley waves, were masked. The density information was obtained from a compensated formation density (FDC) log in borehole 164. An elastic 1D velocity-depth model was used as background (Figure 3.20) with the P/S-wave velocity ratio constant (equal to 1.9) at the reservoir area. Figure 3.20 Initial elastic 1D model. Adapted from Beydoun et al. (1989). 3.3.2.4 Depth elastic images: comparison with density log In this field example, given that P-P scattered waves are not visible in the data and only up-going S-S and S-P scattered waves have enough energy, then P-wave imaging is not possible and only two images, S-wave velocity and density images, could be generated.
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