P- and S-mode separation of three-component VSP data

ABSTRACTIn an offset vertical seismic profile (VSP), elastic wavefields are recorded in three orthogonal components. For a given wave mode P, SV or SH, the amplitude is distributed among the three components because observations close to the target layer enable numerous seismic waves with large emergent angles to be recorded. Therefore, in VSP P and S mode separation, it is necessary to extract the same wave mode from every component and restore its true amplitude, rather than simply removing the P-wave projection from the horizontal component or the S-wave projection from the vertical component. Thus, we investigate a two-step method of VSP wave mode separation in which it is assumed that the P and SV (or SH) modes are not polarised orthogonally. First, the Z- and R-components are rotated into the nonorthogonal polarisation directions of the PV- and SV-waves. Here, the PV-wave is the P-mode projection onto the vertical–radial plane. The PV- and T-components are rotated into the nonorthogonal polarisation...