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

摘要

In 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 directions of the P- and SH-waves. To derive true polarisation directions for each component rotation, we employ a least-squares approach utilising the wave particle motions in different quadrants, which can determine the polarisation direction even under noisy conditions. Synthetic data test and application to VSP data from a coalfield reveal the stability and effectiveness of our method.