Compressional/shear wave separation in vertical seismic profiling

Abstract

Disclosed is separation of the compressional and shear components of the wavefield measured in 3-component vertical seismic profiling. The separation involves working in the zx plane defined by the borehole axis (z) and the surface seismic source, and in successive and overlapping windows each defined by (i) a depth interval centered at the depth level currently being processed and including several depth levels above and below and (ii) a time interval sufficiently long to include the receipt of the relevant seismic energy by the geophones. The total wavefield measurements in a window are rotated to give the energy along the borehole axis (z) and that perpendicular to it in the zx plane of interest, the result is forward Fourier transformed in two dimensions, then filtered in wavenumber-frequency space by means of a matrix filter which depends on previously estimated compressional and shear speeds of the region that produced the VSP measurements being filtered, and the result is inverse Fourier transformed to derive the compressional component. Subtracting the compressional component from the total measured wavefield gives the shear component. The corresponding operations performed entirely in the spatial domain give corresponding results.