A multi-component seismic azimuthal imaging trial: preliminary results

Abstract

In conventional (P) seismic-reflection surveys, a P-wave generated at the surface is reflected to provide an image of the subsurface. Upon reflection some of the energy is converted into a shear (S) wave. This reflection is known as a converted wave or PS-wave, and may improve the geological interpretation. Velseis has acquired a number of 2D PS-wave surveys at the shallow coal-scale (0-400m). Typically, the two images created from positive, and negative, offset rays can lead to differing geological interpretation. The effect has also been documented at the petroleum scale, and may be caused by diodic-illumination and azimuthal anisotropy. This effect is expected to be exacerbated in the 3D case.

To investigate these complexities we have recently acquired a small trial 3D PS-wave survey with the immediate aim of examining such azimuthal effects in detail. The ultimate aim is a commercially viable 3D-3C methodology for coal-scale targets. The trial dataset consists of an approximately 1000m x 300m swath, collected over a target that has previously been investigated with a conventional 2D survey. The geological environment consists of a target coal-seam with a depth ranging from 70-120m, and including significant faulting. The acquired 3D-3C survey has high shot density, compared to conventional single-component data. This will allow us to produce four full-fold datasets constructed using different azimuth rays. This will be used to examine the effects of anisotropy and diodic-illumination. We are simultaneously examining the P-wave sections to determine if these are also affected by anisotropic problems. The preliminary results from this investigation will be presented.