Layer stripping of azimuthal anisotropy from P-wave reflection moveout in orthogonal survey lines

This paper presents a layer-stripping procedure to determine interval measurements of fracture parameters in multi-layered fractured media with vertically varying strike directions. The procedure is based on the P-wave travel time difference between two orthogonal seismic survey lines, and this difference is referred to as the P-wave azimuthal moveout response (AMR). The interval AMR of a fracture target for a fixed offset is a function of cos 2(alpha-phi(i)) with respect to the line azimuth alpha and the fracture-strike azimuth phi(i). Consequently two pairs of orthogonal survey lines can be used to determine the local fracture strike phi(i) if the interval AMR of the target is known. In the case of a weakly fractured overburden underlain by a fractured target, layer stripping can be achieved through the alignment of the top-target event by performing NMO correction separately for all survey lines. The interval AMR of the target layer may then be calculated from the residual moveout of the bottom-target event, if any. In the general case, a ray-tracing procedure, similar to that used in AVO analysis, is required to perform effective layer stripping. Full-wave modelling is used to verify and illustrate these procedures.