Feasibility of characterizing an aligned fracture set from azimuthal amplitude variations of PP and converted waves

Abstract

The amplitude variation with azimuth (AVAz) is commonly used to predict subsurface fracture properties. Published analyses demonstrate the ambiguous characterization from PP-wave AVAz for a vertical set of rotationally variant fractures. Thus, it is worth analyzing the extra information hidden in the converted wave AVAz. We have considered the linear-slip theory to model a vertical or tilted set of rotationally variant fractures in either of two welded half-spaces with polar anisotropy. We investigate the AVAz of PP and converted waves to determine their ability to create unambiguous fracture characterizations. Reflectivity approximations are derived for PP and converted waves to extract their AVAz attributes exclusively depending on fracture properties. We consider pseudo-PSV and pseudo-PSH waves with linear reflectivity approximations transformed from the nonlinear ones for typical converted waves in fractured media. The pseudoconverted waves degenerate into the typical PSV and PSH waves when the incident medium is azimuthally isotropic. For a vertical fracture set, the AVAz of PSV and PSH waves conveys no more deterministic fracture information than that of PP waves. It is demonstrated that the unambiguous characterization is feasible for a fluid-filled vertical fracture set, whereas it might be not for a gas-filled one. For a tilted fracture set, the fracture normal azimuth can be obtained uniquely from the AVAz of PSV or PSH waves but ambiguously from the PP-wave counterpart. Using multiwave AVAz attributes only gives rather vague estimates of the fracture tilt angle. For vertical and titled fracture sets, numerical model tests demonstrate overall better inversions from jointly using PP and PSV waves than solely using either of them.