Estimation of Interval P Wave Orthorhombic Parameters from VSP Walkaway and Walkaround Data Using Differential Evolution

Fractured subsurface formations with anisotropic backgrounds, such as in shale layers, can be effectively modeled as orthorhombic media in many cases. Extracting interval orthorhombic parameters from seismic data (surface or VSP) is, however, a non-unique and nonlinear problem. The extraction is further complicated by the presence of local extrema in the error functional that needs to be optimized. Such parameters, however, can help characterize the formations better, and hence it is necessary to develop a robust methodology for such parameter extraction. In this work, we developed an inversion scheme for obtaining interval orthorhombic parameters from VSP walkaway and walkaround direct P-wave travel time data. We combined a layer (3D planar/non-planar) based anisotropic ray tracer with a global optimization algorithm called Differential Evolution to estimate P-wave anisotropic parameters along with principal stress orientations. We further applied the developed algorithm to a field dataset and demonstrated that it can help obtain orthorhombic media parameters that could be potentially related to geology, in the presence of noise and suboptimal acquisition design.