Shear Wave Birefringence In Reverse VSP: An Approach To 3-D Surface P To S Converted Waves

Abstract

We present an original method to estimate local shear wave birefringence properties for 3-D surface P to S converted waves. To accomplish this we approach the problem in reverse VSP (RVSP), and we show that they are equivalent. The method works in the pre-stack domain and uses the converted P to S waves as hypothetical sources to study the problem under the propagator matrix method in transmission. The importance of this method is that no information is required about layering above the zone of interest to obtain an accurate estimation of the anisotropy parameters. The method involves solving a nonlinear problem in the frequency domain where a global minimization technique called Simulation Annealing is used. The procedure also allows us to estimate the axis of anisotropy independently of the offset angle in the range of angles considered in VSPs and in surface profiles. The proposed method is validated with synthetic RVSP data for two models with different densities of vertical fractures. Results show good accuracy in the estimation of the angle of the fractures for the whole range of offsets. Also, results show a dependence on the frequency and offset ranges considered in the analysis.