We present an original method to estimate local shear wave birefringence properties for3-D surface P to S converted waves. To accomplish this we approach the problem inreverse VSP (RVSP), and we show that they are equivalent. The method works in thepre-stack domain and uses the converted P to S waves as hypothetical sources to studythe problem under the propagator matrix method in transmission. The importance ofthis method is that no information is required about layering above the zone of interestto obtain an accurate estimation of the anisotropy parameters. The method involvessolving a nonlinear problem in the frequency domain where a global minimization techniquecalled Simulation Annealing is used. The procedure also allows us to estimate theaxis of anisotropy independently of the offset angle in the range of angles considered inVSPs and in surface profiles. The proposed method is validated with synthetic RVSPdata for two models with different densities of vertical fractures. Results show goodaccuracy 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 theanalysis.
展开▼
