Robust multi-modal surface wave inversion for shallow velocity and shear statics

A robust surface wave inversion has been developed to invert near surface S wave velocity for depth model building and shear statics correction. The algorithm utilizes mainly two most effective inversion engines: the non-linear least square Levenberg-Marquardt (LM) inversion and the differential evolution (DE) method. Both of them can minimize the dispersion curve distance for simpler cases, or directly solve the Eigen-determinant of the surface wave secular functions for more complex cases. The algorithm has been applied to an OBC 3D4C data from off shore Vietnam, where the near surface geology changes greatly from rapid depth varying reef structures to floodplain alluvial fans. The surface wave, it is the Scholte wave in this case, has demonstrates variety of dispersion patterns, which are used in the inversion and revealed the complex subsurface velocity model underneath the sea floor. The receiver side shear statics has been calculated based on the inverted Vs model and applied to the PS wave data, which not only saves man powers tremendously compared with the conventional shear statics method, but also provides a more accurate solution. The imaging results show much better event continuity and geologically interpretable horizons.