Anisotropic effects on the near surface seismic imaging

Abstract

ABSTRACTAnisotropic effects have been widely considered in seismic subsurface imaging but rarely for the near‐surface area. The near‐surface area is often assumed isotropic. Seismic refraction methods are often applied to infer a shallow velocity model, either for making static corrections or for building the top portion of a pre‐stack depth migration velocity model. To study the anisotropic effects on the refractions and the near‐surface imaging results, we design numerical models with horizontal‐transverse‐isotropy anisotropic parameters assigned in the shallow layers and perform three‐dimensional seismic ray‐tracing with the consideration of horizontal‐transverse‐isotropy anisotropy to calculate the first‐arrival travel times. Then, we assume the near‐surface area “isotropic” and invert a near‐surface velocity model using isotropic travel‐time tomography. Through these tests, we have identified that the footprints of the recording geometry could be mapped into the velocity model, and high‐velocity anomalies are produced right above the refractor if the horizontal‐transverse‐isotropy anisotropy is ignored. By applying the azimuth‐dependent travel‐time tomography, we could account for anisotropic effects and infer the fast and slow velocity models to determine the direction of the symmetry axis. With the same approach, we process a three‐dimensional real data set, and the results are consistent with the local geology.