Localized anisotropic tomography with checkshot : Gulf of Mexico case study

Abstract

Borehole information must be used to build accurate anisotropic depth models. While various techniques exist, almost none of them is extendable to a general case of complex structure and deviated wells. Localized tomographic inversion is a flexible approach that can potentially be applied to most complex cases. It attempts to streamline and automate the estimation process by directly incorporating the available well data into conventional reflection tomography. We present a case study from Gulf of Mexico where we invert for local vertically transversely isotropic (VTI) model using a joint dataset consisting of seismic and checkshot data. Because this area has flatlayered structure, the results can be compared with more traditional manual 1D layer-stripping inversion. We invert for three VTI parameters and search for a smooth velocity field that both fits the checkshot traveltimes and flattens all seismic gathers. To regularize tomographic inversion, we apply smoothing operators that are oriented along geological dip and have large lateral extent. The anisotropic profiles derived by tomography and 1D inversion have similar trends, but differ in high-frequency details. Borehole data require careful conditioning before joint inversion because of potential difference in water velocity between seismic and well surveys. The workflow we present can be applied to calibrating anisotropic parameters in the more general case of 3D models with structural dip and borehole data from deviated wells.