Investigation of Overburden Heterogeneity Effects and Their Removal through High Resolution Tomography and Prestack Depth Migration

Abstract

For contributing to precise depth delineation in the Ichthys giant gas field offshore northwest Australia, we investigated shallow velocity heterogeneity effects using forward modelling and seismic data review. We discovered that their removal through applications of high-resolution tomographic velocity modelling and prestack depth migration (PSDM) enabled correct representation of the target reservoir structure. Analysis of synthetic seismic pre-stack gathers generated from forward modelling demonstrated that small velocity anomalies, such as channels, in shallow overburdens could give rise to apparent seismic RMS velocity artefacts at deeper target levels. Delineation of ``true' velocity anomalies and implementation of PSDM using precise shallow velocity model were required for solving these problems. In accordance with the phenomenon predicted by forward modelling, examination of coinciding patterns of time-thickness, amplitude and deep prestack time migration (PSTM) velocity allowed determination of the shallow heterogeneous layers that caused target velocity undulation. We employed two iterations of tomographic velocity model updating for PSDM velocity model building. First, the shallow heterogeneous velocity patterns were successfully identified by utilising the dense residual moveout picking and the layer-based 3D high-resolution finite-offset tomography. Subsequent grid-based global tomography with constraints was used for updating the entire velocity field and delivered stable velocity pattern at the deep. This two-step approach successfully eliminated deep velocity artefacts.