3D PSDM Model Building in a Well-Rich Environment

The paper describes velocity-depth model building for a marine 3D VSP survey in the Arabian Gulf. OBC seismic data exists but is of poor S/N and contaminated with multiples. We construct a P-wave velocity-depth model using only VSP checkshots and well tops from wireline. The checkshots provide P-wave velocity control and the well tops define the depth intervals. First, well tops are gridded in 3D to extrapolate continuous surfaces over the depth volume. Then time-depth information from checkshots is converted to interval velocity functions at each well. Finally, interval velocity is gridded and smoothed in 3D. The composition of all the layers yield the P-wave model. The P-wave model was used to perform pre-stack depth migration on P-wave gathers from a 3D VSP survey. Model accuracy was verified by QC of common image point gathers. We then use wireline Vp/Vs ratio from area wells as a scaler to convert the P-wave model to S-wave. A Vp/Vs volume was built using 3D kriging of the Vp/Vs ratio logs from available wells. The Vp/Vs volume was then divided into the P-wave volume to yield the S-Wave volume. PSDM results using the S-wave model on S-wave gathers are not currently available.