Cost-effective OBC feasibility studies along with valuable high-resolution VSP data

Abstract

Cost effective OBC Feasibility studies can be performed concurrently with the acquisition of walkaway VSPs (2D or 3D). READ Well Services (RWS) now offers deployment of four-component ocean bottom cables (OBC) as an addition to standard 2D or 3D VSP surveys. The recording is done from a separate vessel or using a remotely operated buoy. The objective is to acquire OBC data simultaneously with the VSP acquisition using the same source vessel and source points. Whereas the VSP yields a high resolution P-P and P-S image over a restricted area, the OBC will yield a slightly lower resolution image with a greater coverage area. Due to the fact that the down-going wavefield is recorded in the VSP, parameters such as direct P-velocity, transmitted S-velocity, Q-values and anisotropy can be estimated with great accuracy. These parameters can be applied to greatly improve the quality of the OBC image. The OBC data from the combined OBC/VSP survey can be used as feasibility study to deternine if a larger scale OBC survey might be conducted over the field. The advantage of acquiring OBC data simultaneously with VSP is in the economics. The seismic crew, seismic source vessel and recording unit have already been deployed offshore for the VSP. There is often waiting time associated with the drilling rig when the 3DVSP crew is not productive. The additional cost for laying a couple of OBC lines and acquiring more data is small compared to mobilizing a complete OBC crew for a separate feasibility study. A simple offshore velocity model has been designed in order to create synthetic data to illustrate the combined VSP/OBC imaging technique.