oa Imaging through gas - a case study from the<br>North Sea. Anisotropic depth migration<br>processing of a 4C time-lapse survey

In late 1994 BP and partners Shell, Total, and Amerada-Hess embarked on what can be described as the industry’s ultimate seismic exercise – the repeated acquisition and processing of 4-component data over the Valhall field, offshore Norway, using a permanent sea-floor receiver installation. The 4D or time-lapse exercise was designed specifically to monitor and manage the Valhall field; a baseline 3D survey and a series of six repeat monitor surveys were acquired and processed at 3-month intervals, in the world’s first “Life of Field” or LoFS seismic program. This case study discusses the processing issues and workflows used to process the baseline and monitor surveys. The primary processing challenge involves imaging the reservoir in depth, in spite of the gas cloud, and required the development of velocity models for both the PP and the converted wave (PS) data. Subsequent 4D processing to compare the baseline survey with the repeated monitor surveys aims to capture true changes in the reservoir.