A new approach to compensate for illumination differences in 4D surveys with different individual acquisition geometries
Successful time-lapse (or 4D seismic) studies require special care when it comes to the removal of undesirable artifacts caused by the differences in acquisition geometries. By attempting to repeat the source and receiver geometries between surveys as precisely as possible any subsequent 4D noise is minimized so that subtle seismic signal variation induced by reservoir production can be detected. It is commonly accepted that the required repeatability accuracy is directly linked to the desired sensitivity and resolution of the 4D signal. Illumination studies prior to any 4D experiments ensure that the reservoir is illuminated in as identical a fashion as possible between base and monitor survey so that the desired 4D effects can be recovered. In fact, it is common practice to plan and design 4D surveys with optimal acquisition repeatability in mind. However, in some cases it is not possible to repeat the survey geometries between vintages. When the geometry differences are small, corrections can be made during data processing by including steps such as 4D binning, which aim to preserve those seismic traces that are associated with the smallest variation in source and receiver positions. The process of 4D binning is particularly effec¬tive when the acquisition for both the base and monitor survey are very similar such as streamer on streamer or OBS (Ocean Bottom Seismic) on OBS surveys. Nevertheless, 4D binning does not perform well when both acquisition vintages comprise significant differences in their respective source and receiver positions. This is for example the case when different streamer acquisition azimuths are involved or when large cable feathering differences at long offsets are observed or indeed when a towed streamer survey is to be compared with an OBS acquisition.