The seismic method is not guaranteed to work. The most difficult overburdens can scatter, absorb and deflect seismic energy so badly that little or no energy from the target returns to the surface. The small amount of energy that does return will usually be exceptionally difficult to process, not least because it will then be difficult even to identify primary energy pre-stack, making it especially difficult to design and test a denoise, demultiple and /or depth imaging strategy. Seismic acquisition and processing in many areas of Berau Bay (Papua Barat) must overcome overburden problems in this ultra-difficult class.
How do we find the best acquisition and processing approach? One approach would be to run a field test using an over-sampled acquisition design with a source and receiver every 12.5m say and covering an area large enough to generate a fully migrated full-fold image, and to process this dataset at various levels of decimation until a design that worked and was affordable emerged. This is very expensive in the shallow waters of Berau Bay, where streamer operations are mostly difficult or impossible, and OBC must instead be used. And there is no guarantee that the result will be applicable a few tens of km away
from the initial pilot study. BP had a successful trenched OBC survey over the Vorwata Field in 2005. Trenching the cable improves receiver coupling and significantly reduces ambient noise. However the cost/km2 is very high, hence trenched OBC is prohibitively expensive except in small areas of very high value such as giant field production cores. This was encouraging BP to search other alternative acquisition methods for Berau Bay.
The acquisition design started with 3D Finite Difference Acoustic Forward Modeling study. The study is making important contributions in Berau Bay seismic programs by suggesting acquisition designs and appropriate seismic processing techniques that can overcome the laterally variable, complex subsurface geologies. Computer simulation is far cheaper and faster than real acquisition testing. With a realistic processing sequence, the synthetic data produce images very similar to the real target structures as a function of field effort, allowing a cost – benefit trade off to be arrived at. The result of the modeling study together with a thorough seismic acquisition technology evaluation and field trial equipment testing lead the decision to use wide azimuth – wide patch – high fold acquisition via OBC parallel geometry. OBC parallel geometry has proven it’s capability to overcome a complex geology and difficult surfac condition at Berau Bay. The flexibility of the geometry has given wide application for any type of seismic survey (exploration, appraisal and development).
Authors : Deny Suryanto, Supriyono