COMBINED DIVING WAVE REFRACTION TOMOGRAPHY AND REFLECTION TOMOGRAPHY FOR PSDM VELOCITY MODEL BUILDING (BEKAPAI FIELD, INDONESIA)

摘要

The offshore Mahakam Delta is well-known for its gas pockets trapped at shallow depth (<400 meters), causing severe degradation of the seismic image. They absorb strongly the seismic energy and lower significantly the signal to noise ratio. Building a reliable velocity model in such areas with short wavelength lateral and vertical velocity variations can be a serious issue. In the Bekapai Field, velocity model building also has to deal with the lack of near angle information in shallow depths as well as weak transmitted signals in deeper intervals.In our case, the velocity model building process combined both a diving wave refraction tomography for shallow intervals (< 400 meters, approximately 1/6 of the streamer cable length) and a reflection tomography for deeper intervals. In this paper, it is shown that diving wave tomography is a robust technique that improves the near surface model. To complete the model for depths greater than 400 meters, the available pre-stack time migration velocity field was combined to the shallow velocity model. The initial velocity field has been further updated with iterative Pre-Stack Depth Migration (PSDM) and reflection tomography with input data having sufficient offset range.The Bekapai final velocity model has enabled a far better focusing of shallow events, including shallow gas amplitude anomalies that were hidden after the 2002 time data-reprocessing. The apparent lateral extension of the anomalies has helped to optimize the design of the upcoming 3D OBC survey on the field. While the diving wave refraction tomography has been suitable for shallow intervals, the reflection tomography has also produced an improved velocity model, contributing to an enhanced focusing ofseismic events and fault planes that enables the interpreter to reduce significantly the remaining structural uncertainty and to deliver reliable structural interpretations for further reservoir model building and field production optimization.