An Integrated Workflow For Imaging Below Shallow Gas: A Trinidad Case Study

Trapped shallow gas severely impacts signal to noise ratio and makes accurate overburden velocity model estimation difficult, since surface P-wave data may be heavily attenuated and further contaminated with both surface related and internal multiples. Additionally, when the seabed is very shallow (<100m), reflection tomography is ill conditioned for reliably estimating near surface velocities due to the lack of short offset traces and the impact of multiples on deeper reflection events. The problems associated with these issues require an integrated approach to imaging ranging from improving data quality to estimating a reliable shallow velocity model, and integration into a robust global velocity model that will adequately image below gas. Building upon the technical strengths of both companies, BP and WesternGeco (WG) undertook a technical collaboration project to evaluate the fundamental causes of data quality degradation in Greater Cassia, offshore Trinidad and to implement appropriate existing solutions to improve imaging below shallow gas. As a product of this collaborative effort, in this paper we present an integrated workflow on real data showing that a targeted demultiple method combined with multiple passes of refraction and reflection tomography is an effective way to deal with data quality and imaging problems below shallow gas.