PP-PS Joint Reflection Tomography for Migration Velocity Analysis – A Case Study from Malaysia

We describe here in detail the workflow that was used for proper imaging of converted waves which should be migrated to depth using full anisotropic ray-tracing. The model building went through two major steps. First, the Vp or P-wave velocity was built followed by the Vs or S-wave velocity model building. To recover high frequency variation in the P-wave velocity model, an amplitude inversion was run to feedback into the model building route. We call this step “gas body insertion”. Following this, to get correct Vs model update, and a true co-location of PP and PS events, a joint PP-PS non-linear tomography was run which uses an S-ray approximation to map-migrate PS events to PP events. In this way, the mis-positioning between the PP and PS events provides a 3D co-location error which is then minimized inside the tomographic update while also minimizing the PS RMO. This process outputs jointly updated Vp, Vs, and Epsilon fields that are then used in the final PP-PS depth imaging. The final converted wave image clearly illuminates events inside the gas cloud, leading to better event interpretation and fault positioning, which helps to delineate the reservoir more confidently.