The Garden Banks model experience

To better understand deepwater imaging challenges in the Gulf of Mexico, we constructed a large 3D model based on the complex salt geology of the Garden Banks protraction area. We simulated a regional wide-azimuth (WAZ) streamer survey with offsets of ±8 km inline and ±4 km crossline over this model. Using the true velocity model, reverse time migration of this data set produced a usable subsalt image nearly everywhere. The same synthetic seismic data set was passed to interpreters and geophysicists to process and image as if it were real field data. They did not get to see the true velocity model. Instead they performed conventional “migrate, pick, and flood” top-down velocity-model building, followed by final imaging through their interpreted velocity model. Where the salt was relatively simple, we found that the interpreted velocity model was reasonably accurate. Reverse time migration of the seismic data through these parts of the velocity model produced an image that was imperfect, but still usable for exploration-scale, structural interpretation. Where the salt structure was complex, however, it was sometimes grossly misinterpreted. The ensuing large-scale errors in the interpreted velocity model resulted in an unusable, shattered subsalt image. We next simulated a low-frequency, ocean-bottom-node (OBN) acquisition, with offsets up to 30 km in all azimuths. We started with the imperfect model produced by the interpreters and investigated what it would take for full-waveform inversion (FWI) to successfully find and fix the gross interpretation errors. Initial results suggest this “interpretation followed by FWI” methodology could produce a velocity model capable of generating an adequate subsalt image, but it may require ultrawide offsets (30 km), ultralow frequencies (below 2 Hz), and improved FWI algorithms to do so.