Suprasalt model building using full-waveform inversion

Abstract

The application of full-waveform inversion (FWI) to bring high resolution to the velocity model is becoming a standard approach in the velocity model-building workflow. Diving wave FWI in conjunction with reflection FWI (RFWI) has been widely used in the Gulf of Mexico (GOM) to optimize the suprasalt model. Accuracy of a velocity model from tomography is dependent on residual moveout (RMO) picking accuracy. In a good signal-to-noise ratio area, the confidence of RMO picking is high. But gathers in areas affected by gas exhibit poor event continuity, which makes it difficult to get accurate RMO picks. In such a geologic regime, FWI can improve the velocity model and therefore the final image quality. There are two main components of a velocity model from the GOM area: the first is the sediment, and the second is salt geometry. In the beginning of the model-building cycle, it is most likely that salt geometry is not accurately defined. This inaccuracy leads to a big mismatch between synthetic and observed data for both diving wave FWI and RFWI. One way to handle this situation is to start with the salt model and iteratively adjust the salt interpretation as FWI model building progresses from lower to higher frequencies. Another approach could be eliminating the salt-related energy from the input and then using the sediment-only model for FWI. We are proposing a desalt approach in which we try to eliminate or reduce the salt-related energy from the input data and then use a sediment-only velocity model as a starting model for the entire suprasalt FWI workflow. We will present a case study in which, by adapting the desalt workflow, we could manage to do more FWI iterations by eliminating salt interpretation.