Suprasalt model building using full-waveform inversion

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 depends on residual moveout (RMO) picking accuracy. In areas with a good signal-to-noise ratio, the confidence in RMO picking is high. However, gathers in areas affected by gas exhibit poor event continuity, making it difficult to obtain accurate RMO picks. In such a geologic regime, FWI can improve the velocity model and, consequently, the final image quality.

There are two main components of a velocity model from the GOM area: sediment and salt geometry. At the beginning of the model-building cycle, salt geometry is often not accurately defined. This inaccuracy leads to a significant mismatch between synthetic and observed data for both diving wave FWI and RFWI.

One way to address this issue 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 involves eliminating the salt-related energy from the input and using a sediment-only model for FWI.

We propose a desalt approach, in which we attempt to eliminate or reduce salt-related energy from the input data and then use a sediment-only velocity model as the starting model for the entire suprasalt FWI workflow.

We will present a case study demonstrating how, by adopting the desalt workflow, we were able to conduct more FWI iterations while eliminating salt interpretation challenges.