Correcting for salt misinterpretation with full-waveform inversion

Using full-waveform inversion (FWI) to update velocity models that contain salt bodies with high velocity contrasts is challenging. It is even harder if erroneous salt geometry is part of the velocity model. Shen et al. (2017) showed a successful FWI application that corrected some misinterpretation of salt structures and resulted in improved subsalt images at the Atlantis field in the Gulf of Mexico. Their study stressed the importance of the low frequencies (usable down to 1.6 Hz), full azimuths, and wide offsets of OBN data. Encouraged by the success at Atlantis, we revisited some aspects of FWI algorithms to minimize cycle skipping and amplitude discrepancy issues that are common in the presence of salt and salt misinterpretation. Here we present the use of travel time misfit measured in frequency-dependent time windows as the FWI cost function. It is devised to minimize the negative impact from the amplitude discrepancy and cycle-skipping between the recorded data and modeled synthetic data. Furthermore, we use the crosscorrelation coefficient between the recorded data and shifted synthetic data as a weight function in gradient computation to promote travel time measurements of higher quality. We demonstrate the effect of our approach using a staggered full-azimuth streamer data set in an area of complex shallow salt bodies in the Gulf of Mexico.