ALBERT

Description: Evidence for gas-hydrate occurrence in the Western Black Sea is found from seismic measurements revealing bottom-simulating reflectors (BSRs) of varying distinctness. From an ocean-bottom seismic data set low-resolution traveltime-tomography models of P-wave velocity vP are constructed. They serve as input for acoustic full-waveform inversion (FWI) which we apply to derive high-resolution parameter models aiding the interpretation of the seismic data for potential hydrate and gas deposits. Synthetic tests show the applicabilityof the FWI approach to robustly reconstruct vP models with a typical hydrate and gas signature. Models of S-wave velocity vS containing a hydrate signature can only be reconstructed when the parameter distribution of vS is already well known. When we add noise to the modeled data to simulate field data conditions, it prevents the reconstruction of vS completely, justifying the application of an acoustic approach. We invert for vP models from field-data of two parallel profiles of 14 km length with a distance of 1 km. Results showa characteristic velocity trend for hydrate and gas occurrence at BSR depth in the first of the analyzed profiles. We find no indications for gas accumulations below the BSR on the second profile and only weak indications for hydrate. These differences in vP signature are consistent with reflectivity behavior of the migrated seismic streamer data of both profiles where a zone of high reflectivity amplitudes is coincident with the potential gas zone derived from the FWI result. Calculating saturation estimates for the potential hydrate and gas zones yields values of up to 30% and 1.2%, respectively.