QUANTITATIVE DETECTION OF METHANE HYDRATE THROUGH HIGH-RESOLUTION SEISMIC VELOCITY ANALYSIS

A laterally extensive, high-resolution travel time velocity analysis a nd acoustic wave form. inversion were used to quantitatively determine methane hydrate content in deep water sediments of the Blake Ridge of f the southeast U.S. coast. The interval acoustic velocity (V(p)) anal yses were performed in the tau-p domain by interactively picking the t au-p trajectories of prominent reflections in each of 50 plane wave-de composed common midpoint gathers. The reflections correspond to seismi c stratigraphic boundaries so that lateral V(p) changes due to litholo gy changes are mitigated, and V(p) changes due to changing hydrate con tent are enhanced. Two separate interval V(p) analyses were performed, one with thick (approximately 0.4 km) layers which yielded lower unce rtainty but also lower resolution, and one with thinner layers (approx imately 0.l km), yielding higher resolution but slightly larger uncert ainties. Results show no correlation between low-sediment reflectivity and V(p). However, in the areas exhibiting a bottom simulating reflec tor (BSR) a high V(p) interval (approximately 2.0 km/s and 0. 15 km th ick) is seen immediately above the BSR. Where the BSR is strongest a 2 56-layer, least squares acoustic wave form inversion reveals the BSR t o be caused by a V(p) decrease from approximately 2.0 to approximately 1.5 km/s, with little or no change in density. The inversion also rev eals a thin (0.025 km) layer of anomalously low V(p) lying immediately below the BSR. Two models of methane hydrate distribution are tested, each indicating that the volume of methane hydrate in the intervals o f elevated V(p) is up to approximately 25% of the total volume.