Quantitative detection of methane hydrate through high‐resolution seismic velocity analysis

Abstract

A laterally extensive, high‐resolution travel time velocity analysis and acoustic wave form, inversion were used to quantitatively determine methane hydrate content in deep water sediments of the Blake Ridge off the southeast U.S. coast. The interval acoustic velocity (Vp) analyses were performed in the τ‐p domain by interactively picking the τ‐p trajectories of prominent reflections in each of 50 plane wave‐decomposed common midpoint gathers. The reflections correspond to seismic stratigraphic boundaries so that lateral Vp changes due to lithology changes are mitigated, and Vp changes due to changing hydrate content are enhanced. Two separate interval Vp analyses were performed, one with thick (∼0.4 km) layers which yielded lower uncertainty but also lower resolution, and one with thinner layers (∼0.1 km), yielding higher resolution but slightly larger uncertainties. Results show no correlation between low‐sediment reflectivity and Vp. However, in the areas exhibiting a bottom simulating reflector (BSR) a high Vp interval (∼2.0 km/s and 0.15 km thick) is seen immediately above the BSR. Where the BSR is strongest a 256‐layer, least squares acoustic wave form inversion reveals the BSR to be caused by a Vp decrease from ∼2.0 to ∼1.5 km/s, with little or no change in density. The inversion also reveals a thin (0.025 km) layer of anomalously low Vp lying immediately below the BSR. Two models of methane hydrate distribution are tested, each indicating that the volume of methane hydrate in the intervals of elevated Vp is up to ∼25% of the total volume.