A method of estimating the amount of in-situ gas hydrates in deep marine sediments
The bulk volume of gas hydrate in marine sediment can be estimated by measuring interval velocities and amplitude blanking of hydrated zones from true-amplitude processed multichannel seismic reflection data. In general, neither velocity nor amplitude information is adequate to independently estimate hydrate concentration. We propose a method that uses amplitude blanking calibrated by interval-velocity information to quantify hydrate concentrations in the Blake Ridge area of the U.S. Atlantic continental margin. On the Blake Ridge, blanking occurs in conjunction with relatively low interval velocities. The model that best explains this relation linearly mixes two end-member sediments, hydrated and unhydrated sediment. Hydrate concentration in the hydrate end-member can be calculated from a weighted equation that uses velocity estimated from the seismic data, known properties of pure hydrate, and porosity inferred from a velocity/porosity relationship. Amplitude blanking can be predicted as the proportions of hydrated and unhydrated sediment change across a reflection boundary. Our analysis of a small area near DSDP 533 indicates that the amount of gas hydrates is about 6% in total volume when interval velocity is used as a criterion and about 9.5% when amplitude information is used. This compares with a calculated value of about 8% derived from the only available measurement in DSDP 533.