Gas hydrate concentration and characteristics within Hydrate Ridge inferred from multicomponent seismic reflection data

A seismic experiment composed of streamer and ocean bottom seismometer (OBS) surveys was conducted in the summer of 2002 at southern Hydrate Ridge, offshore Oregon, to map the gas hydrate distribution within the hydrate stability zone. Gas hydrate concentrations within the reservoir can be estimated with P wave velocity (V-p); however, we can further constrain gas hydrate concentrations using S wave velocity (V-s), and use V-s through its relationship to V-p (V-p/V-s) to reveal additional details such as gas hydrate form within the matrix ( i.e., hydrate cements the grains, becomes part of the matrix frame or floats in pore space). Both V-p and V-s can be derived simultaneously by inverting multicomponent seismic data. In this study, we use OBS data to estimate seismic velocities where both gas hydrate and free gas are present in the shallow sediments. Once V-p and V-s are estimated, they are simultaneously matched with modeled velocities to estimate the gas hydrate concentration. We model V-p using an equation based on a modification of Wood's equation that incorporates an appropriate rock physics model and V-s using an empirical relation. The gas hydrate concentration is estimated to be up to 7% of the rock volume, or 12% of the pore space. However, V-p and V-s do not always fit the model simultaneously. V-p can vary substantially more than V-s. Thus we conclude that a model, in which higher concentrations of hydrate do not affect shear stiffness, is more appropriate. Results suggest gas hydrates form within the pore space of the sediments and become part of the rock framework in our survey area.