Estimation of Seismic Attenuation and Gas Hydrate Concentration From Surface Seismic Data at Hydrate Ridge, Cascadia Margin.

An improved understanding of the effects of gas hydrate presence on seismic attenuation is important for accurate hydrate characterization and quantification. Based on a rock‐physics model recently presented for gas hydrate‐bearing fine‐grained clay‐dominated sediments, here we establish an integrated workflow for surface seismic data from extracting seismic attenuation to estimating gas hydrate concentration (Ch) in the sediment. We apply this workflow to the high‐resolution seismic data acquired at southern Hydrate Ridge, offshore Oregon, to reveal the hydrate distribution and clarify the controlling factor of hydrate formation. We first present an adaptive‐bandwidth spectral ratio method to robustly measure attenuation. The attenuation measurements show that the presence of hydrate suppresses the attenuation of the host sediment. We then calculate Ch by applying the rock‐physics model to the attenuation measurements. The estimated Ch are mostly low (<5%) in Hydrate Ridge and agrees well with the in‐situ Ch measured from core‐ or well log‐based data. Our result also suggests that the lithology and stratigraphic structures together control the distribution of gas hydrate at Hydrate Ridge, where relatively high Ch is found in the region where a gas‐charged conduit exists and in an anticlinal structure overlying a strong bottom simulating reflection. Adjacent to the anticline, however, a low amount of gas hydrate appears present, possibly due to the gas migration blocked by the anticlinal structure or the lack of gas conduits. Our study offers an effective strategy for detecting and quantifying gas hydrate in fine‐grained clayey sediments through surface seismic data. Plain Language Summary: Seismic attenuation represents the decay of seismic energy during the travel of seismic waves. Seismic attenuation is a crucial tool in detecting and evaluating gas hydrates. Our knowledge of the relation between seismic attenuation and how hydrate forms in sediment has been improved in these years. Here we present a workflow for seismic data in which we measure seismic attenuation using a novel method and calculate the hydrate content in clay‐rich sediment based on a rock physics model. We apply this workflow to the seismic reflection data in Hydrate Ridge, Oregon. The attenuation measurements show that gas hydrate suppresses the attenuation of host sediments. The gas hydrate amount estimated in this study agrees well with (a) seismic characteristics on the seismic profile and (b) the in‐situ measurements of core‐ or well log‐based techniques. Our results show that the gas hydrate distribution at Hydrate Ridge is mainly controlled by lithology and stratigraphic structures. This good agreement suggests that our method is able to not only extract attenuation but also obtain reliable estimation of hydrate amount using seismic data. Key Points: We present a workflow from extracting attenuation to estimating gas hydrate concentration for seismic reflection dataGas hydrate presence in fine‐grained clayey sediments suppresses the seismic attenuation of host sediments at Hydrate RidgeGas hydrate distribution at Hydrate Ridge is controlled by both lithology and stratigraphic structures [ABSTRACT FROM AUTHOR]