Mapping of fluid venting and attenuation of hydrate bearing sediments from offshore Oregon

In this article we present regional high-resolution multichannel seismic reflections surveys of the accretionary sedimentary prism off the coast of Oregon where there are strong gas hydrate-associated bottom simulating reflectors (BSRs), and indications of gas and fluid venting from the seafloor. The new data from RR1718 survey conducted in 2017 as part of a chief scientist training cruise complement previous large airgun deeper-penetration reflection surveys and International Ocean Discovery Program (IODP) seafloor drilling, which were mainly in the area of the mid-slope anticline, Hydrate Ridge. The BSRs are at depths of 120–310 ms (102–264 m) below the seafloor with the regional depths controlled by the landward decreasing heat flow, and shallower water depths. In some areas there are double BSRs interpreted to result from the thermal perturbation and sea level depth changes from transient uplift. Below the BSR there is frequent local brightening of the sediment stratigraphy reflectors by free gas. There are indications of fluid and methane gas venting from the seafloor that locally perturb the local thermal regime and the resulting BSR depth, and of seafloor carbonate caps. The lateral variation in amplitudes of the seafloor and BSR has been mapped in spectral decomposed iso-frequency sections. Attenuation estimates for the BSRs using peak frequency analysis indicate quality factors (inverse of attenuation) as low as 17 for a BSR below the anticline of Hydrate Ridge. The high attenuation is interpreted to be a consequence of a thick free gas layer.