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3-D seismic imaging of an active margin hydrate system, Oregon continental margin : report of cruise TTN 112 R/V Thomas Thompson, June 19 - July 3, 2000 Public Deposited

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  • In the past several years, international interest in gas hydrates in marine sediments has increased considerably because of increasing recognition that the large volume of gas stored in hydrates or trapped beneath them is a significant fraction of the global methane budget, and may therefore be a source of energy for the future. Because methane is a powerful greenhouse gas, hydrates are also being studied as a possible causative factor in global change. These studies, however, are hampered by our limited knowledge of how much gas is actually stored in this reservoir, its behavior during changing environmental conditions, and its mechanical effects on slope stability. In June-July, 2000, we conducted a high-resolution 3-D seismic survey of a 4 x 9 km2 region on Hydrate Ridge on the Oregon continental margin. The northern crest of this ridge was drilled during ODP Leg 146, and evidence for hydrate and underlying free gas was found. Multichannel seismic data collected in 1989 as a site survey prior to Leg 146 show that a BSR is ubiquitous beneath this ridge. A closer look at the 1989 seismic data and at seafloor morphology shows systematic coincident variations in stratigraphic and BSR reflectivity that suggest that tectonic activity, venting, and slumping are important controls on the distribution of hydrate and free gas. The subsurface reflectivity patterns are especially well defined in the region imaged in this survey. The volume that we imaged includes: the summit of southern Hydrate Ridge, where massive hydrates were recovered from near the seafloor in 1996 and 1999; an actively venting carbonate tower southeast of the summit that was observed and sampled using DSV Alvin in 1999; several "double BSRs," and crosscutting anomalously bright stratigraphic reflectors; and an adjacent slope basin within which the seismic signature of hydrate presence is markedly different than beneath the ridge. The 3-D survey comprised 81 profiles spaced 50 meters apart. Shots from 2 GI guns fired simultaneously were recorded on a 600-m-long, 48-channel streamer and an array of 21 4-component ocean bottom seismometers. Several longer regional senic reflection/refraction lines were also shot. For data quality control, each line was processed as a 2D profile immediately after acquisition, assuming a water velocity for normal moveout correction and f-k migration. The locations of the ship and of the streamer were determined via differential GPS and compasses, respectively, and 3D fold was monitered during the cruise to identify locations where additional data were needed. This resulted in reshooting 18 lines. Excellent data quality was obtained in spite of strong winds and high seas. Fortunately noise due to sea-state was at a considerably lower frequency than that used to image subsurface structure. Two preliminary OBS record sections were also produced, both of which showed arrivals to ofsets >20 km, including Pn arrivals from the base of the oceanic crust. The GI-guns were demonstrated to be a very effective broadband source, producing useable energy from 10-150 Hz. The data provide considerable new information on the pathways through which methane-rich fluids and free gas migrate from beneath the BSR to the seafloor and on the geological evolution of Hydrate Ridge. Some highlights of the 3D reflection survey include: possible imaging of the conduit feeding the carbonate chimney; observation of a strong acoustic "bubble" plume over the southern summit of Hydrate Ridge; identification of several new coherent secondary "BSRs;" documentation of the development of an apparently gas-rich anticline on the eastern flank of Hyrate Ridge; identifiction of several apparent slump deposits in the adjacent slope basin that can provide constraints on the growth history of Hydrate Ridge. Additional analysis, including modeling of the OBS data and 3D processing of the reflection data, will certainly lead to sharper images of the subsurface and new surprizes.
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