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doi:10.2204/iodp.proc.311.110.2006

Introduction

Gas hydrate fields on the northern Cascadia margin have been comprehensively studied using a multitude of geological, geophysical, and geochemical techniques and data, including reflection seismics. In preparation for Integrated Ocean Drilling Program (IODP) Expedition 311, additional new subsurface imaging data in the form of marine reflection seismics and subbottom profiling with a 3.5 kHz transducer, among other data, were acquired and are presented in this paper. The locations of all site survey data relevant to Expedition 311 are summarized on the map shown in Figure F1. A major objective of this paper is to describe the main features of the subsurface structure at the new drill site locations and provide the rational for selecting these sites.

Two major seismic reflection campaigns in 1985 and 1989 resulted in structural images of the northern Cascadia subduction system and helped identify the widespread distribution of gas hydrate in this area (Hyndman et al., 1994). This was followed by drilling Ocean Drilling Program (ODP) Sites 889/890 (Leg 146) into these structures (Westbrook, Carson, Musgrave, et al., 1994). Because Leg 146 comprised only one drill site over undeformed sediments on the oceanic plate (Site 888) and two closely separated sites on the mid-continental slope (Sites 889 and 890), no systematic changes in gas hydrate formation and occurrence along the gas hydrate stability area of the continental shelf could be observed. A new drilling campaign was proposed to address the question about systematic gas hydrate changes that could be related to the amount and duration of fluid flow through the sediments. By drilling a transect of sites from the undeformed sediments off the deformation front up the slope toward the shallowest parts of gas hydrate occurrence, fundamental new insights into the quantification of gas hydrate formation can be addressed. In addition to this, it was proposed to drill into a nearby vent site to expand into unusual high fluid flux areas. A combined geophysical, geochemical, and also microbiological analysis of the drill sites was proposed to study all aspects including the biodiversity of these regions.

From 26 July to 11 August 1999, Cruise PGC9902 of the Canadian research vessel CCGS John P. Tully took place to survey sites for the new IODP proposal. The data acquired comprise seismic reflection data as well as subbottom profiles from a hull-mounted 3.5 kHz transducer and 12 kHz echo sounding to map the seafloor. For the seismic recording a 40 inch³ (0.65 L) sleeve gun was used. The signals were recorded with the Canadian Oceanographical Acoustic Measurement System (COAMS), a 1140 m long multichannel seismic (MCS) streamer, as well as by a Teledyne single-channel seismic (SCS) streamer. The profiles were along several main separate corridors and along several closely spaced lines to enable three-dimensional (3-D) data analysis.

Other surveys in the area included Cruises SO111 and SO149 of the German research vessel Sonne to collect multibeam bathymetry (Villinger et al., 1996; Spiess et al., 2001) and three SCS surveys between 1999 and 2004. Two surveys, in 1999 and 2000, were acquired across cold vent sites, again using the 40 inch³ sleeve gun and the Teledyne streamer and in a manner for possible 3-D analysis. The last SCS survey took place in 2004, this time with a 40 inch³ air gun and the Teledyne streamer to cover alternate drill sites for the proposed IODP drilling.

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