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doi:10.2204/iodp.proc.311.109.2006

Evidence of gas hydrate

Data from Leg 146 indicates widespread gas hydrate on Canada's West Coast at the northern Cascadia margin, as evidenced by BSRs in seismic reflection data and increased electrical resistivity compared to normal sediments in electromagnetic surveys, and gas hydrate has been sampled at and just below the seafloor by shallow piston coring (see Hyndman et al., 2001, and Spence et al., 2000a, for earlier summaries). Closely related geophysical data are also available for central Cascadia off Oregon from ODP Leg 204 (e.g., Tréhu, Bohrmann, Rack, Torres, et al., 2003, and references therein). After the first discovery of BSRs in MCS data in 1985, the area offshore central Vancouver Island (Fig. F3) has had many interdisciplinary gas hydrate studies. The initial work focused on mapping the regional distribution of the BSRs and determining the associated regional concentrations of underlying free gas or overlying gas hydrate (e.g., Hyndman and Spence, 1992; Yuan et al., 1996). The more recent studies focused on cold-vent structures that in some cases result in high-concentration gas hydrate near the seafloor, especially a vent field on the mid-continental slope near ODP Site 889/890 (Riedel et al., 2002, 2006).

A wide variety of seismic surveys have been used to map and characterize the gas hydrate and underlying free gas on the Northern Cascadia margin. The seismic systems had principal frequencies from 20 to 650 Hz and included conventional MCS survey lines, three-dimensional (3-D) higher resolution MCS surveys, the high-resolution deep-tow acoustics-geophysics system (DTAGS) system that is towed near the seafloor, detailed close-line-spacing single-channel reflection mapping, and studies using ocean bottom seismometers (OBSs). Additional high-resolution 3.5 and 12 kHz subbottom profiling, 12 kHz swath SeaMARC-II acoustic imaging, and piston coring provided data from the upper 1–50 m beneath the seafloor.

The regional distribution of the BSR, which is characteristic of many marine gas hydrate areas, has been well mapped seismically. A BSR occurs beneath much of the middle continental slope off Vancouver Island but is completely absent in the deep Cascadia Basin. The BSR approaches the seafloor on the upper slope and gas hydrate is not stable on the continental shelf for water depths less than ~600 m in this area. The surveys and analyses of the seismic data include

• Estimating the BSR reflection coefficient (Spence et al., 1991a, 1991b, 1995; Hyndman et al., 1994; Yuan et al., 1994),
• Modeling the BSR reflection waveform (Hyndman and Spence, 1992; Fink and Spence, 1999),
• Determining and modeling the frequency dependence of the BSR (Chapman et al., 2002),
• Determining detailed interval velocity-depth profiles (Yuan et al., 1996),
• Determining and modeling the amplitude versus offset (AVO) behavior of the BSR (Hyndman and Spence, 1992; Yuan et al., 1999; Riedel et al., 2002)
• Determining the full waveform inversion (FWI) of the seismic data for velocity-depth profiles (Singh et al., 1993; Singh and Minshull, 1994; Yuan et al., 1996, 1999), and
• Determining geological associations of gas hydrate BSRs (Hyndman and Davis, 1992; Fink and Spence, 1999; Yuan et al., 1999; Mi, 1998; Ganguly et al., 2000).

We have also deployed several other important geophysical surveying techniques to map the gas hydrate distribution and estimate hydrate concentrations in the vicinity of Site 889. An important new technique for mapping subseafloor gas hydrate concentrations is the controlled-source electromagnetic (CSEM) profiling system developed at the University of Toronto, Canada (Edwards, 1997; Yuan and Edwards, 2001). Excellent agreement has been obtained in the resistivity and depth between the ODP downhole resistivity logs and the electrical profiling system data (Yuan and Edwards, 2001). The latest deployments of the CSEM system were focused on the area of the cold vents and yielded very high electrical resistivity over the near-seafloor gas hydrate concentrations (Schwalenberg et al., 2005; Willoughby et al., 2005; see discussion below). A second new technique for studying seafloor gas hydrate and associated free gas is seafloor compliance using the coherence between seafloor pressure variations induced by surface gravity and infragravity waves and the associated deformation of the seafloor (Willoughby and Edwards, 1997, 2000; Willoughby et al., 2005). The data are primarily sensitive to the shear modulus of the sediments in the upper few hundred meters below the seafloor. Cementing of sediments by gas hydrate substantially increases the shear modulus.

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