Tomographic seismic studies of the methane hydrate stability zone in the Cascadia Margin

Hobro, James; Minshull, T. A.; Singh, S. C.

doi:10.1144/gsl.sp.1998.137.01.10

Cited by 15 publications

(9 citation statements)

References 14 publications

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“…Several deployments of OBSs have been carried out near ODP Site 889 in 1993, 1997 data were collected during two deployments of OBSs, recorded with an airgun (120 cu. Simultaneous traveltime inversions were carried out to provide a 2-D velocity model through the drill site [Hobro et al, 1998]. with a dominant frequency of 75 Hz [Spence et al, 1995].…”

Section: Seismic Datamentioning

confidence: 99%

Geophysical Studies of Marine Gas Hydrate in Northern Cascadia

Hyndman

Spence

Chapman

et al. 2013

Natural Gas Hydrates

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This paper reviews the extensive geophysical studies and Ocean Drilling Project (ODP) results that have provided constraints on the occurrence, distribution, and concentration of gas hydrate and underlying free gas beneath the continental slope of the northern Cascadia subduction zone. On this margin there is a large clastic accretionary sedimentary prism, the most common environment for high concentrations of marine gas hydrates. Most information on the gas hydrate has come from a wide range of seismic surveys, including mapping the area of the characteristic bottom-simulating reflector (BSR), determining the depth distribution of hydrate and underlying free gas, and the geological controls of hydrate formation. BSRs are evident beneath about half of the mid-continental slope. Special seismic studies include BSR reflection coefficients, the frequency dependence of the BSR amplitude, BSR amplitude-versus-offset (AVO), and full waveform inversions. Additional information on hydrate concentration is provided by electrical resistivity profiling, and measurements of seafloor compliance. Detailed heat flow surveys have mapped the thermal regime that controls the depth to which gas hydrate is stable. Ocean Drilling Program Leg 146 drilled through the gas hydrate on the midcontinental slope and a reference hole in the adjacent deep sea Cascadia basin. Downhole geophysical logs and core analyses have constrained the concentrations of hydrate and free gas at the drill sites, and provided calibration of the regional geophysical data. In the region of the ODP site, the estimated concentration of hydrate from both geophysical and borehole data is 20-30% of the pore space (10-15% of total volume) over 100 m above the BSR, with an underlying 10-20 m layer of less than 1% free gas. this region, the Juan de Fuca plate is underthrusting the margin approximately orthogonally at about 40 mm/yr. The Juan de Fuca spreading center is located only several 100 km offshore and the oceanic plate is very young, 6-8 Ma, at the margin. The buoyancy of the young hot oceanic plate and the thick incoming sediment section result in a very subdued and shallow trench with a depth of about 2,500 m. Most of the 2-3 km thickness of incoming hemipelagic and turbidite

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Section: Seismic Datamentioning

confidence: 99%

Geophysical Studies of Marine Gas Hydrate in Northern Cascadia

Hyndman

Spence

Chapman

et al. 2013

Natural Gas Hydrates

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“…The BSR is thought to represent the base of the hydrate stability field, marking the transition between hydrate‐bearing sediments above and the occurrence of free gas below the interface. At the Cascadia margin, gas hydrates were first observed in seismic data acquired in 1985 and have been investigated intensively over the last decade, including single‐channel and multichannel seismic surveys [ Fink and Spence , 1999; Yuan et al , 1996, 1999; Hyndman and Spence , 1992; Singh et al , 1993; Chapman et al , 2002], ocean bottom seismometer (OBS) recording [ Spence et al , 1995; Hobro et al , 1998], electrical resistivity and seafloor compliance studies [ Hyndman et al , 1998; Yuan and Edwards , 2000; Willoughby and Edwards , 2000], and Ocean Drilling Program (ODP) Leg 146 drilling and downhole logging [ Westbrook et al , 1994; Jarrard et al , 1995; MacKay et al , 1994]. …”

Section: Introductionmentioning

confidence: 99%

Seismic investigations of a vent field associated with gas hydrates, offshore Vancouver Island

et al. 2002

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[1] A three-dimensional seismic reflection survey was carried out over a gas hydrate field offshore Vancouver Island in the area of Ocean Drilling Program Leg 146, Site 889/890. The area of investigation is located on the accretionary prism of the Cascadia margin in water depths of 1200-1500 m. Several seismic blank zones were observed in the seismic data over a frequency range from 30 Hz to 3.5 kHz, where the degree of blanking increases with seismic frequency. Time slices and selected horizon slices were used to define the spatial distribution of the blank zones. The zones range from 80 m to several hundred meters in width. One blank zone, almost circular with a diameter of $400 m, has a distinct seafloor expression. It shows the characteristics of a mud/carbonate mound and is probably associated with free gas expulsion. Hydrate was found at four sites with a piston corer within this blank zone at depths of 3-8 m below the seafloor. We present a tectonic model that explains the blank zones to be a result of differential uplift along thrust faults. The blank zones represent conduits for fluids and gas migrating upward. Blanking of the seismic energy is believed to be mainly the result of increased hydrate formation in lenses within the faults. The blanking effect is enhanced locally because of scattering at carbonates formed at the seafloor.

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“…Several deployments of OBSs were carried out in 1993 and 1997, recording a 120 in 3 airgun source with a dominant frequency of 75 Hz. Simultaneous travel time inversion of data from four instruments and from coincident normal‐incidence lines gave velocities of 1.83–1.95 km/s just above the BSR, slightly higher than VSP and sonic log data from ODP Site 889 14…”

Section: Detailed Seismic Surveysmentioning

confidence: 74%

“…Although much of this variation may be produced by topographic effects, a component may result from dynamic effects, including the upward migration of fluids along faults and the displacement of isotherms by thrust faulting. Ocean bottom seismometer (OBS) surveys.Several deployments of OBSs were carried out in 1993 and 1997, recording a 120 in 3 airgun source with a dominant frequency of 75 Hz. Simultaneous travel time inversion of data from four instruments and from coincident normal‐incidence lines gave velocities of 1.83–1.95 km/s just above the BSR, slightly higher than VSP and sonic log data from ODP Site 889 14 High resolution deep‐towed multichannel seismic survey.With the deep‐tow acoustics/geophysics system (DTAGS), a survey was carried out in 1997 in the area of ODP Site 889 15–17.…”

Section: Detailed Seismic Surveysmentioning

confidence: 88%

North Cascadia Deep Sea Gas Hydrates

Spence

Hyndman

Chapman

et al. 2000

Annals of the New York Academy of Sciences

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The Cascadia accretionary margin off Vancouver Island is one of the best studied margins world‐wide for the determination of in situ properties of marine gas hydrates. Most quantitative information has come from cores and downhole logs of the Ocean Drilling Program (ODP) Leg 146 and from extensive seismic and other geophysical surveys. As part of the ODP site surveys, large‐offset multichannel seismic lines outlined the regional distribution of hydrate, and seismic velocity analyses coupled with full waveform inversion provided estimates of the vertical distribution of hydrate and gas. High resolution single‐channel seismic surveys indicate correlations between hydrate/gas concentrations and topographic highs, and between geothermal flux and topography; these correlations provide insight into fluid and methane flow through the sediments. A deep‐towed multichannel seismic survey (DTAGS), together with other data from 20–600 Hz, provide constraints on gradients at the base of the hydrate and gas layer. Extensive heat flow measurements, from probes and from depths of the bottom simulating reflector (BSR), have been modelled numerically, including the regional effects of sediment thickening and advective fluid flow in the accretionary prism. Other geophysical surveys include several seafloor electrical sounding experiments and seafloor compliance measurements of hydrate. ODP drilling has provided valuable downhole log data, core physical property data, and detailed pore fluid chemistry and isotopes. Several semi‐independent estimates of hydrate and gas concentrations have been obtained; all are dependent on reference sediment properties for which no hydrate and no gas is present. From a vertical seismic profile and other seismic data, the velocity increase in a hydrated region is consistent with hydrate concentrations of 20–30% of the pore space in a 100‐m interval above the BSR. Similar values are determined from log resistivity data and from core pore fluid chlorinity.

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Tomographic seismic studies of the methane hydrate stability zone in the Cascadia Margin

Cited by 15 publications

References 14 publications

Geophysical Studies of Marine Gas Hydrate in Northern Cascadia

Geophysical Studies of Marine Gas Hydrate in Northern Cascadia

Seismic investigations of a vent field associated with gas hydrates, offshore Vancouver Island

North Cascadia Deep Sea Gas Hydrates

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