On the compliance method and the assessment of three-dimensional seafloor gas hydrate deposits

Latychev, Konstantin; Edwards, R. N.

doi:10.1111/j.1365-246x.2003.02090.x

Cited by 13 publications

(18 citation statements)

References 28 publications

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“…Using 1D forward models in the assessment of the compliance data can be valid even if Bullseye Vent is a 3D heterogeneous system. Latychev and Edwards (2003) demonstrate that a layered model that represents the average properties of a hydrate-filled region is equivalent to the average 3D response of a heterogeneous structure because this too is only sensitive to the bulk hydrate content and not the distribution of the hydrate content in the hydrate stability zone.…”

Section: Interpretation and Discussionmentioning

confidence: 99%

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A temporal trend in compliance measurements near a gas hydrate accumulation, Northern Cascadia

Roach

Edwards

2015

GEOPHYSICS

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Ocean surface gravity waves produce temporally correlated pressure and vertical displacements on the ocean floor. Compliance is the ratio between the pressure and the resulting displacement. Pressures are measured by a differential pressure gauge, whereas a broadband seismometer measures velocities that are directly related to the displacements in the frequency domain. These instruments are installed on the Cascadia accretionary prism as part of NEPTUNE Canada's ocean-floor network. Gas hydrates have the property of stiffening marine sediment, i.e., reducing the compliance. A compliance measurement is dependent on the elastic parameters of the sediment, particularly, the shear modulus. We have computed compliance from raw data, daily, for 203 days and determined the temporal variation in shear modulus of a region approximately 390 m northeast of the undersea area known as Bullseye Vent. Our aim was to investigate the evolution of the elastic properties of the subsurface in this region. The compliance was determined over a bandwidth of 0.01-0.03 Hz, and had peak sensitivity to the sediment elastic structure at approximately 275 m below seafloor, for a water depth of 1250 m -determined by the gravity wave dispersion relation. Bullseye Vent is not a steadystate system! We observed a linear decrease in compliance of approximately 1% between October 2010 and May 2011, corresponding with an average change of 1% in the shear modulus over the depth of investigation. This corresponds to up to a 13% change solely in gas hydrate content in the first 100 m; a change that resulted from a simplified 1D four-layer model, which does not reflect the complexity and three-dimensionality of the hydrate system. A 13% change in hydrate content was likely an overestimate because the compliance function at 1250 m had limited sensitivity to the uppermost sediments.

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Section: Interpretation and Discussionmentioning

confidence: 99%

“…It is displaced only a few atomic radii by the vertical stress applied by the infragravity waves (Latychev and Edwards, 2003). The compliance function is defined as the frequency dependent transfer function between this stress and the resulting seafloor displacement.…”

Section: Seafloor Compliance Theorymentioning

confidence: 99%

A temporal trend in compliance measurements near a gas hydrate accumulation, Northern Cascadia

Roach

Edwards

2015

GEOPHYSICS

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“…It might be most reasonable to employ geophysical imaging methods sensitive to bulk physical properties, like the seafloor compliance method, to determining the average properties over a certain volume. The compliance “anomaly” over structures has the signature and resolution not unlike the corresponding gravity anomaly [ Latychev and Edwards , 2003]. …”

Section: Discussionmentioning

confidence: 99%

“… Latychev and Edwards [2003] investigated the effect of lateral variability of hydrate deposits on compliance data. They describe a 3‐D finite difference code, using control‐volume discretization to predict the compliance response for non‐layered structures.…”

Section: Three‐dimensional Numerical Finite Difference Modelsmentioning

confidence: 99%

Seafloor compliance imaging of marine gas hydrate deposits and cold vent structures

et al. 2008

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[1] Seafloor compliance, the transfer function between pressure induced by surface gravity waves and the associated seafloor deformation, can address outstanding questions about the nature of seismic blank zones, identified as cold vents offshore Vancouver Island, near Ocean Drilling Program (ODP) Site 889 and the recent Integrated Ocean Drilling Program (IODP) Expedition 311. Despite extensive seismic studies, it is not known whether free gas or gas hydrate is primarily responsible for the seismic blanking, nor are the concentration and distribution of either component conclusively determined. Here we use the formation's shear modulus to distinguish between gas hydrate, which can strongly alter the shear modulus, and free gas, which cannot. The depth-dependent gas hydrate distribution and pore-space saturation are estimated from the frequency variation of seafloor compliance measurements sensitive to the formation's shear modulus. Compliance data are consistent with pervasive moderate hydrate concentration over a broad region and significantly higher cylindrical concentrations associated with the cold vents, perhaps as much as 9 Â 10 6 m 3 in the 400 m diameter Bullseye vent, in agreement with controlled-source electromagnetic data. Compliance data can help distinguish between different models for the structure of such gas hydrate-bearing cold vents.Citation: Willoughby, E. C., K. Latychev, R. N. Edwards, K. Schwalenberg, and R. D. Hyndman (2008), Seafloor compliance imaging of marine gas hydrate deposits and cold vent structures,

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“…For most realistic subsurface models, the result is indistinguishable from that obtained using only the static terms. For 2‐D and 3‐D models, seafloor compliance has been calculated using finite difference approximations (Crawford et al 1998; Latychev & Edwards 2003).…”

Section: Seafloor Compliancementioning

confidence: 99%

The sensitivity of seafloor compliance measurements to sub-basalt sediments

Crawford¹

2004

Geophysical Journal International

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S U M M A R YWe investigate the sensitivity of seafloor compliance -the transfer function between the seafloor stress and displacement under ocean waves -to sub-basalt sediments. Seafloor compliance is sensitive to the subsurface shear modulus as a function of depth and is particularly sensitive to low shear modulus regions such as fluid-saturated sediments. We generate synthetic seafloor compliance data for different models of sediments beneath basalts, run geophysical inversions on the data and compare the inversion results with the input models. Our input models are variations on a reference model in which sub-basalt sediments start 3.5 km beneath the seafloor and have an average shear modulus of 6.8 × 10 9 Pa (shear wave velocity ca 1.7 km s −1 ). The sensitivity of seafloor compliance to sub-basalt sediments depends mostly on the water depth and the sub-basalt sediment layer depth, thickness and shear modulus. If the water depth is 1 km or more, seafloor compliance measurements will detect a sub-basalt sediment layer 0.6 km or more thick and will constrain the depth to the top and bottom of a 2-km thick sub-basalt sediment layer to within 0.2 km. If the water depth is 0.25 km, the thinnest detectable layer will be 1.2 km and the depth uncertainty for a 2-km thick sub-basalt sediment layer will be 0.5-0.8 km. Neither interlayered sediments within the basalts nor sediments above the basalt layer have a strong effect on the measurement sensitivity.

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On the compliance method and the assessment of three-dimensional seafloor gas hydrate deposits

Cited by 13 publications

References 28 publications

A temporal trend in compliance measurements near a gas hydrate accumulation, Northern Cascadia

A temporal trend in compliance measurements near a gas hydrate accumulation, Northern Cascadia

Seafloor compliance imaging of marine gas hydrate deposits and cold vent structures

The sensitivity of seafloor compliance measurements to sub-basalt sediments

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