Bottom‐simulating reflectors: Seismic velocities and AVO effects

Carcione, José M.; Tinivella, Umberta

doi:10.1190/1.1444725

Cited by 200 publications

(174 citation statements)

References 19 publications

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“…The second approach has been to use theoretical effective medium models to predict the physical properties of bulk sediment from the elastic properties of individual constituents. Differing models have been developed to account for the inclusion of hydrate within sediment using modified versions of the Gassmann equation [Ecker et al, 1998;Helgerud et al, 1999], modified versions of the Biot equation [Carcione and Tinivella, 2000;Gei and Carcione, 2003] and a differential effective medium approach (DEM) [Jakobsen et al, 2000]. The model outputs depend on whether the relationship between hydrate and sediment is defined either as hydrate cement at grain contacts; uniform hydrate cementation of individual grains leading to cementation between grains; hydrate acting as a mineral grain supporting the sediment frame; or hydrate growing wholly within the pore fluid and not interacting with the sediment frame.…”

Section: Hydrate Quantificationmentioning

confidence: 99%

A laboratory investigation into the seismic velocities of methane gas hydrate‐bearing sand

2005

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[1] Remote seismic methods, which measure the compressional wave (P wave) velocity (V p ) and shear wave (S wave) velocity (V s ), can be used to assess the distribution and concentration of marine gas hydrates in situ. However, interpreting seismic data requires an understanding of the seismic properties of hydrate-bearing sediments, which has proved problematic because of difficulties in recovering intact hydrate-bearing sediment samples and in performing valid laboratory tests. Therefore a dedicated gas hydrate resonant column (GHRC) was developed to allow pressure and temperature conditions suitable for hydrate formation to be applied to a specimen with subsequent measurement of both V p and V s made at frequencies and strains relevant to marine seismic investigations. Thirteen sand specimens containing differing amounts of evenly dispersed hydrate were tested. The results show a bipartite relationship between velocities and hydrate pore saturation, with a marked transition between 3 and 5% hydrate pore saturation for both V p and V s . This suggests that methane hydrate initially cements sand grain contacts then infills the pore space. These results show in detail for the first time, using a resonant column, how hydrate cementation affects elastic wave properties in quartz sand. This information is valuable for validating theoretical models relating seismic wave propagation in marine sediments to hydrate pore saturation.Citation: Priest, J. A., A. I. Best, and C. R. I. Clayton (2005), A laboratory investigation into the seismic velocities of methane gas hydrate-bearing sand,

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Section: Hydrate Quantificationmentioning

confidence: 99%

A laboratory investigation into the seismic velocities of methane gas hydrate‐bearing sand

2005

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“…Yuan et al, 1996;Wood et al, 1941), and using weighted time averaging equations (Lee et al, 1993), (b) rockphysics modeling (e.g. effective medium theory by Helgerud et al, 1999; cementation model by Dvorkin and Nur, 1993, Biot-stoll model by Carcione and Tinivella, 2000), and (c) using empirical porosity-velocity functions and the effective porosity reduction model (e.g. Jarrad et al, 1995;Hyndman et al, 1993;Yuan et al, 1996).…”

Section: Velocity Versus Hydrate Concentrationmentioning

confidence: 99%

Gas hydrate concentration estimates from chlorinity, electrical resistivity and seismic velocity

Riedel

Collett²,

Hyndman

2005

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Gas hydrate beneath the N. Cascadia continental slope off Vancouver Island occurs as a regional diffuse layer above the BSR and as local high concentrations in large vent or upwelling structures. Regional concentrations of gas hydrate beneath the N. Cascadia continental slope off Vancouver Island have been estimated earlier using multichannel seismic, seafloor electrical, and IODP Leg 146 downhole data. The concentrations of between 15 and 30% of pore saturation in a 100 m thick layer above the BSR are much higher than estimated elsewhere where there is good data, especially the Blake Ridge and central Cascadia off Oregon on ODP Leg 204. Although both of these other studies involved different sediment environments, a careful re-evaluation of the N. Cascadia estimates seemed desirable. We have re-evaluated the methods used to calculate the gas hydrate concentrations from pore-water chlorinity (salinity), electrical resistivity, and seismic velocity, describing in detail the assumptions and uncertainties. Use of the porewater chlorinity/salinity and electrical resistivity directly have low reliability because of the effect on the no-hydrate reference of hydrate formation and dissociation, and the effect of pore fluid freshening by clay dehydration. At ODP Site 889/890 hydrate concentrations range from 5-10% to 30-40%, depending on the no-hydrate reference salinity used. Use of core salinity data along with the downhole and seafloor electrical resistivity data allows calculation of both the in situ reference salinity and the hydrate concentrations. The most important uncertainty in this method is the relation between 2 resistivity and porosity, i.e., Archie's Law parameters. Significantly different relations were determined from the ODP Leg 146 core and downhole log data, the log data resistivity-porosity relation giving much lower concentrations. Finally, seismic velocities from sonic-logs and multichannel data can be used to calculate gas hydrate concentrations, if an appropriate no-hydrate velocity-depth profile can be estimated. A velocity-hydrate concentration relation is also required. Depending on which nohydrate/no-gas velocity baseline is used, estimated hydrate concentrations range from as low as 5% to above 25% saturation. In spite of having three nearly independent methods of estimating hydrate concentrations, it is concluded that the data allow regional concentrations in the 100 m layer above the BSR from less than 5% to over 25% saturation (3-13% of sediment volume). ODP drilling in the region scheduled for the fall of 2005 should help resolve the uncertainties.

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“…The presence of a BSR remains the primary proxy for gas hydrate presence in the absence of direct observations in spite of large uncertainties in determining hydrate and gas concentration from seismic data. These uncertainties result from uncertainties about the velocity structure of comparable sediments in the absence of hydrate and free gas [Yuan et al, 1996], from pressure dependence and nonlinear sensitivity of seismic parameters to gas and hydrate concentrations [e.g., Carcione and Tinivella, 2000;Han and Batzle, 2002], from uncertainties in estimating in situ concentrations in those few places where seismic estimates can be ground-truthed by direct sampling [e.g., Paull and Ussler, 2001], and from the fact that gas hydrate has been found in cores obtained from sites where no BSR is observed [e.g., Kvenvolden and Kastner, 1990;Paull et al, 1996].…”

Section: Introductionmentioning

confidence: 99%

Seismic and seafloor evidence for free gas, gas hydrates, and fluid seeps on the transform margin offshore Cape Mendocino

et al. 2003

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[1] Seismic data and seafloor samples indicate the presence of free gas, gas hydrate, and fluid seeps south of the Gorda Escarpment, a topographic feature that marks the eastern end of the Gorda/Pacific transform plate boundary southwest of Cape Mendocino, California. In spite of high sedimentation rates and high biological productivity, direct or indirect indicators of gas hydrate presence had not previously been recognized in this region, or along transform margins in general. Gas is indicated by a bottom simulating reflection (BSR) observed near the Gorda Escarpment, by ''bright spots'' and ''gas curtains'' scattered throughout the sedimentary basin to the south, and by d C and d18 O isotopes of carbonates, which are similar to those recovered from other hydrate-bearing regions. The BSR reflection coefficient of À0.13 ± 0.04 and interval velocities as low as 1.38 km/s indicate that free gas is present beneath the BSR. Local shallowing of the BSR toward the north facing Gorda Escarpment and beneath a channel near the crest suggests fluid flow toward the seafloor. Integrating these various observations, we suggest a scenario in which methane is formed in thick Miocene and Pliocene deposits of organicrich sediments that fill the marginal basin south of the transform fault. Dissolved and free gas migrates toward the escarpment along stratigraphic horizons, resulting in hydrate formation and in channels, slumps and chemosynthetic communities on the face of the escarpment. We conclude that the BSR appears where hydrate-bearing sediments are uplifted because of current triple junction tectonics.

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Bottom‐simulating reflectors: Seismic velocities and AVO effects

Cited by 200 publications

References 19 publications

A laboratory investigation into the seismic velocities of methane gas hydrate‐bearing sand

A laboratory investigation into the seismic velocities of methane gas hydrate‐bearing sand

Gas hydrate concentration estimates from chlorinity, electrical resistivity and seismic velocity

Seismic and seafloor evidence for free gas, gas hydrates, and fluid seeps on the transform margin offshore Cape Mendocino

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