Seismic characterization of hydrates in faulted, fine‐grained sediments of Krishna‐Godavari Basin: Full waveform inversion

Jaiswal, Priyank; Dewangan, P.; Ramprasad, T.; Zelt, C. A.

doi:10.1029/2012jb009201

Cited by 42 publications

(17 citation statements)

References 83 publications

(78 reference statements)

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“…This resolution can be significantly improved by applying full waveform inversion to the OBS data (Minshull & Singh, ; Singh et al, ; Westbrook et al, ; Xia et al, ). Depending on the type and the quality of the data set, several 1‐D (Korenaga et al, ; Pecher et al, ; Westbrook et al, ; Xia et al, ) and 2‐D full waveform inversion (Delescluse et al, ; Jaiswal et al, ; Wang et al, ) approaches have been used in the past to study the distribution of gas hydrates. In this study, OBS stations are not spaced closely enough to have overlap of raypaths in layers below the seafloor; hence, 1‐D full waveform inversion is more suitable for this data set.…”

Section: Methodsmentioning

confidence: 99%

“…A BSR only provides evidence for the presence of gas hydrates but does not allow for estimation of the amount of gas hydrates trapped in sediments. Gas hydrates fill pore space by forming a sediment‐hydrate microstructure strengthening the matrix of unconsolidated sediments that in turn increases the bulk modulus, thereby resulting in higher P wave seismic velocities compared to sediments not saturated with gas hydrates (Bünz et al, ; Chand et al, ; Helgerud et al, ; Jaiswal et al, ; Lee et al, ; Lee & Collett, ; Lu & McMechan, ; Stoll et al, ; Yuan et al, ). Hence, high P wave seismic velocities are observed above the BSR, whereas the presence of free gas in sediments below the BSR reduces the P wave velocity.…”

Section: Introductionmentioning

confidence: 99%

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Constraints on Gas Hydrate Distribution and Morphology in Vestnesa Ridge, Western Svalbard Margin, Using Multicomponent Ocean‐Bottom Seismic Data

2019

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Gas hydrates occur within sediments on the western Svalbard continental margin and the Vestnesa Ridge, a large sediment drift that extends in a west-northwest direction from the margin toward the mid-ocean ridge. We acquired multicomponent ocean-bottom seismic (OBS) data at 10 locations on the crest area of the eastern segment of the Vestnesa Ridge, an area with active gas seepage. P and S wave velocities are estimated using traveltime inversion, and self-consistent approximation/differential effective medium rock physics modeling is used to estimate gas hydrate and free gas saturation at OBS stations. We apply 1-D full waveform inversion at a selected OBS station to study detailed variations of P wave velocity near the bottom simulating reflection (BSR). High interval P wave velocity (Vp ≈ 1.73-1.82 km/s) and S wave velocity (>0.35 km/s) are observed in a layer above the BSR and low interval P wave velocity (Vp ≈ 1.28-1.53 km/s) in a layer below the BSR. We estimate 10-18% gas hydrate and 1.5-4.1% free gas saturation at different OBS stations in a layer above and below the BSR, respectively. We find significant variation in gas hydrate and free gas saturation across faults suggesting a structural control on the distribution of gas hydrate and free gas in the Vestnesa Ridge. Differences in gas hydrate saturation derived from P wave velocities and earlier estimates obtained from electromagnetic surveys indicate the presence of gas hydrates in faults and fractures. Moreover, beneath some OBS sites, the combined study of P and S waves, resistivity and seismic quality factor (Q), suggests the coexistence of free gas and gas hydrates.

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Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Constraints on Gas Hydrate Distribution and Morphology in Vestnesa Ridge, Western Svalbard Margin, Using Multicomponent Ocean‐Bottom Seismic Data

2019

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“…Because gas hydrate increases the stiffness of the matrix (Jung et al, 2012) and P-wave velocity, it was normally assumed that the sediments saturated with gas hydrates will show lower attenuation (Wood et al, 2000). Unlike P-wave velocity, no unique trend of seismic attenuation in gas hydrates can be observed from the literature; thus, making attenuation characteristic of the gas hydrate bearing sediments a debatable topic (Guerin et al, 1999;Wood et al, 2000;Chand et al, 2004;Rossi et al, 2007;Sain et al, 2009;Sain and Singh, 2011;Jaiswal et al, 2012;Dewangan et al, 2014). Laboratory experiments in hydrate bearing sediments indicated increase of attenuation with hydrate saturation (Priest et al, 2006;Best et al, 2013), whereas attenuation estimates from field experiments on gas hydrates indicated contradicting results.…”

Section: Introductionmentioning

confidence: 99%

Gas hydrate and free gas detection using seismic quality factor estimates from high-resolution P-cable 3D seismic data

et al. 2016

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We have estimated the seismic attenuation in gas hydrate and free-gas-bearing sediments from high-resolution P-cable 3D seismic data from the Vestnesa Ridge on the Arctic continental margin of Svalbard. P-cable data have a broad bandwidth (20-300 Hz), which is extremely advantageous in estimating seismic attenuation in a medium. The seismic quality factor (Q), the inverse of seismic attenuation, is estimated from the seismic data set using the centroid frequency shift and spectral ratio (SR) methods. The centroid frequency shift method establishes a relationship between the change in the centroid frequency of an amplitude spectrum and the Q value of a medium. The SR method estimates the Q value of a medium by studying the differential decay of different frequencies. The broad bandwidth and short offset characteristics of the P-cable data set are useful to continuously map the Q for different layers throughout the 3D seismic volume. The centroid frequency shift method is found to be relatively more stable than the SR method. Q values estimated using these two methods are in concordance with each other. The Q data document attenuation anomalies in the layers in the gas hydrate stability zone above the bottom-simulating reflection (BSR) and in the free gas zone below. Changes in the attenuation anomalies correlate with small-scale fault systems in the Vestnesa Ridge suggesting a strong structural control on the distribution of free gas and gas hydrates in the region. We argued that high and spatially limited Q anomalies in the layer above the BSR indicate the presence of gas hydrates in marine sediments in this setting. Hence, our workflow to analyze Q using high-resolution P-cable 3D seismic data with a large bandwidth could be a potential technique to detect and directly map the distribution of gas hydrates in marine sediments.

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“…The estimated hydrate concentration and the gas saturation for these CDPs are close to 20% and 10%, respectively. Interestingly, the velocity model obtained using unified imaging and waveform inversion [ Jaiswal et al ., , ] confirms the presence of free gas in the vicinity of CDP 450.…”

Section: Discussionmentioning

confidence: 98%

“…Anisotropy Due to Fracture-Filled Gas Hydrate [21] The distribution of gas hydrates is often governed by lithology; for example, they get preferentially deposited in the sandy layers at the Mallik site [Dallimore et al, 1999] and in the Nankai trough [Matsumoto et al, 2001] and within the coarse ash-layers in the Andaman basin [Collett et al, 2008]. The gas hydrate accumulation may also be governed by structure controls; for example, hydrate gets preferentially deposited in the fault/fracture network in the KG offshore basin [Collett et al, 2008;Dewangan et al, 2011;Jaiswal et al, 2012aJaiswal et al, , 2012b. Such fracture-filled deposits may exhibit significant lateral heterogeneity.…”

Section: Ava Pattern Of Bsr Using Tpbe Modelmentioning

confidence: 99%

Anisotropic amplitude variation of the bottom‐simulating reflector beneath fracture‐filled gas hydrate deposit

et al. 2013

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[1] For the first time, we report the amplitude variation with angle (AVA) pattern of bottom-simulating reflectors (BSRs) beneath fracture-filled gas hydrate deposits when the effective medium is anisotropic. The common depth point (CDP) gathers of two mutually perpendicular multichannel seismic profiles, located in the vicinity of Site NGHP-01-10, are appropriately processed such that they are fit for AVA analysis. AVA analysis of the BSR shows normal-incidence reflection coefficients of À0.04 to À0.11 with positive gradients of 0.04 to 0.31 indicating class IV pattern. The acoustic properties from isotropic rock physics model predict class III AVA pattern which cannot explain the observed class IV AVA pattern in Krishna-Godavari basin due to the anisotropic nature of fracture-filled gas hydrate deposits. We modeled the observed class IV AVA of the BSR by assuming that the gas hydrate bearing sediment can be represented by horizontally transversely isotropic (HTI) medium after accounting for anisotropic wave propagation effects on BSR amplitudes. The effective medium properties are estimated using Backus averaging technique, and the AVA pattern of BSRs is modeled using the properties of overlying HTI and underlying isotropy/HTI media with or without free gas. Anisotropic AVA analysis of the BSR from the inline seismic profile shows 5-30% gas hydrate concentration (equivalent to fracture density) and the azimuth of fracture system (fracture orientation) with respect to the seismic profile is close to 45 . Free gas below the base of gas hydrate stability zone is interpreted in the vicinity of fault system (F1).Citation: Sriram, G., P. Dewangan, T. Ramprasad, and P. Rama Rao (2013), Anisotropic amplitude variation of the bottomsimulating reflector beneath fracture-filled gas hydrate deposit,

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Seismic characterization of hydrates in faulted, fine‐grained sediments of Krishna‐Godavari Basin: Full waveform inversion

Cited by 42 publications

References 83 publications

Constraints on Gas Hydrate Distribution and Morphology in Vestnesa Ridge, Western Svalbard Margin, Using Multicomponent Ocean‐Bottom Seismic Data

Constraints on Gas Hydrate Distribution and Morphology in Vestnesa Ridge, Western Svalbard Margin, Using Multicomponent Ocean‐Bottom Seismic Data

Gas hydrate and free gas detection using seismic quality factor estimates from high-resolution P-cable 3D seismic data

Anisotropic amplitude variation of the bottom‐simulating reflector beneath fracture‐filled gas hydrate deposit

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