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

Abstract

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.

Detection of Gas Hydrates in Faults Using Azimuthal Seismic Velocity Analysis, Vestnesa Ridge, W‐Svalbard Margin

“…In survey1, higher seismic velocities obtained in one half of azimuths toward the northeast (~315°–135°) in layers in the GHSZ (L1 to L5; Figure ) are consistent with higher concentrations of hydrates toward the ridge crest as reported based on multi‐channel seismic, 2‐D OBS surveys and 3‐D seismic attenuation studies (Hustoft et al, ; Singhroha et al, ; Singhroha et al, ). However, elevated (~0.03‐0.04 km/s) seismic velocities in layer L5 toward the downslope (southwest) side of Fault1 compared to seismic velocities in the shadow zone (marked as S in Figure ), that is, toward the upslope (northeast) side of Fault1 show the effect of faults on the distribution of gas hydrates in the region (Figure ).…”

“…This upslope migration of fluid and gases can be potentially obstructed by Fault1 and Fault2, thus leading to decreased gas hydrate saturations in the shadow zones due to the lack of availability of gases to form hydrates. Earlier estimates also show decreased gas hydrate saturations in the northeast direction from Fault2 (Singhroha et al, ). This theory is further concurred by higher seismic velocities in the free gas zone (layer LL6 in survey2) potentially suggesting lower free gas saturations in the northeastern half (i.e., in the upslope direction to the fault; Figures b and c).…”

“…In survey1 and survey2, a seismic velocity increase (up to ~0.06–0.08 km/s in layers L5 and LL5) in layers in the GHSZ with azimuths corresponding to fault orientations compared to other azimuths (Figures ) suggests a preferential accumulation of hydrates within faults. A seismic velocity decrease (~0.04 km/s in layer L6 and ~0.06–0.08 km/s in layer LL6) within the free gas zone along the fault direction compared to other directions (Figure ) further proves the role of faults in directing gas migration toward the GHSZ (Bünz et al, ; Plaza‐Faverola et al, ;Singhroha et al, ; Singhroha et al, ). In addition, near the BSR depth in survey2, increase (~0.06‐0.08 km/s) and decrease (~0.06‐0.08 km/s) of seismic velocities occur between fault planes above and below the BSR, respectively (Figures b and 1c).…”

“…We use high‐resolution 3‐D P‐Cable seismic data (Plaza‐Faverola et al, ; Figure c) and OBS data (Singhroha et al, ) acquired in this area to get a starting model for azimuthal seismic velocity analysis. Seismic velocity models estimated using OBS data (Singhroha et al, ) are used to convert the 3‐D P‐Cable seismic data in the depth domain. Two‐dimensional seismic velocity models for different azimuths are then extracted from the depth‐converted 3‐D seismic data.…”

“…The presence of a gas hydrate system in Vestnesa Ridge is well documented (Bünz et al, 2012;Goswami et al, 2015;Hustoft et al, 2009;Petersen et al, 2010;Singhroha et al, 2019;Smith et al, 2014;Vogt et al, 1994). Seafloor pockmarks and gas chimneys along the ridge are linked to faults and fractures (Plaza-Faverola et al, 2015;Singhroha et al, 2016).…”