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

Abstract

Joint analysis of electrical resistivity and seismic velocity data is primarily used to detect the presence of gas hydrate‐filled faults and fractures. In this study, we present a novel approach to infer the occurrence of structurally controlled gas hydrate accumulations using azimuthal seismic velocity analysis. We perform this analysis using ocean‐bottom seismic data at two sites on Vestnesa Ridge, W‐Svalbard Margin. Previous geophysical studies inferred the presence of gas hydrates at shallow depths (up to ~190–195 m below the seafloor) in marine sediments of Vestnesa Ridge. We analyze azimuthal P‐wave seismic velocities in relation with steeply dipping near‐surface faults to study structural controls on gas hydrate distribution. This unique analysis documents directional changes in seismic velocities along and across faults. P‐wave velocities are elevated and reduced by ~0.06–0.08 km/s in azimuths where the raypath plane lies along the fault plane in the gas hydrate stability zone (GHSZ) and below the base of the GHSZ, respectively. The resulting velocities can be explained with the presence of gas hydrate‐ and free gas‐filled faults above and below the base of the GHSZ, respectively. Moreover, the occurrence of elevated and reduced (>0.05 km/s) seismic velocities in groups of azimuths bounded by faults suggests compartmentalization of gas hydrates and free gas by fault planes. Results from gas hydrate saturation modeling suggest that these observed changes in seismic velocities with azimuth can be due to gas hydrate saturated faults of thickness greater than 20 cm and considerably smaller than 300 cm.

Diagenesis of Magnetic Minerals in Active/Relict Methane Seep: Constraints From Rock Magnetism and Mineralogical Records From Bay of Bengal

“…We put forward following plausible mechanisms to explain the formation and preservation of greigite in Z-I and Z-II: 1) decline in methane flux either due to massive hydrate accumulation (Mazumdar et al, 2019;Dewangan et al, 2020) or hindering of upward migrating fluid/gas by the carbonate layers in the subsurface sediments most likely resulted in low sulfide production and preferentially favoured formation and preservation of greigite in the Z-I. Similar observations were reported in fault-controlled cold seep-hydrate Woolsey Mound in the Northern Gulf of Mexico (Simonetti et al, 2013), gas hydrate field offshore Vancouver Island (Riedel et al, 2002), Vestnesa Ridge, W-Svalbard Margin (Singhroha et al, 2020), and in the K-G basin (Badesab et al, 2017;Dewangan et al, 2011). They explained that plugging of an active fault system due to massive hydrate formation can also cause significant drop in methane flux.…”

Thermal Characterization of Pockmarks Across Vestnesa and Svyatogor Ridges, Offshore Svalbard

“…A very similar P wave velocity depth structure was defined by Goswami et al (2015) in a profile along line JR211_17 (Figure 7b) across Vestnesa Ridge and the Lunde pockmark using traveltime inversion of reflection data obtained with two OBS instruments. New and more detailed results of the P wave velocity structure beneath the crest of Vestnesa Ridge across the pockmark chain were obtained by Singhroha et al (2019, 2020). In general, the depth profile of velocity is similar to the previous work, but some local anomalies with P wave velocities as high as 1900 m/s above the BSR were found.…”

“…In general, the depth profile of velocity is similar to the previous work, but some local anomalies with P wave velocities as high as 1900 m/s above the BSR were found. P wave velocity anisotropy induced by gas hydrate‐bearing faults was also studied (Singhroha et al, 2020) and velocity can be up to 5% higher (~80 m/s) for seismic rays that cross a fault plane perpendicular. However, given these studies as well as the general nature of sediments and their depositional character as contourite deposits along and across the ridge makes a drastically higher velocity to deepen the BSR depth from seismic travel time observations rather unrealistic.…”

“…In general, the depth profile of velocity is similar to the previous work, but some local anomalies with P wave velocities as high as 1900 m/s above the BSR were found. P wave velocity anisotropy induced by gas hydrate-bearing faults was also studied (Singhroha et al, 2020) and velocity can be up to 5% higher (~80 m/s) for seismic rays that cross a fault plane perpendicular. However, given A pure methane (CH 4 ) system (blue line) results in the shallowest base of hydrate stability (sI) for any thermal gradient.…”

Origin and Periodic Behavior of Short Duration Signals Recorded by Seismometers at Vestnesa Ridge, an Active Seepage Site on the West-Svalbard Continental Margin

“…Gas hydrate is not the main controller of the seepage at the site, however. Systems of faults and fractures control the underlying fluid migration pathways and chimneys, and thus, the distribution of pockmarks (Figure 1C) (Plaza-Faverola et al, 2015;Singhroha et al, 2019Singhroha et al, , 2020. The spatial and temporal distribution of seepage features along the sedimentary ridge has been linked to dynamic forcing from mid-ocean ridge spreading and from glacial isostatic rebound (Schneider et al, 2018;Himmler et al, 2019;Plaza-Faverola and Keiding, 2019).…”