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.

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.

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.

The presence of a gas hydrate reservoir and free gas layer along the South Shetland margin (offshore Antarctic Peninsula) has been well documented in recent years. In order to better characterize gas hydrate reservoirs, with a particular focus on the quantification of gas hydrate and free gas and the petrophysical properties of the subsurface, we performed travel time inversion of ocean-bottom seismometer data in order to obtain detailed P- and S-wave velocity estimates of the sediments. The P-wave velocity field is determined by the inversion of P-wave refractions and reflections, while the S-wave velocity field is obtained from converted-wave reflections received on the horizontal components of ocean-bottom seismometer data. The resulting velocity fields are used to estimate gas hydrate and free gas concentrations using a modified Biot‐Geertsma‐Smit theory. The results show that hydrate concentration ranges from 10% to 15% of total volume and free gas concentration is approximately 0.3% to 0.8% of total volume. The comparison of Poisson’s ratio with previous studies in this area indicates that the gas hydrate reservoir shows no significant regional variations.

No abstract

Fine-grained marine sediments along continental slopes and rises are saturated with gas rich fluids. The mechanisms of gas transport from the source toward the seafloor influence the sediment geomechanical properties as well as gas hydrate accumulations and seepage dynamics (e.g.,

Seafloor hydrocarbon seepage is a natural fluid release process that occurs worldwide on continental shelves, slopes, and in deep oceanic basins. The Vestnesa sedimentary ridge in the eastern Fram Strait hosts a deep-water gas hydrate system that became charged with hydrocarbons ∼2.7 Ma and has experienced episodic seepage along the entire ridge until a few thousand years ago, when seepage activity apparently ceased in the west but persisted in the east. Although it has been documented that faults and fractures play a key role in feeding the seeps with thermogenic gases, the mechanisms controlling seepage periodicity remain poorly understood. Here we integrate high-resolution P-cable 3D seismic and Chirp data to investigate the spatial and temporal evolution of high-resolution fractures and fluid flow features in the west of the Vestnesa Ridge. We characterize sediment deformation using a fracture density seismic attribute workflow revealing two highly deformed stratigraphic intervals and associated small-scale pockmarks (<20 m diameter). Chronostratigraphic constraints from the region show that these two highly deformed intervals are influenced by at least three major climatic and oceanic events during the last 1.2 million years: the Mid-Pleistocene Transition (∼1.25–0.7 Ma), the penultimate deglaciation (∼130 ka) and the last deglaciation (Heinrich Stadial 1: ∼16 ka). These periods of deformation appear associated with seismic anomalies potentially correlated with buried methane-derived authigenic carbonate and have been sensitive to shifts in the boundary of the free gas-gas hydrate interface. Our results show shifts (up to ∼30 m) in the depth of the base of the gas hydrate stability zone (GHSZ) associated with major changes in ocean bottom water temperatures. This ocean-driven effect on the base of the GHSZ since the Last Glacial Maximum coincides with the already highly deformed Mid-Pleistocene Transition sedimentary interval and likely enhanced deformation and gas leakage along the ridge. Our results have implications for understanding how glacial cycles impact fracture formation and associated seepage activity.

Short duration events (SDEs) are reported worldwide from ocean-bottom seismometers (OBSs). Due to their high frequency (4–30 Hz) and short duration, they are commonly attributed to aseismic sources, such as fluid migration related processes from cold seeps, biological signals, or noise. We present the results of a passive seismic experiment that deployed an OBS network for 10-month (October 2015–July 2016) at an active seepage site on Vestnesa Ridge, West Svalbard continental margin. We characterize SDEs and their temporal occurrence using the conventional short-time-average over long-time-average approach. Signal periodograms show that SDEs have periodic patterns related to solar and lunar cycles. A monthly correlation between SDE occurrences and modelled tides for the area indicates that tides have a partial control on SDEs recorded over 10 months. The numbers of SDEs increase close to the tidal minima and maxima, although a correlation with tidal highs appears more robust. Large bursts of SDEs are separated by interim quiet cycles. In contrast, the periodicity analysis of tremors shows a different pattern, likely caused by the effect of tidally controlled underwater currents on the instrumentation. We suggest that SDEs at Vestnesa Ridge may be related to the dynamics of the methane seepage system which is characterized by a complex interaction between migration of deep sourced fluids, gas hydrate formation and seafloor gas advection through cracks. Our observation from this investigated area offshore west-Svalbard, is in line with the documentation of SDEs from other continental margins, where micro-seismicity and gas release into the water column are seemingly connected.