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

Abstract

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.

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 ).…”

“…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).…”

“…This potentially shows the control of faults on the upslope migration of fluids. Gases and other fluids trapped below impermeable gas hydrate saturated sediments in a layer below the base of the GHSZ move upslope (within the same layer below the GHSZ) toward the ridge crest from deeper (>2 km) depths (Bünz et al, ; Hustoft et al, ; Singhroha et al, ). 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.…”

“…The presence of a gas hydrate system in Vestnesa Ridge is well documented (Bünz et al, ; Goswami et al, ; Hustoft et al, ; Petersen et al, ; Singhroha et al, ; Smith et al, ; Vogt et al, ). Seafloor pockmarks and gas chimneys along the ridge are linked to faults and fractures (Plaza‐Faverola et al, ; Singhroha et al, ). Considering the suggested presence of thermogenic gas (Panieri et al, ; Plaza‐Faverola et al, ; Smith et al, ) in the study area, the study of fault/fracture systems is even more important as thermogenic gases have deeper origins (often below the GHSZ) and structural features affect the migration pathways of thermogenic gases.…”

Frenulate siboglinids at high Arctic methane seeps and insight into high latitude frenulate distribution

“…From a geological perspective, Vestnesa is significant; it is close to an ultraslow spreading center where hydrothermal vents exist (Pedersen et al, ; Sweetman, Levin, Rapp, & Schander, ), and tectonic stress from rifting of ridges and shear motion from transform faults appear to modulate seafloor gas release (Plaza‐Faverola et al, ). Therefore, Vestnesa has been intensively studied from a geological and geophysical perspective (e.g., Bünz et al, ; Consolaro et al, ; Hansen, Hoff, Sztybor, & Rasmussen, ; Plaza‐Faverola et al, ; Singhroha, Bünz, Plaza‐Faverola, & Chand, ; Sztybor & Rasmussen, ), while biological studies of living faunas are limited (e.g., Åström et al, ). Any higher‐order understanding of the ecology of Vestnesa seeps requires knowledge of the dominant community members, particularly since frenulates can be considered ecosystem engineers due to their influence on the biology as well as the physical characteristics of their habitats (Dando, Southward, Southward, Lamont, & Harvey, ; Sen, Åström, et al, ; Sen et al, ).…”