A joint electromagnetic and seismic study of an active pockmark within the hydrate stability field at the Vestnesa Ridge, West Svalbard margin

• Published in: Journal of Geophysical Research: Solid Earth
• Main Authors: Goswami, Bedanta K., Weitemeyer, Karen A., Minshull, Timothy A., Sinha, Martin C., Westbrook, Graham, Chabert, Anne, Henstock, Timothy J., Ker, Stephan
• Format: Article in Journal/Newspaper
• Language: English
• Published: Amer Geophysical Union 2015
• Subjects: gas hydrate; Svalbard; CSEM; saturation; pockmark; Vestnesa; The Chimney; Svalbard margin
• Online Access: https://archimer.ifremer.fr/doc/00283/39458/37928.pdf; https://doi.org/10.1002/2015JB012344; https://archimer.ifremer.fr/doc/00283/39458/

We acquired coincident marine controlled-source electromagnetic (CSEM), high-resolution seismic reflection and ocean-bottom seismometer (OBS) data over an active pockmark in the crest of the southern part of the Vestnesa Ridge, to estimate fluid composition within an underlying fluid-migration chimney. Synthetic model studies suggest resistivity obtained from CSEM data can resolve gas or hydrate saturation greater than 5% within the chimney. Acoustic chimneys imaged by seismic reflection data beneath the pockmark and on the ridge flanks, were found to be associated with high-resistivity anomalies (+2-4 m). High-velocity anomalies (+0.3 km/s), within the gas hydrate stability zone (GHSZ) and low-velocity anomalies (-0.2 km/s) underlying the GHSZ, were also observed. Joint analysis of the resistivity and velocity anomaly indicates pore saturation of up to 52% hydrate with 28% free gas, or up to 73% hydrate with 4% free gas, within the chimney beneath the pockmark assuming a non-uniform and uniform fluid distribution respectively. Similarly, we estimate up to 30% hydrate with 4% free gas or 30% hydrate with 2% free gas within the pore space of the GHSZ outside the central chimney assuming a non-uniform and uniform fluid distribution respectively. High levels of free-gas saturation in the top part of the chimney are consistent with episodic gas venting from the pockmark.