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

We acquired coincident marine controlled source electromagnetic (CSEM), high-resolution seismic reflection and ocean-bottom seismometer(OBS) data over an active pockmarkin 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 chimneysimaged by seismic reflection data beneath the pockmarkand 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 chimneybeneath the pockmarkassuming a nonuniform 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 chimneyassuming a nonuniform and uniform fluid distribution, respectively. High levels of free-gas saturation in the top part of the chimneyare consistent with episodic gas venting from the pockmark.