A joint electromagnetic and seismic study of Arctic hydrates and fluid escape features, offshore Svalbard

Abstract

The west Svalbard continental margin has been in the spotlight since 2008, when more than 200 active methane seeps were reported near the landward edge of the gas hydrate stability zone (GHSZ). Several additional seeps have since been reported in the continental shelf area, some of which are likely to be associated with shallow hydrate dissociation and ocean warming. In addition, active seeps and seafloor pockmarks were also reported along the crest of the deep water Vestnesa Ridge area, which is also a known area of gas hydrate presence. However, the seeps at the ridge are unlikely to be related to ocean warming. Nevertheless, improved estimates of the amount of methane trapped within hydrate and gas in the west Svalbard continental margin are necessary to evaluate seafloor methane fluxes and its impact on future climate.
Hydrate presence in the two study areas, the Vestnesa Ridge and the continental slope of the margin were traditionally inferred from the presence of bottom simulating reflectors (BSR) in seismic reflection data and high seismic velocity anomalies observed within the GHSZ. Bulk resistivity estimates obtained from marine controlled source electromagnetic (CSEM) data are highly sensitive to the presence of both hydrate and free gas within the sediment pore spaces. The presence of hydrate leads to an increase in the bulk resistivity and seismic velocity, whereas the presence of free gas leads to an increase in bulk resistivity but decrease in seismic velocity. These complementary attributes, combined with seismic reflection data provide a unique opportunity to obtain improved constraints on hydrate and free gas saturations in the margin. Therefore, CSEM data were acquired in 2012 at the two locations, to complement coincident seismic reflection and seismic refraction data.
The CSEM data were acquired using a 100 m long horizontal electric dipole antenna that transmitted a 81 A, 1 Hz pseudo-square wave current. It was recorded using two sets of CSEM receivers: ocean bottom electric-field (OBE) sensors recorded the long offsets (850-3000 m), whereas a towed receiver, Vulcan recorded the data at a constant offset of 300 m. Synthetic model studies presented here suggest a maximum depth sensitivity of 200 m for Vulcan and about 1.5 km for the OBEs.
Resistivity models obtained from the inversion of CSEM data show high resistivities (2-30 ?m) within the GHSZs of the Vestnesa Ridge and continental slope sediments. The high resistivities and presence of BSRs provide further evidence for the presence of hydrates within the west Svalbard margin. Significant heterogeneity in hydrate distribution can also be inferred across the margin, based on lateral resistivity variation in these models.
The Vestnesa Ridge contains several pockmarks along the ridge crest. Periodic gas re-lease are reported through these pockmarks, based on observations of high intensity flares near the seabed on hydro-acoustic data. Fluid escape through the GHSZ to the seafloor pockmarks were also suggested by previous studies, which is supported by the observations of coincident high resistivity (from CSEM), high velocity (from seismic refraction) and acoustic chimneys (from seismic reflection). In addition, the observations of coincident high resistivity features and high amplitude reflectors at the continental shelf edge of the margin and beneath the crest of the Vestnesa Ridge suggests the presence of free gas outside the GHSZ of the west Svalbard margin.
A joint resistivity and velocity analysis shows the presence of high hydrate (up to 73%) and gas saturations (up to 28%) within a fluid flow feature beneath a pockmark at the Vestnesa Ridge. Variation in resistivity and seismic reflection characteristics beneath different pockmarks indicate differences in fluid composition within each chimney feature. At the continental slope area, saturation estimates based on resistivity models, using modified Archie’s Law suggest highest hydrate saturations (around 40%) within the lower slope sediments. A BSR is also observed within these sediments, whereas the BSR is absent within sediments beneath water depths shallower than 700 m. Based on high resistivities observed within the upper slope sediments, around 20% hydrate is estimated within the GHSZ and around 30% free gas is estimated at the edge of the continental shelf.

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ORCID for Tim Minshull: orcid.org/0000-0002-8202-1379

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