First results from a marine controlled‐source electromagnetic survey to detect gas hydrates offshore Oregon

Abstract

Submarine gas hydrate is a hazard to drilling, a potential hydrocarbon resource, and has been implicated as a factor in both submarine slope stability and climate change. Bulk in situ electrical resistivities evaluated from electromagnetic surveys have the potential to provide an estimate of the total hydrate volume fraction more directly than by using seismic and well log data. We conducted a marine controlled‐source electromagnetic sounding at Hydrate Ridge, Oregon, USA, in August, 2004. Electromagnetic fields transmitted by a deep‐towed horizontal electric dipole source were measured by a linear array of 25 seafloor electromagnetic receivers, positioned 600 m apart to produce a dense coverage in the recorded electric field data. Results are presented in simple form by apparent resistivity pseudosections, which produce an approximate image of lateral resistivity variations across the study region. Resistivity values are consistent with those from well logs collected in the area and pseudosection features are correlated with seismic reflectors. Archie's Law, based on pseudosection apparent resistivities, predicts volumetric hydrate concentrations vary from 0–30% across the ridge.

Electrical Resistivity Investigation of Gas Hydrate Distribution in Mississippi Canyon Block 118, Gulf of Mexico

“…Receivers consist of three-component fluxgate magnetometers and two component electrode dipoles. Because of the extensive experience with CSEM in the marine environment, it was the natural choice for marine hydrate studies (Weitemeyer et al, 2006). In the largest-scale implementations, the source dipole is towed through the water near the bottom, while the receiver stations are left at fixed positions on the seafloor.…”

On electric fields produced by inductive sources on the seafloor

“…In a marine setting, electromagnetic systems have been applied successfully in exploring for oil and gas reserves (see Constable 2010 for a review), gas hydrates (Yuan and Edwards, 2000;Weitemeyer et al, 2006;Schwalenberg et al, 2010a,b;Constable et al, 2016), and submarine massive sulfide (SMS) deposits (Cairns et al, 1996;Kowalczyk, 2008;Lipton, 2008;Kowalczyk et al, 2015;Hölz et al, 2015;Mueller et al, 2016). SMS deposits contain valuable resources of Cu, Pb, Zn, Au, and Ag (Herzig, 1999), and some of these deposits, such as the Solwara 1 deposit in the Bismark Sea (Lipton, 2008), may have potential to be economically mined.…”

“…Controlled source electromagnetic (CSEM) methods have already been applied in investigating marine gas hydrates, both for hazard mitigation purposes and as a potentially economic resource of natural gas (Yuan and Edwards, 2000;Weitemeyer et al, 2006;Schwalenberg et al, 2010a,b;Constable et al, 2016).…”

Rapid resistivity imaging for marine controlled-source electromagnetic surveys with two transmitter polarizations: An application to the North Alex mud volcano, West Nile Delta

“…The recorded electromagnetic fields are functions of the seafloor resistivity structure. This approach is suitable for large-scale targets, such as conventional petroleum reservoirs, but we felt that the relatively small scale of North Alex (which is approximately 1000 m in diameter) would make such a survey difficult to perform (note however that Weitemeyer et al [2006] successfully use this method to map even smaller scale shallow marine gas hydrates, and therefore the approach would not have been logistically impossible for North Alex). An alternative technique for small-scale targets, as suggested in theory by Edwards (1997) and successfully performed in the field by Schwalenberg et al (2005) for gas hydrate exploration, is to tow the TX and RXs along the seabed on a single cable.…”