Vulcan: A deep‐towed CSEM receiver

Abstract

We have developed a three‐axis electric field receiver designed to be towed behind a marine electromagnetic transmitter for the purpose of mapping the electrical resistivity in the upper 1000 m of seafloor geology. By careful adjustment of buoyancy and the use of real‐time monitoring of depth and altitude, we are able to deep‐tow multiple receivers on arrays up to 1200 m long within 50 m of the seafloor, thereby obtaining good coupling to geology. The rigid body of the receiver is designed to reduce noise associated with lateral motion of flexible antennas during towing, and allows the measurement of the vertical electric field component, which modeling shows to be particularly sensitive to near‐seafloor resistivity variations. The positions and orientations of the receivers are continuously measured, and realistic estimates of positioning errors can be used to build an error model for the data. During a test in the San Diego Trough, offshore California, inversions of the data were able to fit amplitude and phase of horizontal electric fields at three frequencies on three receivers to about 1% in amplitude and 1° in phase and vertical fields to about 5% in amplitude and 5° in phase. The geological target of the tests was a known cold seep and methane vent in 1000 m water depth, which inversions show to be associated with a 1 km wide resistor at a depth between 50 and 150 m below seafloor. Given the high resistivity (30 Ωm) and position within the gas hydrate stability field, we interpret this to be massive methane hydrate.

Resistivity image beneath an area of active methane seeps in the west Svalbard continental slope

“…The CSEM profiles were acquired using a CSEM transmitterdeep-towed active source instrument (DASI; Sinha et al 1990), a deep-towed tri-axis electric-field receiver-Vulcan (Weitemeyer & Constable 2010;Constable et al 2016) and 14 ocean-bottom electric-field (OBE) sensors (Minshull et al 2005;Fig. 2).…”

“…Schwalenberg et al 2005;Weitemeyer et al 2006a;Schwalenberg et al 2010a,b;Weitemeyer & Constable 2010;Weitemeyer et al 2011;Goswami et al 2015;Attias et al 2016). It was also used commercially in Japan for hydrate exploration (referenced within Constable et al (2016)). In this paper, 2-D resistivity cross-sections obtained from inversion of CSEM data are presented for two lines acquired in the area of methane seeps, on the west Svalbard continental slope (Fig.…”

Marine Mineral Exploration With Controlled Source Electromagnetics at the TAG Hydrothermal Field, 26°N Mid‐Atlantic Ridge

“…The CSEM system presented here is comprised of the Deep‐towed Active Source Instrument (DASI; Sinha et al, ) and two three‐axis electric field receivers (Vulcan; Constable et al, ) towed at 350 and 505 m behind DASI. The DASI‐Vulcan array was towed between 50 and 150 m above the seafloor at a constant speed of 1.852 km/hr.…”

A first application of a marine inductive source electromagnetic configuration with remote electric dipole receivers: Palinuro Seamount, Tyrrhenian Sea

“…Marine CSEM systems typically employ a ship-towed electric dipole transmitter and electric dipole receivers placed on the seafloor. Marine CSEM methods have been successfully applied in detecting and characterizing SMS deposits and other small targets (Wolfgram et al 1986;Cairns, Evans and Edwards 1996), oil and gas reserves (see Constable 2010 for a review) and gas hydrates (Yuan and Edwards 2000;Weitemeyer, Constable and Key 2006a;Schwalenberg et al 2010a, b;Hölz et al 2015b;Swidinsky, Hölz and Jegen 2015;Constable, Kannberg and Weitemeyer 2016).…”