Vulcan: A deep-towed CSEM receiver

TitleVulcan: A deep-towed CSEM receiver
Publication TypeJournal Article
Year of Publication2016
AuthorsConstable S, Kannberg P.K, Weitemeyer K.
JournalGeochemistry Geophysics Geosystems
Date Published2016/03
Type of ArticleArticle
ISBN Number1525-2027
Accession NumberWOS:000375144700023
Keywordsconductivity; field; Gas hydrate; hydrate; instruments and techniques; inversion; marine csem; Marine electromagnetics; new-zealand; ridge; sea-floor; source electromagnetic data; system

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 degrees in phase and vertical fields to about 5% in amplitude and 5 degrees 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 m) and position within the gas hydrate stability field, we interpret this to be massive methane hydrate.

Short TitleGeochem. Geophys. Geosyst.
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