Using CSEM techniques to map the shallow section of seafloor: From the coastline to the edges of the continental slope

Abstract

Many important processes occur within the shallow section of the seafloor on the continental shelf and slope, yet conventional geophysical constraints on the physical properties within this critical boundary layer are limited. Some of the key constraints involve quantification of fluids within the seafloor, which can be provided by electrical methods. This paper reviews the application of a towed EM system to map the uppermost 20 m of seafloor in a variety of settings ranging from nearshore regions in water depths of approximately 10 m on the continental shelf out to water depths of 1300 m. The system is a mapping tool that provides areal maps of seafloor resistivity and has been used for a variety of purposes, including sedimentary characterization and facies mapping, evaluation of groundwater discharge, and mapping seafloor mounds in the Gulf of Mexico, thought to contain massive deposits of gas hydrate.

A short electromagnetic profile across the Kane Oceanic Core Complex

“…Converting the apparent resistivities to apparent porosities using Archie's Law [Evans, 2007], assuming an ideally connected pore fluid, yields values of around 46% in the least resistive area and close to zero at the contact. Typical seafloor porosities in volcanic or hard-rock settings are <20%, with the upper values reported for basalt breccia [Evans et al, 1991], although there are no data from true brecciated rubble.…”

“…The objective was to survey a longer profile and to discriminate, on the basis of electrical resistivity, between areas of serpentinized peridotite and gabbro. The towed EM system used consists of a frequencydomain magnetic dipole-dipole array, roughly ∼40 m long, towed in contact with the seafloor at 1-2 knots that measures structure to depths of around 30m below the seafloor [Evans, 2007]. The system has been used extensively in sedimented settings where it has been shown to record changes in relative porosity on the order of 1-2% quite reliably.…”

Geophysical and geochemical survey of a large marine pockmark on the Malin Shelf, Ireland

“…Measurements are made roughly every 10 m along the towline, so sensitivity to lateral variations in structure is high. Each receiver measures data at 3 distinct frequencies [Evans, 2007] and in general the information from the 6 pieces of information (3 amplitudes and 3 phases) on each receiver are combined into a single apparent resistivity measurement -the resistivity of the uniform seafloor that best matches the observations. The frequencies are chosen based on skin-depths between the transmitter and receiver for a range of likely seafloor conductivities.…”

Permeability anisotropy and resistivity anisotropy of mechanically compressed mudrocks

“…Daigle and Dugan, 2011) or pore orientations (e.g. Yang andAplin, 1998 &2007) have been developed but have not yet been experimentally validated. These models suggest that permeability anisotropy is low (-1) at high porosities and will increase rapidly (ranging from 1 to > 10) as mudrocks compress and particles become more aligned.…”

“…Evans, 2007), and can be analyzed to yield the porosity and saturation of formations through relations such as Archie's Law (Archie, 1942). Resistivity is commonly used to delineate oil bearing formations by noting abnormalities.…”