Coastal and Continental Shelf Electromagnetics
|A photograph of the towed EM system on deck. The system consists of a transmitter (large cylinder at right) connected to the ship by a 0.680 conducting cable. Three receivers (smaller cylinders) to behind the transmitter at separations of 4m, 13m and 40m and provide information to depths of about 20m subsurface.|
Many long-standing and emerging scientific problems in coastal processes require knowledge of the geologic structure underlying the coastal zone. For example, understanding groundwater exchange between land and sea requires knowledge of the local hydrologic conditions that are controlled by formation permeability, and, hence, lithology. The development of reliable seafloor surveying tools has made the use of electrical resistivity a viable geophysical approach for studying sedimentary processes on the continental shelf. Within seafloor sediments, the electrical resistivity is controlled by the amount and distribution of seawater, and so provides a useful handle on porosity.
Over the last 10 years, I have become increasingly involved in studies of coastal and continental shelf sediments. This work has utilised an extremely effective CSEM developed by colleagues at the Geological Survey of Canada and which we have duplicated and updated at WHOI. The system is a frequency-domain magnetic dipole-dipole array, that is towed along the seafloor, and measures resistivity profiles to depths of about 20m.
The system is a mapping tool, and is especially powerful when combined with high-resolution chirp seismic techniques. The profiling nature of the system makes it an attractive means of interpolating structure between core locations, providing facies information over greater areas of seafloor than is possible by coring alone. Also encouraging is that porosity estimates from EM surveys provide a useful additional constraint to that from acoustic backscatter, allowing more accurate sediment classification over large areas of seafloor (Evans, 2001).
A survey completed on the Eel River shelf, California (Evans et al., 1999) found one region, now known to be associated with the northern limb of an anticline system, that exhibits dramatic variability in structure with incredibly low porosities (less than 15%) in places. We proposed several explanations for this structure, but the most enduring has been that fresh groundwater is leaking to the seafloor channeled through faults associated with the anticline. We have used existing literature on pore water salinity to show that the EM system could be used to detect fresh submarine groundwater (Hoefel & Evans, 2001). Based on this result, we collected coincident EM and chirp seismic coverage off North Carolina, focusing on nearshore structures with a particular emphasis on groundwater discharge (Evans and Lizarralde, 2003).
We have used the system to map physical properties within paleo-channels in a mid-shelf setting off New Jersey (Evans et al., 2000). These channels had been previously mapped using high resolution seismic techniques. Our survey found two distinct EM responses associated with two different sets of buried channels that were imaged seismically: one that was easily identified with those channels (through an increase in apparent porosity within the confines of the channel) and another that failed to respond to a seismically observed channel at all (Evans et al., 2000). This survey demonstrated that EM data are perfectly complementary to high resolution seismic reflection methods, with some reflectors caused by only modest changes in physical properties across their boundary.