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Marine Geophysics

EM system
The towed EM system used for measuring subseafloor resistivity in coastal settings. Enlarge »

The Coastal Systems Group uses a variety of geophysical techniques in order to understand coastal structre and processes. Some specific applications of the techniques are shown on adjacent pages.

On land we have used ground penetrating radar (GPR) with a high degree of success for imaging relict coastal inlets and for identifying patterns of beach progradation or erosion. GPR uses high frequency (around 200 MHz) electromagnetic waves which reflect from interfaces with contrasting dielectric properties. Dielectric properties are influenced by heavy mineral (metallic) contents, changes in porosity and groundwater salinity. GPR units are commercially available and can easily be used to collect long profiles of data, probing to depths of several meters beneath the surface.

 

EM and Chirp data

An example of EM Data (top two panels) and chirp seismic (lower two panels) from Offshore Carolina. Enlarge »

Offshore, we commonly use chirp seismic profiling techniques to image sedimentary structure. These methods, also using commercially available equipment, send sound pulses into the seafloor which reflect from surfaces with different acoustic impedances. Layers with different porosities or sediment type will cause reflections and images of these reflections can be interpreted in terms of sedimentary history, much the same as the GPR data.

Seismic reflection provides details of stratal geometry but little about physical properties. Knowledge of subseafloor physical properties (porosity, grain size etc) can be linked to depositional processes and also to processes of groundwater transport which can preferentially occur through high permeability zones. We have built our own seafloor electromagnetic profiling tool which measures subseafloor electrical resistivity. Resistivity is particularly useful on the continental shelf as it provides a first order measure of porosity, which is closely linked to sediment grain size. Resistivity may also be used to locate areas of fluid flow through the seafloor.

near surface sediment porosities
An example from the STRATAFORM area, off Eureka California, of how the EM system can be used to build up a map of near surface sediment porosities. Enlarge »

The EM system we use 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 sediment core locations, providing information on shallow properties over greater areas of the seafloor than is possible by coring alone.

For more information please contact:

Rob Evans (revans@whoi.edu)

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