MAPPING BELOW THE SEAFLOOR: ELECTROMAGNETIC METHODS FOR MEASURING PHYSICAL PROPERTIES

Electromagnetic (EM) techniques, which measure the electrical resistivity of the seafloor, are particularly useful in nearshore sedimentary settings as resistivity can be fairly easily interpreted in terms of sediment porosity. Surveys with good areal coverage can therefore be used to compile facies maps. Sub-bottom resistivity can also be impacted by the presence of fresh groundwater offshore and so can be used to assess submarine groundwater discharge to the seafloor. Here we describe a system that can be used in coastal and continental shelf settings to measure the subseafloor porosity.

The system we use is a frequency domain magnetic dipole-dipole array that can measure sub-bottom resistivity to about 20m below the seafloor. The array consists of a large coil, which generates magnetic fields over a range of frequencies, and three receivers, which measure the amplitude and phase of these fields after propagation through the seafloor. The system is towed along the seafloor at speeds of about 1ms-1 and makes a measurement of seafloor resistivity every 10m or so along track. The receivers are spaced 4m, 13m and 40m behind the transmitter and provide information at different depth intervals, generally to about one-half the source-receiver separation. Each receiver measures amplitude and phase at three frequencies, chosen so that the source-receiver separation is greater than a skin-depth in the ocean (a skin depth is the length over which fields decay to 1/e of their original value). Since higher frequency fields decay more rapidly, the 4m receiver measures frequencies of 20kHz to 200kHz, while the 40m receiver measures lower frequencies from 200Hz to 2kHz. Apparent resistivities on each of the three receivers are converted to apparent porosities using empirical relationships. We have used the system off Northern California, New Jersey, North Carolina and Massachusetts in very different sedimentary environments, and will present examples of data from each.