Fluvial Channels on the Continental Shelf: Using EM Understand Their Role in Hydrology.

We present data from EM surveys of the continental shelf which have been able to add to existing information on the internal structure of fluvial channels formed during sealevel lowstands. The data were collected using a towed EM surveying system owned and operated by the Geological Survey of Canada. The system uses frequency domain magnetic fields to measure the electrical resistivity of the uppermost 20m or so of the seafloor in a continuous profiling mode, and represents a powerful addition to coastal and shelf surveying tools. On the mid-shelf off New Jersey, two sets of channels imaged seismically show very different EM signatures dependent on the nature of the infilling material and the surface into which the channels are carved. We have also surveyed a coastal environment, around Wrightsville Beach, North Carolina. Here, a number of channels incise an Oligocene sequence, which we interpret as a hydrologic capping unit, and in places appear to intersect the top surface of a limestone aquifer. Based on the complex geophysical signature of these channels, we hypothesize that they act as high permeability conduits, allowing groundwater to flow from the limestone up to the seafloor. If this is true, then it has important implications for groundwater discharge and nutrient supply to the seafloor.