MRI: Development of a Towed Streamer Controlled Source Electromagnetic (CSEM) System for Groundwater Mapping on the Continental Shelf

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The presence and circulation of groundwater on continental shelves is a new frontier in the hydrological sciences that has important links to continental shelf geochemistry, the deep biosphere, global biogeochemical cycles and environmental and climate change. Considerable attention is currently being paid to submarine groundwater discharge in near coastal zones, but very little is known about potentially enormous fossil fresh and brackish groundwater bodies that exist up to a hundred kilometers offshore and to sub-seafloor depths of hundreds of meters in many parts of the world. A necessary and missing piece of information is the spatial distribution of such deposits across continental margins. Drilling has identified brackish or fresh water layers in many places, but gives only point measurements, and is prohibitively expensive when large areas need to be characterized. Electromagnetic (EM) methods measure the electrical resistivity of the subsurface, and offer the only non-invasive, geophysical means to detect the presence of resistive fresh water in the subsurface, and map the layers over length scales of many tens of kilometers. A recent modeling study suggests that as much as 1300 km3 of fresh water could be trapped off the New England shore, and 3.5 x 105 km3 within passive continental margins globally. For reference, the city of New York consumes roughly 1.5 km3 per year. As fresh water resources become increasingly stressed, many nations will need to exploit these valuable resources in a manner that does not prematurely salinate the fresh water system.A towed streamer controlled source electromagnetic (CSEM) surveying system will be designed and built to map fresh water deposits over large areas of the continental shelf. The system will be a modification to a successful approach utilized by the petroleum industry that is adapted in scale and transmission frequency. The marine CSEM system will consist of three subsystems: the transmitter and receiver, along with a coordinating control computer element that manages the transmission and reception of data and data analysis/display for quality control. Data will be acquired onboard ship in real time, permitting adaptive surveying if targets of specific interest are identified. The transmitter will generate a multi-frequency signal with a high current (up to 500 A) over a 200-400 m source dipole towed at 10 m water depth. The receiver string will consist of up to 20 receiver elements connected to multiple electrodes on a 1 km streamer towed at half of the water depth. The receivers and transmitter will be connected to the ship, allowing synchronization by a single clock and removal of the source signature at the receivers by deconvolution. The towed streamer CSEM system will expand the pool of EM surveying equipment available to academic researchers in the US. In addition to supporting groundwater research, the same methodology could be used to study other resistive structures such as in geotechnical surveys or for offshore CO2 sequestration monitoring.