Groundwater on continental shelves is a new frontier in hydrological science that has important links to continental shelf geochemistry, the deep biosphere, global biogeochemical cycles, and environmental and climate change. Some attention has been 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 in many parts of the world up to ~100 km offshore and to sub-seafloor depths of hundreds of m.

Electromagnetic methods determine the electrical resistivity of the subsurface, and offer the only non-invasive, geophysical means to detect the presence of fresh water in the subsurface, and to map the layers over distances of many tens of km or more. This proposal supported the design and testing of a towed streamer controlled source electromagnetic (CSEM) system specifically built to survey for groundwater on the continental shelves.

The towed streamer marine CSEM system consists of three subsystems: the transmitter and receivers, along with a coordinating control computer element that manages the transmission and reception of data and data analysis/display for quality control. The transmitter source is a custom-fabricated unit that takes 480 VAC, 100 A, 3 φ input and produces a bipolar 250 V, 250 A DC output that is controllable via a fiber optic link to the control computer. The transmit dipole comprises two 10’ long copper electrodes with 200 m spacing connected using 2/0 welding cable and towed behind a research vessel. The transmit dipole was hung from surface floats on 10 m long tethers. The current flowing in it was measured synchronously with the receivers to facilitate removal of the source signature by deconvolution. The receiver array comprises a 1 km long custom streamer cable containing 10 receiver elements (each separated by 100m) and an additional 350 m streamer cable without receiver elements immediately behind the ship to provide a minimum offset from the source dipole. The receive streamer is towed at half of the water depth. During the test cruise, 5 receiver elements were deployed, so that the maximum offset between ship and receiver was ~850m. Each receiver element contains a micro-electromechanical attitude sensor providing pitch, roll, heading and depth information on the receiver element, a data acquisition system and fiber optic communication and power connection with the towing ship. Data from an electrode pair are acquired at each receiver element via silver-silver chloride electrodes over a 10 m span connected to low noise amplifiers coupled to an A/D converter. Sampling is controlled by the control computer to ensure synchronicity. A 1 pps timing signal traceable to GPS is sent to the receiver elements on a second optical fiber to synchronize internal SeaScan clocks. The single board computer puts together and return a data stream consisting of a timestamp, a data sample and attitude information at a 300 Hz rate, and also stores the data internally. The control computer consists of a Linux box that manages the data acquisition process ranging from synthesizing the source waveform to acquiring the data streams from the receiver elements. The source waveform was produced digitally and sent to the pulse modulator over a fiber optic link.  Communication to/from the receiver elements was carried out over a full duplex fiber optic link via a fiber optic media converter. The transmit waveform comprised a base frequency of 0.5 Hz and harmonics out to 9.5 Hz, with 1.5 and 3.5 Hz containing most of the energy. Transmission consisted of 60 s “shots” separated by 10 s intervals with the transmitter off to enable estimation of the signal-to-noise ratio.

A cruise to test the towed streamer system took place over 5-13 Aug 2019 aboard R/V Thomas G. Thompson. A region offshore Martha’s Vineyard was surveyed that coincides with a northeast offshore-onshore line previously studied. The system worked at turn on, except that the 1 pps fiber was broken in the junction box, and so the receiver elements had to rely on internal clocks, and the receiver sections proved to be too buoyant, so that the streamer was not horizontal. The tow speed was slowly raised to 5 knots, with no problems encountered. The system was operated for ~1.5 d, when a lobster pot was snagged, resulting in tangling of the transmitter and receiver cables and the loss of a transmitter electrode. The system was recovered, repaired and re-deployed with additional weight placed on each receiver element. About 4.5 d of additional data were then collected, ending offshore Long Island. The total data set comprises nearly 700 line km and about 56 Gbyte overall.

Last Modified: 01/28/2021
Modified by: Alan D Chave