Analysis of an Extensive Towed Streamer Dataset Collected on the New England Continental Shelf

Woods Hole Oceanographic Institution, Woods Hole MA

Investigators

Abstract

Groundwater on continental shelves is a new frontier in water sciences. A modeling study suggests that over a thousand cubic kilometers of fresh water could be trapped offshore New England, and hundreds of thousands of cubic kilometers exist within passive continental margins globally. For reference, the City of New York consumes roughly a cubic kilometer per year. Under recent NSF support, a towed electromagnetic (CSEM) system was designed, constructed, and tested that is aimed at surveying offshore groundwater. This technique builds on experience in the petroleum industry using a similar technology. CSEM is preferentially sensitive to resistive bodies, which include fresh water. During the course of an 8-day test cruise in August 2019, more than 700 km of data were collected over a region extending from the east end of Martha’s Vineyard to offshore Long Island and from coastal waters to the edge of the continental shelf. This proposal seeks to analyze, model, and interpret this exciting data set. Specifically, the inference that the entire New England shelf is underlain by freshwater is a hypothesis that can be partially tested. The result will contribute to national health and welfare, and has societal relevance given the increasing scarcity of fresh water. The towed streamer CSEM system is composed of a 1-km long receive streamer towed at half of the water depth, and a 250-m long transmit dipole towed near the surface. The transmit streamer is powered by a 250-A source producing a bespoke waveform between 0.1-10 Hz. Data are available in real time via optical fibers in the streamer cable. Data are collected in “shots”, each 60 s, of transmitted waveforms and 10 s with the transmitter off to measure noise levels. The work to be carried out under this proposal includes: 1) merging the navigation and streamer attitude data to produce a model for receiver location as a function of time, 2) computing the frequency response of each shot, removing the source response in the process, 3) devising a robust method for culling out the shots that are noisy to produce a final data set, 4) merging the navigation and shot data to produce a final data set containing location, depth, amplitude and phase, and 5) inverting the data to produce 2D resistivity maps along the various lines where data were collected. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.