Freshwater Beneath the US Atlantic Continental Shelf

In 2015 we conducted a pilot study of large-scale electromagnetic (EM) surveying of offshore groundwater at locations off New Jersey and Martha’s Vineyard. EM methods remotely measure bulk electrical conductivity, which is strongly a function of groundwater salinity. The large conductivity contrast between sediments containing resistive freshwater and conductive seawater makes offshore aquifers good targets for EM methods. Using equipment designed and built by Scripps Institution of Oceanography, we recorded seafloor magnetotelluric (MT) and controlled-source EM (CSEM) data using 10 receivers deployed at 10 to 20 km spacing. We augmented that with continuous CSEM data recorded by an array of four receivers surface-towed 0.6 to 1.4 km behind the transmitter antenna.

Submarine ground water contained in continental shelves may be a significant global phenomenon, yet little is known about the distribution of these fresh and brackish water bodies.  As freshwater resources diminish onshore, characterizing these submarine hydrologic systems will be necessary in understanding the sustainability of coastal freshwater as a resource. Off the US Atlantic coast, a scattering of boreholes has revealed low salinity groundwater, but the limited data do not provide enough information to meaningfully characterize the aquifers.

Team:
Chloe Gustafson
Kerry Key
Rob Evans (Woods Hole Oceanographic Institution)

Funding Source: NSF Award 1458392

Check out these images of the project and the HD drone video of the survey in action:

EM survey location on the US Mid-Atlantic Margin. Blue squares show the location of seafloor EM/MT receiver stations and red lines show the locations of the controlled-source EM transmitter and towed-EM receiver tows. Green circles along the New Jersey profile show the locations of drill holes from IODP expedition 313. Yellow triangles show older AMCOR drill holes.

Diagram of survey instrumentation. We surface-towed a 300 m long transmitter dipole with about 100A current output. Four inline electric dipole receivers were surface-towed at ranges of about 500 to 1400 m. Passive magnetotelluric and controlled-source EM data were also recorded by an array of 10 seafloor EM receivers at each location.
A Scripps broadband EM/MT receiver being deployed from the R.V. Marcus G. Landseth.
Winch for the surface towed EM receiver array and the dipole EM antenna.
EM transmitter controller and power-supply.
A surface-towed electric dipole receiver called a Porpoise. The gray tube holds a data logger modified from a seafloor EM receiver and the yellow pipe is a 2m long dipole that holds silver silver-chloride electrodes at each end.