How does an EMF detector, or something similar, detect freshwater under the seafloor? In the news recently, Woods Hole and Columbia University said they found a gigantic aquifer of freshwater underneath the seabed off the Northeast coast.  They said they used electric field detectors to sense the fresh water, which is less conductive than seawater.
But how do you remotely sense whether something nearby is electrically conductive?
There is no Wikipedia page for 'electric detector' like there is is for metal detector.....  Although there are pages for 'capacitive sensing' and 'EMF measurement'....
I'm just a little confused....
 A: 
We used a recently developed surface-towed controlled source EM (CSEM) system16 in combination with the passive-source magnetotelluric (MT) method to map low-salinity groundwater systems in two locations on the US Atlantic coast: offshore Martha’s Vineyard and New Jersey (Fig. 1). CSEM and MT methods are preferentially sensitive to resistors and conductors, respectively (Methods), so we use both data sets to obtain a more robust resistivity model.

So searching one finds what these CSEM are 

We compare two approaches for acquiring marine controlledsource electromagnetic (CSEM) surveys: one towing the horizontal electric dipole antenna close to the seabed (deep towing), and the other one towing the antenna just below the sea
  surface (surface towing). 

So it is electromagnetic detectors used usually to find hydrocarbons (oil).

Marine CSEM surveying has become an established method
  for hydrocarbon exploration during the last 10 years (Eidesmo
  et al., 2002; Ellingsrud et al., 2002; Constable, 2010). It uses
  low-frequency (0.1 – 10 Hz) electromagnetic fields that are
  usually generated by a horizontal electric dipole (HED) source
  towed in the water and recorded by seafloor receivers. The
  measured fields carry information about the distribution of electrical resistivity in the subsurface and are especially sensitive
  to thin resistive layers typical for hydrocarbon-filled reservoirs

modified for use for fresh water.
