Monday June 19, 2023

Scientific American

On a clear September day in 2015, after 10 years of working to get funding, my colleague Kerry Key and I stepped aboard the R/V Langseth, a research ship docked at the Woods Hole Oceanographic Institution in Massachusetts. We were about to lead a 10-day expedition to map a deposit of fresh water, size unknown, hidden 100 meters (about 330 feet) under the rocky seafloor.

Back in the 1960s the U.S. Geological Survey had drilled a series of vertical boreholes off the New Jersey coast, looking for sand deposits and other resources. They unexpectedly struck fresh water, which was baffling. Years later researchers obtained water samples from the same location and analyzed the chemistry, finding to their surprise that the liquid was a mix of recent rainwater and seawater. Rainwater, 65 kilometers (40 miles) out to sea—under the seafloor?

That’s where we were headed. Once the R/V Langseth was in position, we spooled out a long, floating line that held a special transmitter. It sent electromagnetic fields hundreds of meters down through the ocean and into the seabed. The fields passed through the seafloor and created secondary, return signals captured by other sensors on the line. We slowly towed the array for 130 kilometers over the region where drilling had been done. We also dropped instruments that sank to the bottom and recorded the signals from our transmitter, as well as naturally occurring electromagnetic fields. We could use all these readings to create an image of what was underneath the seafloor. Once we had completed the survey off New Jersey, we sailed up toward Martha’s Vineyard—where researchers had suggested there might also be fresh water—and ran a long sensing profile there, too.

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