Collaborative Research: A Pilot Study for Electromagnetic Surveying of Freshwater Resources Beneath the US Atlantic Continental Shelf

新鲜饮用水的供应通常来自地表以下深处的含水层。这些含水层单元并不停留在海岸线上，而是带着淡水向近海延伸数百公里。在某些情况下，人们怀疑可能存在孤立的淡水水体，它们是在海平面比今天低得多的时期形成的。虽然在大陆架下面存在大量淡水可能是一种全球现象，但对潜在资源程度的估计差别很大。最近的一项模拟研究表明，新英格兰海岸可能会有多达1300立方千米的淡水被困。作为参考，纽约市每年消耗大约1.5立方千米。随着淡水资源日益紧张，许多国家将需要以不污染淡水供应的方式开发这些宝贵的资源。尽管我们怀疑近海存在大量淡水，但我们对在哪里可以找到水的了解仅限于收集样本的钻井地点。例如，大约40年前人们就知道，在新泽西州近海可以找到大量的淡水。然而，钻井只能提供点测量，当需要研究大面积时，成本过高。缺少的是能够绘制对新鲜地下水敏感的地下地图的工具，以及能够绘制陆上供水稀缺的新地区沉积物的工具。本研究利用了最初由学术界开发，但已被石油工业采用和扩展的电磁测量工具。这些工具可以测量海底传导电流的能力，因此对地下海水盐度的变化很敏感，因为淡水传导电流的能力比海水小。在这个项目中使用的电磁方法是唯一一种非侵入性的方法，可以探测地下是否存在淡水，并绘制数十公里长的地层地图。在这项研究中，该项目将描述美国大西洋沿岸大陆架下两个地点海底淡水的空间分布：马萨诸塞州玛莎葡萄园岛附近和新泽西州近海。这项工作的更广泛影响包括提供了一个概念证明，即电磁可以有效地用于探测近海海洋环境中的淡水储量。

Supplies of fresh drinking water typically come from aquifers deep below the surface. These aquifer units do not stop at the shoreline, but instead extend offshore for perhaps hundreds of kilometers, carrying freshwater with them. In some cases, it is suspected that isolated bodies of freshwater may exist, emplaced during periods of time when sea level was much lower than it is today. Although the presence of large volumes of freshwater beneath the continental shelf is likely a global phenomenon, estimates of the extent of potential resources vary widely. A recent modeling study suggests that as much as 1300 km3 of fresh water could be trapped off the New England shore. For reference, the city of New York consumes roughly 1.5 km3 per year. As freshwater resources become increasingly stressed, many nations will need to exploit these valuable resources in a manner that does not result in contamination of the freshwater supply. Although we suspect that extensive freshwater exists offshore, our knowledge of where the water can be found is limited to drilling sites where samples have been collected. For example, it has been known for ~40 years that a substantial body of freshwater can be found offshore New Jersey. However, drilling gives only point measurements and is prohibitively expensive when large areas need to be studied. What is missing are tools capable of mapping the subsurface that are sensitive to fresh groundwater and for mapping deposits in new areas where water supplies onshore are scarce. This research takes advantage of electromagnetic surveying tools that were initially developed by academia, but which have been adopted and expanded by the petroleum industry. These tools measure how well the seafloor can conduct electrical current and, as a result, are sensitive to changes in the salinity of water in the subsurface because fresh water conducts current less than seawater. The electromagnetic methods that will be used in this projet are the only non-invasive means available to detect the presence of fresh-water in the subsurface and to map these layers over length-scales of tens of kilometers. In this research, the project will characterize the spatial distribution of sub-seafloor fresh water beneath the continental shelf of the US Atlantic coast in two locations: off Martha's Vineyard, Massachusetts, and the offshore New Jersey. Broader impacts of the work include providing a proof of concept that electromagnetics can be effectively used to detect freshwater reserves in the offshore marine environment.