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

淡水的供应通常来自地表深处的含水层。这些含水层单元不会止步于海岸线，而是向近海延伸数百公里，携带淡水。在某些情况下，人们怀疑可能存在孤立的淡水体，部署在海平面比今天低得多的时期。尽管大陆架下存在大量淡水很可能是一种全球现象，但对潜在资源范围的估计差别很大。最近的一项模拟研究表明，多达1300千米的淡水可能被困在新英格兰海岸外。作为参考，纽约市每年的消费量约为1.5千米。随着淡水资源日益紧张，许多国家将需要以不导致淡水供应受到污染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，但我们对哪里可以找到淡水的了解仅限于采集了样本的钻探地点。例如，人们知道新泽西州近海存在大量淡水资源已有约40年的历史。然而，钻探只能给出点的测量，当需要研究大片区域时，成本高得令人望而却步。缺少的是能够绘制对新鲜地下水敏感的地下地图的工具，以及能够在陆上供水稀缺的新地区绘制沉积物地图的工具。这项研究利用了最初由学术界开发，但已被石油行业采用和推广的电磁测量工具。这些工具测量海底传导电流的程度，因此对地下水面水的盐度变化很敏感，因为淡水传导的电流比海水小。这个项目中将使用的电磁方法是唯一可以用来探测地下淡水存在并绘制数十公里长度范围内这些层的非侵入性手段。在这项研究中，该项目将在两个地点表征美国大西洋海岸大陆架下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一个概念证据，证明可以有效地利用电磁学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.