MRI: Development of a Towed Streamer Controlled Source Electromagnetic (CSEM) System for Groundwater Mapping on the Continental Shelf

大陆架地下水的存在与循环是水文学研究的一个新前沿，它与大陆架地球化学、深层生物圈、全球生物地球化学循环、环境与气候变化有着重要的联系。目前，人们对沿海地区附近的海底地下水排放给予了相当大的关注，但对世界许多地方存在于离岸100公里处和海底数百米深处的潜在巨大的化石新鲜和微咸地下水体知之甚少。一个必要而缺失的信息是这些沉积物在大陆边缘的空间分布。钻井已经在许多地方发现了咸淡水或淡水层，但只能进行点测量，而且当需要对大面积区域进行表征时，成本过高。电磁（EM）方法测量地下的电阻率，并提供了唯一的非侵入性的地球物理手段来检测地下是否存在电阻淡水，并绘制出数十公里长度范围内的地层图。最近的一项模拟研究表明，新英格兰海岸可能有多达1300立方千米的淡水，全球被动大陆边缘可能有3.5 × 105立方千米的淡水。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使淡水系统过早盐化的方式开发这些宝贵的资源。将设计并建造拖曳拖缆控制源电磁（CSEM）测量系统，用于绘制大陆架大面积淡水沉积物。该系统将是对石油工业采用的成功方法的改进，该方法在规模和传输频率上进行了调整。海洋CSEM系统将由三个子系统组成：发射器和接收器，以及一个协调控制计算机元件，用于管理数据的传输和接收以及数据分析/显示，以进行质量控制。数据将在船上实时获取，如果确定了特定兴趣目标，则允许自适应测量。发射机将在10米水深的200-400米源偶极子上产生高电流（高达500 a）的多频信号。接收器串将由多达20个接收器元件组成，连接到一个1公里长的拖缆上的多个电极，拖缆在水深的一半处拖曳。接收器和发射器将连接到船上，允许通过单个时钟进行同步，并通过反卷积去除接收器的源信号。拖曳拖缆CSEM系统将扩大美国学术研究人员可用的EM测量设备池。除了支持地下水研究外，同样的方法还可用于研究其他电阻结构，如岩土工程调查或近海二氧化碳封存监测。

The presence and circulation of groundwater on continental shelves is a new frontier in the hydrological sciences that has important links to continental shelf geochemistry, the deep biosphere, global biogeochemical cycles and environmental and climate change. Considerable attention is currently being paid to submarine groundwater discharge in near coastal zones, but very little is known about potentially enormous fossil fresh and brackish groundwater bodies that exist up to a hundred kilometers offshore and to sub-seafloor depths of hundreds of meters in many parts of the world. A necessary and missing piece of information is the spatial distribution of such deposits across continental margins. Drilling has identified brackish or fresh water layers in many places, but gives only point measurements, and is prohibitively expensive when large areas need to be characterized. Electromagnetic (EM) methods measure the electrical resistivity of the subsurface, and offer the only non-invasive, geophysical means to detect the presence of resistive fresh water in the subsurface, and map the layers over length scales of many tens of kilometers. A recent modeling study suggests that as much as 1300 km3 of fresh water could be trapped off the New England shore, and 3.5 x 105 km3 within passive continental margins globally. For reference, the city of New York consumes roughly 1.5 km3 per year. As fresh water resources become increasingly stressed, many nations will need to exploit these valuable resources in a manner that does not prematurely salinate the fresh water system.A towed streamer controlled source electromagnetic (CSEM) surveying system will be designed and built to map fresh water deposits over large areas of the continental shelf. The system will be a modification to a successful approach utilized by the petroleum industry that is adapted in scale and transmission frequency. The marine CSEM system will consist of three subsystems: the transmitter and receiver, along with a coordinating control computer element that manages the transmission and reception of data and data analysis/display for quality control. Data will be acquired onboard ship in real time, permitting adaptive surveying if targets of specific interest are identified. The transmitter will generate a multi-frequency signal with a high current (up to 500 A) over a 200-400 m source dipole towed at 10 m water depth. The receiver string will consist of up to 20 receiver elements connected to multiple electrodes on a 1 km streamer towed at half of the water depth. The receivers and transmitter will be connected to the ship, allowing synchronization by a single clock and removal of the source signature at the receivers by deconvolution. The towed streamer CSEM system will expand the pool of EM surveying equipment available to academic researchers in the US. In addition to supporting groundwater research, the same methodology could be used to study other resistive structures such as in geotechnical surveys or for offshore CO2 sequestration monitoring.