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

大陆架上地下水的存在和循环是水文科学中的一个新前沿，与大陆架地球化学、深层生物圈、全球生物地球化学循环以及环境和气候变化有重要联系。目前对近海地区的海底地下水排放给予了相当大的关注，但对世界许多地区近海100公里处和海底深处数百米处可能存在的巨大的淡水和咸水地下矿体化石知之甚少。一条必要且缺失的信息是这类矿藏在大陆边缘的空间分布。钻探已经在许多地方发现了微咸水或淡水层，但只给出了点测量，当需要确定大片区域的特征时，成本高得令人望而却步。电磁(EM)方法测量地下的电阻率，并提供唯一的非侵入性地球物理手段来检测地下是否存在阻性淡水，并绘制数十公里范围内的层图。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，而全球被动大陆边缘的淡水可能被困住3.5×105千米。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，许多国家将需要以一种不过早使淡水系统盐碱化的方式开发这些宝贵的资源。将设计和建造一个拖缆可控源电磁(CSEM)探测系统，以绘制大陆架大片区域淡水沉积物的地图。该系统将是对石油行业采用的一种成功方法的修改，该方法在规模和传输频率上进行了调整。船用CSEM系统将由三个子系统组成：发射机和接收机，以及一个协调控制计算机部件，负责管理数据的发送和接收以及用于质量控制的数据分析/显示。数据将在船上实时获取，如果识别出特定感兴趣的目标，则允许进行自适应测量。发射机将在10米水深的200-400米源偶极子上产生大电流(高达500安)的多频信号。接收器串将由多达20个接收器元件组成，这些接收器元件连接到1公里长的拖缆上的多个电极，拖曳深度为水深的一半。接收器和发射器将连接到舰船上，允许通过单个时钟进行同步，并通过去卷积消除接收器处的源签名。拖曳拖缆CSEM系统将扩大美国学术研究人员可用的电磁测量设备池。除了支持地下水研究外，同样的方法还可以用于研究其他阻性结构，例如用于岩土工程勘测或用于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.