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

MRI：开发用于大陆架地下水测绘的拖缆控制源电磁 (CSEM) 系统

• 批准号: 1726798
• 负责人: Alan Chave
• 金额: $ 81.25万
• 依托单位: Woods Hole Oceanographic Institution
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2017
• 资助国家: 美国
• 起止时间: 2017-09-01 至 2020-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1726798&HistoricalAwards=false
• 关键词: MRI; Development; Towed; Streamer; Controlled

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.

大陆架地下水的存在和循环是水文科学的一个新领域，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着重要联系。目前，人们对沿海地区的海底地下水排放给予了相当大的关注，但对世界许多地方离岸100公里和海底以下数百米深处潜在的巨大的化石淡水和半咸水体却知之甚少。一项必要但缺失的资料是这种矿床在大陆边的空间分布情况。钻探已经在许多地方发现了咸水层或淡水层，但只能进行点测量，而且当需要对大面积区域进行特征描述时，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电阻淡水的存在，并绘制数十公里长度尺度的地层。最近的一项模拟研究表明，多达1300立方公里的淡水可能被困在新英格兰海岸之外，全球被动大陆边缘内有3.5 × 105立方公里。作为参考，纽约市每年消耗大约1.5立方公里。随着淡水资源的日益紧张，许多国家将需要以不使淡水系统过早盐渍化的方式开发这些宝贵的资源。将设计和建造拖曳拖缆受控源电磁（CSEM）测量系统，以绘制大面积大陆架的淡水沉积物。该系统将是对石油工业所采用的成功方法的修改，该方法在规模和传输频率方面进行了调整。海上CSEM系统将由三个子系统组成：发射机和接收机，沿着协调控制计算机元件，用于管理数据的发送和接收以及数据分析/显示，以进行质量控制。数据将在船上以真实的时间获得，如果确定了特别感兴趣的目标，就可以进行适应性测量。发射机将产生一个多频信号与高电流（高达500 A）超过200-400米源偶极子拖在10米水深。接收器串将由多达20个接收器元件组成，这些元件连接到在水深一半处拖曳的1公里拖缆上的多个电极。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，并通过反卷积消除接收器处的源特征。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，同样的方法可用于研究其他阻力结构，如岩土工程勘测或近海二氧化碳封存监测。