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的联系。目前，人们对沿海地区海底地下水的排放给予了相当多的关注，但人们对潜在的巨大化石淡水和微咸地下水体知之甚少，这些地下水体存在于离岸一百公里处，许多地方的海底深度达数百米。世界的。一个必要且缺失的信息是此类矿床在大陆边缘的空间分布。钻探已经在许多地方识别出了咸水或淡水层，但只能提供点测量，并且当需要表征大面积区域时，成本高昂。电磁 (EM) 方法可测量地下的电阻率，并提供唯一的非侵入性地球物理方法来检测地下电阻淡水的存在，并绘制数十公里长度范围内的地层地图。最近的一项模型研究表明，新英格兰海岸可能滞留多达 1300 平方公里的淡水，全球被动大陆边缘可能滞留 3.5 x 105 平方公里的淡水。作为参考，纽约市每年消耗约 1.5 平方公里。随着淡水资源的压力越来越大，许多国家需要以一种不会过早使淡水系统盐化的方式开发这些宝贵的资源。将设计和建造拖曳式拖缆控制源电磁 (CSEM) 测量系统来绘制淡水地图大陆架大面积沉积物。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。船上数据将实时获取，如果识别出特定目标，则可以进行自适应测量。发射器将在 10 m 水深拖曳的 200-400 m 源偶极子上生成高电流（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，这些接收器元件连接到拖曳在一半水深的 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，并通过反卷积去除接收器处的源签名。拖曳式 CSEM 系统将扩大美国学术研究人员可用的电磁测量设备库。除了支持地下水研究之外，相同的方法还可用于研究其他电阻结构，例如岩土工程勘察或近海二氧化碳封存监测。