Analysis of an Extensive Towed Streamer Dataset Collected on the New England Continental Shelf

大陆架地下水是水科学研究的新领域。一项模拟研究表明，超过一千立方公里的淡水可能被困在新英格兰近海，而全球被动大陆边缘存在数十万立方公里的淡水。作为参考，纽约市每年消耗大约一立方公里。在美国国家科学基金会（NSF）最近的支持下，设计、建造和测试了一个拖曳式电磁（CSEM）系统，旨在测量近海地下水。这项技术建立在石油工业使用类似技术的经验基础上。CSEM对包括淡水在内的电阻体优先敏感。在2019年8月为期8天的测试巡航过程中，从玛莎葡萄园岛东端到长岛近海，从沿海水域到大陆架边缘，收集了700多公里的数据。本提案旨在分析、建模和解释这一令人兴奋的数据集。具体来说，整个新英格兰大陆架被淡水覆盖的推论是一个可以部分验证的假设。结果将有助于国民健康和福利，并具有社会相关性，因为淡水日益稀缺。拖曳拖缆CSEM系统由拖曳在一半水深的1公里长的接收拖缆和拖曳在水面附近的250米长的发射偶极子组成。发射拖缆由250-A源供电，产生0.1-10 Hz之间的定制波形。数据可通过拖缆中的光纤实时获取。数据以“镜头”的形式收集，每60秒一次传输波形，10秒一次关闭发射机以测量噪声水平。根据这项建议进行的工作包括：1)合并导航和拖缆姿态数据，生成接收器位置作为时间函数的模型；2)计算每个镜头的频率响应，在此过程中去除源响应；3)设计一种鲁棒方法，剔除有噪声的镜头，生成最终数据集；4)合并导航和镜头数据，生成包含位置、深度、幅度和相位的最终数据集；5)对数据进行反演，沿数据采集线生成二维电阻率图。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Groundwater on continental shelves is a new frontier in water sciences. A modeling study suggests that over a thousand cubic kilometers of fresh water could be trapped offshore New England, and hundreds of thousands of cubic kilometers exist within passive continental margins globally. For reference, the City of New York consumes roughly a cubic kilometer per year. Under recent NSF support, a towed electromagnetic (CSEM) system was designed, constructed, and tested that is aimed at surveying offshore groundwater. This technique builds on experience in the petroleum industry using a similar technology. CSEM is preferentially sensitive to resistive bodies, which include fresh water. During the course of an 8-day test cruise in August 2019, more than 700 km of data were collected over a region extending from the east end of Martha’s Vineyard to offshore Long Island and from coastal waters to the edge of the continental shelf. This proposal seeks to analyze, model, and interpret this exciting data set. Specifically, the inference that the entire New England shelf is underlain by freshwater is a hypothesis that can be partially tested. The result will contribute to national health and welfare, and has societal relevance given the increasing scarcity of fresh water. The towed streamer CSEM system is composed of a 1-km long receive streamer towed at half of the water depth, and a 250-m long transmit dipole towed near the surface. The transmit streamer is powered by a 250-A source producing a bespoke waveform between 0.1-10 Hz. Data are available in real time via optical fibers in the streamer cable. Data are collected in “shots”, each 60 s, of transmitted waveforms and 10 s with the transmitter off to measure noise levels. The work to be carried out under this proposal includes: 1) merging the navigation and streamer attitude data to produce a model for receiver location as a function of time, 2) computing the frequency response of each shot, removing the source response in the process, 3) devising a robust method for culling out the shots that are noisy to produce a final data set, 4) merging the navigation and shot data to produce a final data set containing location, depth, amplitude and phase, and 5) inverting the data to produce 2D resistivity maps along the various lines where data were collected.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.