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

大陆架地下水是水科学的新前沿。一项模拟研究表明，超过1000立方公里的淡水可能被困在新英格兰近海，而全球范围内被动的大陆边缘存在数十万立方公里的淡水。作为参考，纽约市每年大约消耗1立方公里。在NSF最近的支持下，设计、建造和测试了一个旨在调查近海地下水的拖曳电磁(CSEM)系统。这项技术建立在石油行业使用类似技术的经验基础上。CSEM优先对包括淡水在内的阻性物体敏感。在2019年8月为期8天的试航过程中，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的区域收集了700多公里的数据。该提案旨在分析、建模和解释这一令人兴奋的数据集。具体地说，整个新英格兰大陆架被淡水覆盖的推断是一个可以部分检验的假设。这一结果将有助于国民的健康和福利，并具有社会意义，因为淡水日益稀缺。拖曳拖缆CSEM系统由一个1公里长的接收拖缆和一个250米长的发射偶极子组成，前者在水深的一半处被拖曳，后者在海面附近拖曳。发射拖缆由一个250-A电源供电，可产生0.1-10赫兹之间的定制波形。数据可通过拖缆中的光纤实时获取。数据以发射波形的“镜头”形式收集，每个发射波形60个S，10个关闭发射器的S以测量噪声水平。在该方案下要执行的工作包括：1)合并导航和拖缆姿态数据以产生作为时间函数的接收器定位模型，2)计算每个炮击的频率响应，在该过程中去除震源响应，3)设计用于剔除噪声的炮击的稳健方法以产生最终数据集，4)合并导航和炮击数据以产生包含位置、深度、幅度和相位的最终数据集，5)将数据倒置，沿着收集数据的不同线路生成2D电阻率图。这一裁决反映了NSF的法定使命，并通过使用基金会的智力优势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.