Development of GNSS-Acoustic Surveying for Shallow Water

板块构造，也被称为大陆漂移，是构成大陆和洋盆海底的部分地壳以每年几英寸的速度在地球上缓慢移动的过程。在陆地上，这些运动可以通过精确的GPS测量来检测到。然而，GPS(也被称为全球导航卫星系统的GNSS)信号无法穿透海水。因此，必须采用另一种手段来探测海底的构造运动。一种已建立的方法是使用海面上的平台(如船只或浮标)，其位置由GPS和声纳同时观测。海底接收器的位置可以通过测量它们与GPS导航的水面平台之间的声纳脉冲的行进时间来确定。这种方法是利用海底接收器开发的，设计的海底接收器位于深海--海平面以下1000米或更深的地方，在那里它们不会受到人类活动的干扰。然而，用于探测海底构造运动的许多有趣的科学目标都在较浅的水域--几百米深。这一研究项目旨在开发一种保护性海底结构，使接收者免受拖网捕捞(否则将危及调查)的安全。GNSS-声学方法依赖于一个假设，即海洋中的声速结构是一维的，只取决于深度。从很长的时间(天)来看，这一点平均而言是正确的。利用这一假设，即使不完全了解一维声速分布的细节，也可以在全球参考系中以厘米级的精度确定对称放置在海面询问器周围的声学应答器阵列的中心坐标。在浅水中，声速变化不那么重要，因为声学范围较小。这应该会消除对应答器阵列的需求--只需要一个就足够了。然而，拖网捕鱼的风险依然存在。该项目的双重目标是1)确定用环波滑翔机询问器确定单个应答器坐标的精度，直到300米水深；2)测试一个抗拖网结构的设计，该结构可以容纳单个应答器，不受人为干扰。该奖项反映了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。

Plate tectonics, also known as continental drift, is the process where parts of the earth’s crust that make up continents and ocean basin seafloor move slowly around the globe at a few inches per year. On land, these motions can be detected with precise GPS measurements. However GPS (also called GNSS for “Global Navigation Satellite Systems”) signals cannot penetrate seawater. Consequently an alternative means must be invoked to detect tectonic motion of the seafloor. An established method uses a platform on the sea surface (such as a ship or buoy) whose position is simultaneously observed with GPS and sonar. The positions of seafloor receivers can be determined by measuring the travel time of sonar pulses between them and the GPS-navigated surface platform. The method has been developed with seafloor receivers designed to reside in the deep ocean – 1000 meters or greater beneath the surface, where they are safe from disturbance by human activity. However much of the interesting scientific targets for detecting seafloor tectonic motion are in shallower water – a few hundred meters deep. This research project aims to develop a protective seafloor structure that keeps a receiver safe from trawl fishing (which would otherwise compromise the survey). The GNSS-Acoustic method relies on an assumption that the sound speed velocity structure in the ocean is one dimensional, dependent only on depth. Over long times (days) this is true on average. With this assumption, the center coordinates of an array of acoustic transponders positioned symmetrically around a sea surface interrogator can be determined with cm level accuracy in a global reference frame even with imperfect knowledge of the details of the one-dimensional sound velocity profile. In shallow water, sound velocity variations are less important because the acoustic range is smaller. This should remove the need for an array of transponders – just one should suffice. However the risk from trawling remains. The project’s dual aim is to 1) determine the accuracy with which a single transponder’s coordinates can be determined up to 300 m water depth with a circling Wave Glider interrogator and 2) test the design of a trawl-resistant structure that can house a single transponder, free from disturbance from anthropogenic interactions.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.