Development of GNSS-Acoustic Surveying for Shallow Water

板块构造，也被称为大陆漂移，是构成大陆和海洋盆地海底的地壳部分以每年几英寸的速度围绕地球仪缓慢移动的过程。 在陆地上，这些运动可以通过精确的GPS测量来检测。 然而，GPS（也称为“全球导航卫星系统”的GNSS）信号不能穿透海水。 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在海面上使用一个平台（如船或浮标），其位置同时由GPS和声纳观测。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传播时间来确定。 该方法已被开发与海底接收器设计驻留在深海- 1000米或更大的表面下，在那里他们是安全的人类活动的干扰。 然而，许多用于探测海底构造运动的有趣的科学目标都在较浅的水中--几百米深。 该研究项目旨在开发一种保护性海底结构，使接收器免受拖网捕鱼的影响（否则会影响调查）。 全球导航卫星系统声学方法依赖于这样一个假设，即海洋中的声速结构是一维的，仅取决于深度。 在很长一段时间（天）内，平均来说这是真的。 利用这一假设，即使对一维声速剖面的细节不完全了解，也可以在全球参考系中以厘米级的精度确定围绕海面浮标对称定位的声应答器阵列的中心坐标。 在浅水中，声速变化不太重要，因为声波范围较小。 这样就不需要一组转发器了--只要一个就足够了。 然而，拖网捕鱼的风险仍然存在。 该项目的双重目标是：1）确定在300米水深的情况下，用一个盘旋的波浪滑翔机探测器确定单个应答器坐标的准确性; 2）测试一个可以容纳单个应答器的抗拖网结构的设计，该奖项反映了NSF的法定使命，并通过使用基金会的学术价值和更广泛的影响审查标准。

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.