Development of GNSS-Acoustic Surveying for Shallow Water

University Of California-San Diego Scripps Inst Of Oceanography, La Jolla CA

Investigators

Abstract

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.