Mapping of ocean currents in shallow water using moving ship acoustic tomography

Abstract

This study investigates techniques for the acoustic mapping of ocean currents in a shallow-water environment using moored and ship-towed stations. The currents are estimated using the differential travel times (DTTs) observed in the reciprocal acoustic transmissions between those stations. Due to the relative motion induced by the ship-towed station, the Doppler shift estimated from the measured channel impulse response is used to compensate for the arrival patterns and to correct the influence of the relative speed on the DTTs. Furthermore, to estimate the DTTs from the arrival patterns received in the reciprocal directions, a method is devised which involves the time-evolving cross-correlation function of the reciprocal arrival patterns. A feasibility test was carried out at Sizhiwan Marine Test Field, Kaohsiung, Taiwan in September of 2015. The currents were estimated using the data collected from one ship-towed and four moored stations. The estimated currents are consistent with the direct measurements from a shipboard acoustic Doppler current profiler and reveal the spatial distribution of the currents.