Shallow-Water Acoustic Mapping of Ocean Currents Using Towed Transceivers

This study uses four moored {acoustic transceivers (a device consisting of a source and a receiver)} along with one towed by a ship to demonstrate the acoustic mapping of ocean currents with the mobile station in a shallow-water environment. The basic principle of the acoustic mapping for currents is that the difference in the travel times (DTT) of oppositely traveling acoustic signals is proportional to the integrated current velocity along the acoustic ray path. Thus the currents can be estimated with the DTT data. When the towed transceiver is incorporated, the acoustic signal is Doppler-distorted and the DTTs are affected due to the relative instrument motion. Since the transmitted signal, $m$-sequence, is highly sensitive to Doppler, the method based upon the delay-Doppler ambiguity function can determine the Doppler shift and the arrival patterns simultaneously. For the estimation of DTT, the conventional approach using the peak-picking method is subject to the uncertainties in identifying and resolving acoustic rays in the shallow-water environment. Hence, this study proposed a method based on the cross-correlation function (CCF) of the reciprocal arrival patterns. Ideally, the DTT could be obtained by the lag time corresponding to the maximum correlation. However, due to the multiple acoustic arrivals the CCF exhibits multiple peaks with similar heights. To address this issue, we utilize the time-evolving CCFs to select appropriate peaks for the determination of DTT. Using the data collected at Sizhiwan Marine Test Field, Kaohsiung, in September of 2015, the current field was estimated. The towed transceiver in the experiment provided additional reciprocal travel-time data for sensing the water volume at many angles and increased the coverage of the mapping area. Besides augmenting the number of DTT data for the current estimate, some of the data from the towed transceiver are used to validate the estimated field. The experiment site is dominated by the semi-diurnal tidal currents with the principal current direction flowing {along the isobaths}. The time evolution of the estimated current agrees well with the shipboard-ADCP measurements, and the spatial variations are observed when the currents change the direction.