A reciprocal acoustic transmission experiment for precise observations of tidal currents in a shallow sea

Abstract Observations of ocean currents are important information in the fields of ocean science and engineering. Reciprocal acoustic transmissions have the potential to operationally measure tidal currents in coastal seas. This study investigates the capability of the reciprocal transmissions to measure tidal currents, particularly in relation to high-frequency variations. We conducted a reciprocal acoustic transmission experiment in an area of complex coastlines in the Seto Inland Sea, Japan, in October 2019. During this experiment, a burst transmission was used, and the system performed six transmissions with 1-min intervals between them every 10 min. The estimated path-averaged currents agreed with the currents obtained from a shipboard acoustic Doppler current profiler (ADCP) on an hourly basis and had captured high-frequency variations that were not captured by the hourly ADCP observations. The measurement noise of the estimated currents, which was determined as the variation within the six transmissions, was 2.5 cm s−1 and was small enough to trace the variations in the currents over the time series even for the variations with periods considerably shorter than M2 tide. The reciprocal transmission’s capability to capture high-frequency variations will improve the capability to reproduce current velocity fields when inversion or data assimilation methods are applied.