Reciprocal Sound Propagation Experiment in Very Shallow Water Area of Hashirimizu Port

Abstract

A fundamental experiment for environmental monitoring using reciprocal sound propagation was conducted in a very shallow water area in Hashirimizu Port near the entrance to Tokyo Bay. Sound propagation systems with a 12.5 kHz transducer were placed at opposite piers with a distance of about 120 m. Sound travel time was monitored using M-sequence phase-modulated signals so that an accurate arrival time could be detected even in a noisy environment. Arrival time changed throughout the experiment period about 3 weeks almost in accordance with the changes in seawater temperature. However, under certain conditions, such as a low tide and a high negative gradient of the temperature profile in the depth direction, the arrival time was longer than expected. Ray tracing calculation using Bellhop revealed that all rays radiated by the transducer were refracted downward because of the sound speed profile changes. It turns out that the ray path changes depending on ocean environmental changes, especially changes in temperature and the tides. Current speed estimation from reciprocal travel time showed a trend similar to the measurement result of the acoustic doppler current profiler (ADCP). The low reliability of current can be improved by changing the peak tracking method. It also has a potential to get wide area of ocean environment using reflected waves.