Small scale reciprocal sound propagation analysis in Hashirimizu port

Small scale reciprocal sound propagation was carried out at Hashirimizu port in front of Tokyo Bay, Japan. A pair of the transducers with the distance about 120 m was set at the bank of the port. The average depth at the area is about 4 m. There were so many surface and bottom reflections. In this study, authors investigate the effects of ocean changes such as temperature, tidal level and current, to the reciprocal sound propagations. The 7th order M-sequence was sent every 5 minutes with carrier frequency of 80 kHz. The travel time mainly varied according to the water temperature. But sometimes, it shifted rapidly which could not be considered the effect of the water temperature. As there was almost 1.5 m depth changes because of the tide, the strength of receiving signals also changed according to the interferences of surface and bottom reflections. The biggest peak of the correlated signal was shifted under the conditions of the depth and temperature. It was also confirmed by calculations by finite-difference time-domain (FDTD) method. Because of these complicated interference, it was difficult to estimate current along the propagation path although it was improved by the peak tracing method. But there is still possibility to monitor water flows with few transducers. This method will be possible to monitor the average changes along the sound propagation area. Moreover, it will enable to monitor more accurate temperature or flow distributions using more transducers to create tomography system in the future.