Reciprocal acoustic transmission experiment at Mihara-Seto in the Seto Inland Sea, Japan

In this Letter, we reported the results of our reciprocal acoustic transmission experiment, which was conducted using four acoustic transceivers at Mihara-Seto of the Seto Inland Sea, Japan. In the study, we introduced two ideas on signal transmission. The first idea is about the periodicity of msequences: the transmission signal in the experiment was one period of an m-sequence of 2,047 digits, but the last 63 and first 64 digits were respectively added to the head and tail of the m-sequence to achieve the original auto-correlation property of repeated m-sequences. The other idea is about the transmission timing of the signal: the start time of each pulse was synchronized among the stations by using the 1 pps signal of GPS, but time-shifted slightly so that the arrival signals do not overlap at all stations. We showed examples of fluctuations in the SNR of the observed arrival peak and the obtained path-averaged currents. Although we did not fully investigate how the two modifications improve the results because of the limited time for the experiment preparation, the SNRs of the observed signal were sufficiently high in general, and the estimates of the path-averaged currents were precise enough to trace the fluctuations over the time series. The path-averaged currents of the two transmission paths synchronously exhibited rapid and short-period fluctuations with slight time lags. It is expected that the capability of the reciprocal transmission to capture short-period fluctuations over long periods can contribute to reproducing precise spatiotemporal variations in current velocity fields in the Seto Inland Sea and other shallow waters. The two-month data analysis is still ongoing. We will investigate the details of spatiotemporal variations in current velocity fields at the observation sites, and the results will be reported elsewhere. © 2021 Acoustical Society of Japan. All rights reserved.