Realization of acoustic monitoring of rip currents based on passive reciprocal sound transmissions

In order to measure the velocity of rip current, we have developed an acoustic monitoring system consisting of two transducers and several sound reflectors. The two transducers are connected to a modified two-frequency fish-finder which can alternatively transmit and receive the preset sound signals of 50kHz or 200kHz at an interval of several seconds. Sound reflectors were composed of commercial steel rods. The sea trial of this system was carried out in the Uradome Beach in early September 2012. The transducers were placed at a height of 0.5m over the sea floor at a depth of 1m to 1.5m and horizontally separated at a distance of 30m offshore. Six reflectors were placed at a distance of 15m, 30m, 45m, 60m, 75m and 90m along the shoreline. The sound signal of 50 kHz burst waves were transmitted by one transducer and received by the other, and vice versa. The SNR´s of the signals through the direct base line and through reflective paths of the 30m, 45m and 60m reflectors were sufficient for the analysis as expected. The preliminary result suggested to be able to reconstruct a current velocity distribution in the beach based on passive reciprocal sound transmissions.