An acoustic monitoring system of the rip current based on passive reciprocal transmissions

In the beach, the rip current is one of dangerous swift currents to carry swimming persons offshore. Sometimes, they are drowned because of this current. So it is desirable to detect and monitor the occurrence of the rip current in realtime. In general, the acoustic method is powerful at the view of underwater remote-sensing, and the reciprocal transmission is one tool to measure direct currents. But, usually acoustic transceivers are too expensive. Therefore, we propose an acoustic monitoring system combined with two acoustic transceivers and several sound reflectors for the measurement of the rip current. The core idea is shown in figure 1. An acoustic signal is transmitted by Tx₁ and/or Tx₂ and reflected by the sound reflector R₁ and received by Tx₂ and/or Tx₁. The difference of travel times between forward transmission Tx₁-R₁-Tx₂ and reverse transmission Tx₂-R₁-Tx₁ is given by the following relation: δ t= 2VL/C² or V=C²(2L)^-1δ t where L is the distance between Tx₁ and Tx₂, V is the velocity of the mean flow over the triangular area Tx₁-R₁-Tx₂, and C is the sound speed over the area. This relation is applicable to the other sound reflectors in figure 1. The sound reflector R₂ gives a mean velocity of V₁ and V₂. The sound reflector R₃ gives a mean velocity of V₁, V₂ and V₃. Therefore, this set of three different mean velocities is converted to a zonal current field. If the configuration is properly designed, the current field can be expected to monitor when and where the rip current will occur. The prototype of the monitoring system was designed and composed of two 14 kHz acoustic transceivers and a non-directional sound reflector. The acoustic test was carried out i- the sea of Hiratsuka beach. As a result, it was found out that the bottom backscattering and reverberation of transmitted sound waves was stronger than expected ones, and disturbed receiving sound signals transmitted by each transceiver and reflected by sound reflectors in a short range. The reflected signals were detected at a range of 100m. The application of the replica cross correlation was successfully carried out. The result was effective to determine the precise travel time of direct and reflective waves. In order to resolve these problems properly, the frequency of sound waves should be changed to a higher frequency. A new acoustic monitoring system composed of an echo sounder, two broadband acoustic transducers and several sound reflectors for the measurement of the rip current is being developed. This system can use the 50kHz transducer for a long range and the 200kHz for a short range. The more detailed analysis will be discussed in the presentation.