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

Content Provider	IEEE Xplore Digital Library
Author	Nakano, I. Ishida, H.
Copyright Year	2012
Description	Author affiliation: Global Ocean Development Inc., Yokohama, Japan (Nakano, I.) \|\| Kobe University, Kobe, Japan (Ishida, H.)
Abstract	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 Tx and/or Tx and reflected by the sound reflector R and received by Tx and/or Tx. The difference of travel times between forward transmission Tx and reverse transmission Tx is given by the following relation: δ t= $2VL/C^{2}$ or $V=C^{2}(2L)^{−1}δ$ t where L is the distance between Tx and Tx, V is the velocity of the mean flow over the triangular area Tx, and C is the sound speed over the area. This relation is applicable to the other sound reflectors in figure 1. The sound reflector R gives a mean velocity of V and V. The sound reflector R gives a mean velocity of V, V and V. 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 in 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.
Starting Page	1
Ending Page	6
File Size	1159636
Page Count	6
File Format	PDF
ISBN	9781457720895
e-ISBN	9781457720918
DOI	10.1109/OCEANS-Yeosu.2012.6263446
Language	English
Publisher	Institute of Electrical and Electronics Engineers, Inc. (IEEE)
Publisher Date	2012-05-21
Publisher Place	Korea (South)
Access Restriction	Subscribed
Rights Holder	Institute of Electrical and Electronics Engineers, Inc. (IEEE)
Subject Keyword	Acoustics Transceivers Sea measurements Correlation Monitoring Acoustic measurements Current measurement sound reflector rip current current velosity recirocal transmission
Content Type	Text
Resource Type	Article

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