Real-time monitoring of coastal current and throughflow by ocean acoustical tomography with a time reversal responder

Abstract

The conventional Coastal Acoustic Tomography (CAT) system works in a controlled timing of sound transmission from acoustic stations surrounding an observation site. Data received in each station are sent to an integrated data center via communication network and a real-time mapping of current and sound speed is realized. In a new time reversal mirror CAT system (TRM-CAT) proposed, data received at a few land stations by time reversal responders gather all data transferred to the data center. The SNR of received signals is increased and the cost of field experiment is reduced. The TRM-CAT consists of two types of station, land and response stations. The land station is a conventional CAT station that sends coded signals to the response station and receives time reversal signals released. The response station work as a responder, which does not transmit any acoustic signals, until a signals coming from the land station are recorded in a memory, and then reconstructed as a time reversal data to transmit toward the land station at a pre-determined timing. One-way travel time and travel time difference are determined by a matched filter with codes. As a results, the average sound speed and flow velocity can be calculated in land stations. The TRM-CAT is designed and the system simulation is presented. Application to monitoring volume and heat transports through straits and rivers in real time is planned now. Finally, the TRM-CAT serves as a cost effective system for real-time mapping of velocity structures in the coastal sea.