Experiment of acoustic ranging from GNSS buoy for continuous seafloor crustal deformation measurement

The next challenge of the measurement system of the seafloor crustal deformation through the GNSS/acoustic method is the continuous measurement of seafloor crustal deformation using this method. Then we started to develop a measurement system on a buoymoored 32 km off Cape Ashizuri, Japan, named as Kuroshio-Bokujyo buoy No. 18, operated by Kochi prefecture.An acoustic transducer was fixed at about 3 m under the sea surface with a pole mounted on the buoy.Three ocean bottom acoustic transponders had been installed around the moored point of the buoy. The acoustic signals are sequentially transmitted to these three transponders every three minutes. The recorded data on the buoy are transmitted to a ground base station through a commercial communication satellite operated by Thuraya Telecommunications Company.

We performed a test for the continuous acoustic ranging between the buoy and the ocean bottom transponders from March 28 to 31 and June 2 to July 12, 2018. We succeded to perform the acoustic ranging throughout the above periods. The acoustic signals were transmitted 20643 times in total, and the buoy-mounted acoustic ranging unit received 16406 signals from the ocean bottom transponders. However, the acoustic data sent to the ground base station was only 17 % because of unexpected trouble of the communication port of the satellite modem.

The arrival time of the acoustic signal is usually recognized by the cross-correlation computation with the transmitted waveform. When the acoustic signal is contaminated by high amplitude reflected waves from the sea-surface and/or the buoy's body, the cross-correlation coefficient attains maximum at the timing of the reflected wave arrival; then we set an antireflection cover to the transducer. Nevertheless, subsequent waves with the maximum cross-correlation coefficient are found after about 0.3 ms from the direct wave. These subsequent waves are arisen from the reflection at the antireflection cover. We coded a program for detecting the arrival time of direct acoustic wave automatically on the basis of thresholds of cross-correlation coefficient and energy ratio (the ratio of the squared amplitudes preceding and following a test point). The program successfully picked the correct direct wave onset for 99 % of the recorded waveforms.