A sub-bottom profiler system for remotely operated vehicle in deep sea was developed to observe surface sedimentary layers of several thousand meter-depth. Using a parametric acoustic array, we realized a very narrow beam width of 4 degrees with the frequency of 3.5 kHz. In this report, first, the dependency of the secondary pressure levels of parametric acoustic sources on the depth of water is calculated by the use of the physical data obtained in real sea. As the result, it is shown that the second pressure levels become slightly higher in deep sea of 6, 000 m than the values at sea surface. Next, a prototype of the parametric source for the experiments under ordinary ambient pressure is developed and tested. The experimental data obtained in water tank agree with the calculated values. Then, in the field experiments of the sub-bottom profiling at the wharf in comparison with the conventional source. Finally, we developed a parametric sub-bottom profiler for 10, 000 m class ROY, and acoustic characteristics of these equipments measured in water tank and experimental results in shallow water are shown.
This paper proposes a new method in sonar signal processing using “Overlapping sectional synthetic aperture.” This method is developed to provide high azimuthal resolution in a synthetic aperture operation even when a ship cannot keep moving on a straight “base” line. Three errors in a ship positioning are assumed, i.e., in travel distance, lateral location and aperture direction. The aperture direction is at first detected with a gyroscope and made parallel to the base line using mechanical feedback system. The travel distance error is measured with an accelerometer and fed into the signal processing procedure. The lateral distance is then corrected as follows. Signals from the overlapping part of two successive apertures overlapping in the base line direction with the lateral deviation in position are compared and the phase difference between two signals are detected. Signal of the non-overlapping part of the second aperture is corrected using this phase difference. It is possible to obtain much higher azimuthal resolution than a single aperture by repeating this procedure. This method does not require to change most of conventional sonar system and the signal processing is performed mostly using Fourier transform. Results of computer simulation prove the effectiveness of this method.