Bathymetry by new designed interferometry sonar mounted on AUV

AUV (autonomous underwater vehicle) "r2D4" that can act under the sea up to a depth of 4,000 m was built to explore the seabed and observe hydrothermal activities. The r2D4 was designed to be equipped with side scan sonar SYSTEM2000 as one of many observation sensors. Because side scan sonar is mainly used for getting geological data, we have been developing a system to get bathymetric data using combination of side scan sonar and three hydrophones. We are aiming at bathymetric survey using the AUV r2D4 equipped with this system. Some groups have already developed interferometric side scan sonar system for bathymetric survey. Most systems are equipped with conventional side scan sonar system and two hydrophones whose distance is set at nth wavelength of the transmitter signal. However such an interferometric side scan sonar system using two hydrophones has inherent defects and technical issues. For example, the longer we extend the distance of two hydrophones, even though we can get higher resolution data of elevation angle of received signals, the more difficult it is to discriminate which data are correctly expressive of the real seafloor. This is why we could not get bathymetric data in high resolution using this inteferometric side scan sonar system with two hydrophones. Consequently we started to develop a new interferometric side scan sonar system which has three hydrophones that are arranged in a two-dimensional array of L-shape. Each length of the two legs of the L-shape is 3.4 times and 13.6 times the wavelength of the transmitter signal (100 kHz). With this new system we could get resolution enhancement and eliminate the defect, described above, that previous interferometric side scan sonar systems with two hydrophones have. In this paper we describe advantages of the new system and show 3D-mappings as the results of experiments and surveys. We describe the method, now being tried, to get higher resolution data.