Sub-bottom synthetic aperture imaging sonar system using an AUV and an autonomous surface tracking vehicle for searching for buried shells of toxic chemicals

On Kanda port construction in Fukuoka prefecture, no fewer than 2,800 harmful chemical bombs have been discovered beneath the sea bottom so far. In advance of dredging in the port, we have carefully carried out magnetic inspection for looking for chemical bombs. Since a true target often gets mixed in a lot of suspicious metallic objects, we must cautiously draw up detected objects one by one. Use of a magnetic sensor only is uneconomical and inefficient. On account of these, we started a new three-year project of developing a sub-bottom synthetic aperture imaging sonar (sub-bottom SAS) system by using an autonomous underwater vehicle (AUV) and an autonomous surface vehicle for tracking the AUV. The AUV Tri-Dog 1, which was developed by T. Ura and H. Kondo, is going to be used. The AUV will be installed with a new sub-bottom SAS system and could perform stable and suitable navigation for SAS imaging in shallow water. The surface autonomous tracking vehicle can compensate for positioning error of the AUV and monitor sub-bottom imaging. The synthetic aperture sonar technology for sub-bottom imaging is now in the worldwide limelight, and several projects are ongoing. Since most of chemical bombs were buried up to 5 meters beneath the sea bottom, operating frequency 10 kHz to 30 kHz is expected to be useful for sub-bottom SAS. At the beginning of the project, we measured acoustical characteristics of sub-bottom sediments in Kanda port in 2010 May using a 33kHz single beam echo sounder RESON-Navisound 410 and a parametric sub-bottom profiler Innomar SES2000 Compact. Based on the acoustical characteristics, we will design a sonar projector and hydrophone arrays and a new surface autonomous tracking vehicle in this year.