Next R&D of measurement method of offshore minerals using the low-frequency acoustic Mills-cross transmission and receiving system ∼ multibeam sub-bottom profiling method and "Natsushima" cruise experiment ∼

This paper discusses new approach about a new sub-bottom-profiling method for the precise measurement of the mineral source layer which exists under the hydrothermal deposits. In Japan, the seafloor hydrothermal deposits in domestic exclusive economic zone (EEZ) have attracted a lot of attention in recent years because it contains much offshore minerals under them. The technique which can measure the ore reserve of offshore minerals of seafloor hydrothermal deposits accurately and effectively above the seafloor is desired. To realize these technologies, authors perform a research and development of the exploration method to measure sub-bottom structure and the seafloor mineral distribution under the seafloor hydrothermal deposit by the high resolution. For this purpose, it is most preferable that underwater cruising platform such as AUV performs a more accurate sub bottom profiling in the order of one meter. To realize these aims, authors propose and plan to develop the method of the acoustic transmission and receiving system which adopt the Mills-cross method while maintaining a low water altitude by installing the low frequency acoustic source in AUV. We developed the low frequency acoustic source at the feasibility study in 2009. And we succeeded to develop the highly-accurate sub-bottom profiling method and showed the feasibility of this measurement method. Based on these results, authors conducted a cruise experiment in the vicinity area of Iheya knoll in Okinawa trough by the research vessel "Natsushima" of JAMSTEC in February 2011. There were a lot of seafloor hydrothermal deposits in that area. In this experiment, authors installed our prototype acoustic source in the experimental towing vehicle "Deep-Tow" for the deep water exploration. And we performed data acquisition of the sub-bottom profiling under seafloor of the actual seafloor hydrothermal deposit at a 100m altitude from the seafloor. This cruise was an observation that verifies the practicality of a new sub-bottom profiling technology that has high resolution in a low altitude of 100m from the sea floor. Authors' group succeeded to acquire the data of the layer of the mineral source under an actual hydrothermal deposit by this experiment. In this paper, authors show the plans, conditions of our cruise test, and display the proposed approach of new sub-bottom profiling method.