Séminaire de Eiichi Asakawa (J-MARES/JGI, Tokyo, Japan)

“Cross-ministerial Strategic Innovation Promotion Program (SIP)” was launched by the Council for Science, Technology and Innovation (CSTI) in 2014. It has addressed eleven issues selected considering the critical social needs. “Next-generation technology for ocean resources exploration (Zipangu in the Ocean) “is one of the SIP issues. In this project, we aim to establish the survey protocol for seafloor mineral resources and have been developing technologies for ocean resources exploration at efficient and low cost.

The primary target is seafloor massive sulphide (SMS). Many hydrothermal activities have been found in submarine volcanic areas distributed along the Izu-Bonin Arc and the Okinawa Trough in Japan. SMS ore deposits exist in deep water (>1500m) and the target depth is less than 50m below the sea bottom. Therefore the survey platforms that can reach close to the sea bottom, such as AUV and ROV, are important. For example, the technology of multiple AUVs operation using ASV enables us the efficient and minute bathymetry measurement that is the first step of SMS exploration in the deep-sea floor. Figure 1 shows the fleet of AUVs and the specifications developed by National Maritime Research Institute (NMRI) in SIP. Development of ROVs equipped with multipoint coring system and sonar imaging system enables efficient, low cost sampling.

In my presentation, I particularly focus on seismic surveys. High resolution seismic technologies are essential to investigate the concealed SMS ore deposit. Our patented technology, developed for high resolution seismic survey systems are shown in the Figure below. ACS is a survey system using a deep-towed streamer with surface and/or deep- towed seismic sources. We carry out ACS at the first stage of the exploration to extract possible hydrothermally active areas ranging from tens square kilometers. ZVCS consists of a deep-towed vertical receiver cable with surface and/or deep-towed seismic sources. There are two kinds of ZVCS, using deep-tow(ZVCS-DT) or ROV(ZVCS-R). Compared to ZVCS-DT, ZVCS-R enables high lateral density shooting and very flexible operation. As a second stage survey, ZVCS-DT and ZVCS-R are planned to narrow down the prospective areas which has been extracted by ACS. In 3DVCS, multiple vertical cables are moored on the seafloor. A circular shooting at the sea surface can clearly visualize detailed 3D sub-seafloor images. To obtain a precise depth image, a 3D pre-stack depth migration (3D-PSDM) is applied to the 3DVCS data. The velocity model for 3D-PSDM can be built by the velocity analysis of common scattering point gathers obtained by the equivalent offset migration and the result of refraction tomography.