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Sulfide deposits with extremely high Au concentrations (up to 275 ppm; avg. 102 ppm, n = 15), high Au/Ag ratios (0.24, n = 15), and low Cu/(Cu + Zn) ratios (0.03, n = 15) were discovered in 2015 in active hydrothermal fields at a water depth of 760 m in a basalt-dominated submarine caldera in the Izu-Ogasawara frontal arc, Japan. Native gold grains occur in massive sulfide fragments, concretions, and metalliferous sediments from a sulfide mound (40 m across and 20 m high) with up to 30-m-high black smoker chimneys. Tiny native gold grains up to 14 μm in diameter are mainly present in sulfide fallouts from chimney orifices and plumes. Larger native gold grains up to 150 μm long occur mostly as discrete particles and/or with amorphous silica and sulfides. The larger gold grains are interpreted to represent direct precipitation from Au-bearing hydrothermal fluids circulating in and/or beneath the unconsolidated sulfide mound deposits. Sulfur isotope compositions from a limited number of sulfide separates (n = 4) range from 4.3 to 5.8‰ δ34S, similar to the quaternary volcanic rocks of the arc. Barite separates have values of 22.2 and 23.1‰, close to modern seawater values, and indicate probable seawater sulfate origin. The Cu, Zn, and Pb concentrations in bulk samples of sulfide-rich rocks are similar to those of volcanogenic massive sulfides formed in continental crustal environments. The gold is interpreted to have formed by low-temperature hydrothermal activity, perhaps genetically different from systems with documented magmatic contributions or from seafloor hydrothermal systems in other island arc settings. Its presence suggests that basalt-dominated submarine calderas situated on relatively thick continental crust in an intraoceanic arc setting such as the Higashi-Aogashima knoll caldera may be perspective for gold mineralization.

The exploration of buried deposits under the sea sediment requires high directivity and low attenuation beams. The parametric sub-bottom profiler meets these requirements; moreover, it is comparatively small and can be easily mounted on autonomous underwater vehicles (AUVs). For this reason, we are developing a new PSBP system for AUVs. This paper presents the recent results of the PSBP system for AUV during the sea trial conducted under the “Zipangu of the Sea” Project as a sub-program of SIP (Strategic Innovation Promotion), managed by the Cabinet Office of the Japanese government.

The exploration of buried deposits under the sea sediment requires high directivity and low attenuation beams. The parametric sub-bottom profiler (PSBP) meets these requirements; moreover, it is comparatively small and can be easily mounted on autonomous underwater vehicles (AUVs). For this reason, we are developing a new PSBP system for AUVs. In this study, the performance of the PSBP system is estimated by using a sonar equation based on the transformed beam equation (TBE) and the Biot-Stoll model to optimize the measurement system.

Small AUV “IK-A” equipped with IRSAS was developed for submarine hydrothermal deposits exploration. Observed IRSAS data could acquire the back scatter image and bathymetry. One of the features of the seafloor hydrothermal deposits is the chimney and the mound-like terrain. Back scatter image can distinguish these terrain and other terrains. IRSAS has increased much measurable terrain by switching the pointing angle and real/synthetic aperture. On the other hand, perturbations of small IK-A, it was detected that affects the synthetic aperture processing. At the moment, there is a need to solve the problems of the following two points. Improving motion correction for synthetic aperture processing. Determination of the switching criteria of the narrow beam transmitter or wide beam transmitter.

This paper focuses on developing a NEW Autonomous Underwater Vehicle (AUV) equipped with Interferometric Real and Synthetic Aperture Sonar (IRSAS) for the exploration of submarine hydrothermal ore deposits. After the construction, the first sea trial carried out in Feb 2016. From successfully results, we feel certain that new AUV play an important role in surveys of hydrothermal deposits.

We are developing a technique for the systematical exploration of submarine hydrothermal ore deposits. In the Izu-Ogasawara Arc area these deposits are usually characterized by the existence of tower-like vents 2∼20m high by 2∼5m wide. We are using a set of Interferometric Real Aperture Sonars (InRAS) to generate 3D acoustic images of the areas surrounding the vents. We not only improved the resolution thanks to a long interferometric baseline, but we also efficietnly unwrapped the phase in this zone of large elevation differences by applying a two-stage approach (Branch Cut and SSPU methods). These improvements allow us to map the seafloor bathymetry around the submarine hydrothermal deposits. Since April 2014 we are conducting in-situ observations of both known and potential zones of hydrothermal deposits. Moreover, in November 2015 we will gather validation data in an experimental pool.

Digging is necessary to detect plant roots under the water bottom. However, such detection is affected by the transparency of water and the working skills of divers, usually requires considerable time for high-resolution sampling, and always damages the survey site. We developed a new automatic non-destructive acoustic measurement system that visualizes the space under the water bottom, and tested the system in the in situ detection of natural plant roots. The system mainly comprises a two-dimensional waterproof stage controlling unit and acoustic measurement unit. The stage unit was electrically controlled through a notebook personal computer, and the space under the water bottom was scanned in a two-dimensional plane with the stage unit moving in steps of 0.01 m (±0.0001 m). We confirmed a natural plant root with diameter of 0.025–0.030 m in the reconstructed three-dimensional acoustic image. The plant root was at a depth of about 0.54 m and the propagation speed of the wave between the bottom surface and plant root was estimated to be 1574 m/s. This measurement system for plant root detection will be useful for the non-destructive assessment of the status of the space under the water bottom.