Kenji Nagahashi

Expanded interferometry and synthetic aperture applied to a side scanning sonar for seafloor bathymetry mapping

A new designed Autonomous Underwater Vehicle r2D4 for operation to 4,000 meters was built for the purpose of seafloor mapping on the oceanic ridge having hydrothermal activity in July 2003. An expanded interferometry sonar with operating frequencies of 100 kHz was installed on the AUV for high resolve bathymetry mapping, not to mention the backscatter imaging. In effect, the interferometry sonar is simply composed of three hydrophones, which are arranged at intervals of three and thirteen wavelengths in the L-shape at each side, in addition, to a side scanning sonar Klein System 2000 with operating frequencies of 100 kHZ and 500 kHz. It explains features of low cost and low power consumption. An initial survey test of the AUV was carried out up to 500 meters deep off Ryotsu port of Sado Island in July 2003. In advance of the sea test of the AUV, a test of the interferometry sonar with the barge had been carried out. L-shape array of hydrophones enables us to make a phase difference measurement successfully with high resolution. It follows that a detailed swath bathymetry was brought about. Consequently, a seafloor mapping was carried out to reveal the detail topography on the top of Kuroshima knoll in the vicinity of Okinawa islands in December 2003. The AUV is installed with the PHotonic Inertial Navigation System (PHINS), which provides inertial navigation and motion measurements of the AUV. Furthermore, since the AUV is able to move underwater very stably along planed courses, it is a proper platform in materializing the synthetic aperture of the side scanning transducers. We have been devoting ourselves to process the observation data. As a result, the synthetic aperture technique was very useful for the improvement of the seafloor bathymetry mapping as expected.

High‐resolution acoustic mapping to understand the ore deposit in the Bayonnaise knoll caldera, Izu‐Ogasawara arc

We collected deep-sea multibeam, side scan, and subbottom profiler data using an autonomous underwater vehicle at the Bayonnaise knoll, a submarine caldera located in the rift zone of the Izu-Ogasawara arc. We aimed to reveal topographic and geological features and the origin of a hydrothermal field called the Hakurei site in the caldera. We performed seafloor classification by textural analysis using calibrated side-scan sonar data, which provided an effective means to understand the geology and to highlight potential areas of hydrothermal constructions. The high-resolution bathymetric map illustrates that the Hakurei hydrothermal field is distributed over a landslide landform in the caldera wall. The distribution of hydrothermal vents indicates that the slip surface has served as a major route of hydrothermal fluids. The radial alignment of chimneys and mounds indicates radial routes of hydrothermal fluid and/or belching along fragile lines in the landslide landform. Various postcaldera activities are inferred including the formation of a lava dome, a pyroclastic cone, and subsequent phreatic explosions. A general volcano-tectonic structure extending across the caldera in a NW-SE direction is interpreted as an inferred boundary fault of the North Myojin Rift. Analogous to the Hokuroku basin and land kuroko deposits, it is suggested that the main contributing factor in the formation of kuroko deposits was volcano-tectonic activity that dominated the margin of the back-arc rift basin. The intersections between the margin of a rift basin and the surrounding knolls have a high potential for ore-forming areas.

Development of an autonomous underwater vehicle for survey of cobalt-rich manganese crust

AUV BOSS-A with visual and acoustic instrument device has been developed to survey manganese crusts located at the depth of 1,000 to 2,400 m. The AUV navigates at constant speed and altitude along waypoints with primary set, and automatically observes the seafloor by developed instruments. Visual instrument device mounted on the AUV creates 3D color reconstruction which shows the condition and the distribution of manganese crusts. Mounted acoustic instrument device is able to measure manganese crusts thickness. This paper describes the system and survey method of the AUV BOSS-A and survey results of sea trial for the demonstration of the manganese crust survey using the AUV. Survey results shows that manganese crusts with thickness of 15 to 45 mm are distributed at Katamaya sea mount. coustic instrument devices.

Design and concept of a Biointeractive Autonomous Underwater Vehicle “BA-1”

To keep the precious protein source, it is thus extremely important to conserve the biodiversity of the ocean and at the same time to make use of its limited space as much as possible without doing any environmental harm. With this broad objective in mind, Tokyo University of Marine Science and Technology (TUMSAT) and Mitsui Engineering & Shipbuilding Co., Ltd. have started a research and development project, “Pen-free Offshore Aquaculture System using Biointeractive Autonomous Underwater Vehicles.” This project is part of a more comprehensive project, “Marine Biotechnology Innovation,” which has been funded by the Japanese government since 2007. In this paper, we propose a concept of a biointeractive AUV that monitors and takes care of schools of fish just like a sheep dog in a ranch. In a future plan of this system, multiple biointeractive AUVs monitor the environment spatially and accurately, feed the fish, monitor the growth of the fish, guide them, and report the data through satellite to the land while charging their batteries by docking with buoys. A first model of the biointeractive AUV “BA-1” has launched in early 2009. The design of the biointeractive AUV “BA-1” is described in this paper. The basic test result of the interaction system for fish is also described briefly.

Evaluation result of new seafloor mirror transponder and AUV observation system in seafloor geodetic observation

Intending to capture sea-bottom crustal deformations with high accuracy, we have been developing new technologies for seafloor geodetic observations with an AUV. This has led to the development of a small high-performance geodetic observation system and an ideal spherical acoustic transducer, both to be mounted on an AUV, and the tuning and evaluation tests of them have yielded good results.

Development and sea trial of an Autonomous Underwater Vehicle equipped with a sub-bottom profiler for surveying mineral resources

In recent years, Autonomous Underwater Vehicle (AUV) has received much attention as a tool of ocean survey. One of the method for the efficient of hydrothermal deposits investigation is multiple vehicle operation of AUVs. The aim of our work is to development the small and inexpensive AUV and operate together with other AUVs. This paper introduce the first AUV in our project, and show the result for the sea trial of the AUV.

Investigation method for the biomass of kichiji rockfish by hovering type AUV

We surveyed the biomass of valuable kichiji rockfish using hovering type AUV Tuna-Sand which developed for observation of the seafloor, in Kitami-Yamato bank in northern ocean of Japan at June 2013. The AUV took about 5,300 photography of the seafloor for over 24 hours. The photograph had 37 kichiji rockfishes 90 mm to 340 mm long. Kichiji rockfishes which were taken several times by the AUV stayed on the seafloor at same attitude. TS_076 dive point of 652 m depth had most number of kichiji rockfish and many brittle stars. TS_077 dive point of 305 m depth had two fishes although the AUV observed the broadest range in all dive points. In TS_079 dive, the AUV observed ten fishes although observation area was the smallest, and fish density (= number per 1 ha) was over 220 /ha. Made photomosaic shows that a kichiji rockfish stayed in oneself without troop and shortest distance between two fishes was about 4.0 m.

Fundamental Developments of New Generation Seafloor Geodetic Observation System Based on AUV Technology

Institute of Industrial Science, University of Tokyo (IIS) has started a project to develop the fundamental technologies for constructing new-generation seafloor geodetic observation system. The current observational method using research vessel cannot help being subjected to annual cruise schedule of research vessels. It has been difficult for us to change the cruise schedule as appropriate according to weather and sea condition, GPS satellite distribution and so on. The new system, which we are developing, based on AUV technology will give us opportunities for observation with choosing favorable conditions of sea and GPS satellite distribution, much more frequent observations and flexible planning of observation in response to sudden geodetic events. Trial models of the sea surface and seafloor units were finished. We conducted several performance evaluation experiments in the sea and dam site, in order to bring the observation system to completion.

Development of the parametric sub-bottom profiler for autonomous underwater vehicles — Prediction of the reflection level using sonar equation based on TBE and Biot-Stoll model

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.

AUV IRSAS for submarine hydrothermal deposits exploration

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.