The deep sea cruising AUV URASHIMA is an autonomous underwater vehicle for deep-sea exploration developed and operated since 1998 by JAMSTEC. The vehicle can be pre-programmed to travel autonomously, using its onboard computer. As it can get closer to the seabed during exploration than a research vessel can, AUV URASHIMA acquires extremely high-resolution seafloor topographic data and sub-bottom profiler data. The AUV is able to obtain sufficient data to form a detailed bathymetric map of a 5 km²-wide area per dive by travelling about 100 meters above the seabed.
Hydrothermal deposits have received much attention as important seabed resources in recent years. For exploration of hydrothermal deposits, a wide area is first surveyed with a vessel to draw a rough bathymetric map. A target area selected based on the map is then surveyed with an autonomous underwater vehicle so as to create a high-resolution bathymetric map. Seafloor topographic and morphologic structures and sometimes hydrothermal venting are identified in this detailed map. A target dive site is then determined to survey with a manned research submersible or remotely operated vehicle, and collect samples of hydrothermal deposits and fluids. These are the standard procedures for seabed topographic surveys, and an AUV has a key role in obtaining the data necessary to draw a detailed bathymetric map for deep-sea exploration. The features of research vessels, AUVs and ROVs in such surveys are listed below.
- Vessels can obtain the mapping data of a wide area.
- The resolution of the data is low because 10 kHz-order acoustic signals are used, and vessels are distant from the seabed.
- AUVs can obtain the mapping data of a relatively wide area.
- The resolution of the data is high because AUVs can approach the seafloor, and 100 kHz-order acoustic signals are used.
- AUVs collect the mapping data, not mud or rock samples.
|Manned research submersibles and ROVs|
- Submersibles and ROVs are equipped with cameras and manipulators which enable researchers to take images and collect geological and biological samples.
- A target dive site of submersibles and ROVs is small so they are less suitable for surveys covering extensive areas.
- Cables between ROVs and vessels limit the mobility of ROVs.
Vehicles for deep-sea exploration used to be always connected with their support vessels by cable during surveys. The cable(s) between a vehicle and its support vessel limit(s) the mobility of the vehicle, allowing it to move around only in a relatively small area. In addition, rolling and pitching of the support vessel and/or connected components, if any, might affect the vehicle. Therefore, the autonomous navigation and stable observation capabilities raise the importance of AUVs.
AUV URASHIMA started its life as an experimental vehicle. A number of tests and trials were conducted to collect performance and observational data, which were utilized to refine the vehicle for better usability. In 2009, URASHIMA was finally put into practical use, and JAMSTEC began to accept applications from both JAMSTEC and external researchers seeking the use of the vehicle.
Acoustic mapping is not the only key feature. URASHIMA can carry large-sized observational instruments by taking advantage of its large body of 1.3 m in width and 10 m in length. Its payload space is large enough for four adults. Therefore, a variety of instruments such as the following have been loaded on the AUV.
Furthermore, the built-in control system was modified so it can transmit and receive acoustic signals to and from instruments mounted by researchers. It is now possible for researchers to send commands (Send, Record, Lock, etc.) to their own instruments underwater and confirm the operability of instruments.
Many autonomous underwater vehicles are commercially available in recent years. Very few AUVs, however, are as large as the 10 meter-long AUV URASHIMA. The vehicle is likely to be only one AUV in the world which can carry large-sized instruments, can be towed, and can send and receive commands to and from instruments.
In order to cruise autonomously, AUVs must be able to determine their position by themselves, and to calculate how far they have moved. The global positioning system (GPS) is available for personal or automobile navigation on land, but the radio waves used by GPS cannot propagate underwater, making it impossible to use this type of navigation below the sea surface. URASHIMA, therefore, uses a navigational system that combines the strengths of hybrid navigation and acoustic navigation. The former is inertial navigation together with the data of a Doppler velocity meter. The inertial navigation system measures inertial motion of a body with robust, stable, high-precise ring-laser gyros and motion sensors in tri-axes from time to time, and calculate moved distance based on the Newton's law of motion. The latter, acoustic navigation, works by calculating distance based on round travel time of acoustic signals between the AUV and acoustic transponder deployed at known location before cruise.
The power source for URASHIMA is a lithium-ion battery. Lithium-ion batteries are high-performance energy storing devices characterized by the high energy density and long service life, and therefore, used in mobile phones and electric cars. The battery URASHIMA carries is the same as the ones SHINKAI 6500 does. A lithium-ion battery is recycled and loaded onto URASHIMA after being used in SHINKAI 6500. URASHIMA used to carry a closed-cycle polymer electrolyte membrane fuel cell in the years of operating as an experimental vehicle. On February 28, 2005, the AUV set a world record for the continuous cruising distance of 317 kilometers.
AUV URASHIMA is equipped with a multi-beam echo sounder to obtain topographic data of the seafloor. Echo sounders emitting higher-frequency acoustic waves provide higher-resolution imagery, but have less detection range. AUVs can keep the altitude or depth and transmit acoustic waves with a high frequency of 400 or 200 kHz near the seabed, resulting in higher-resolution topographic images than the ones based on the data obtained through MBES surveys using vessels on the sea surface.
A side scan sonar transmits acoustic waves not from the bottom of but the side(s) of a vehicle's body, and an image of the seabed is produced based on backscattered acoustic signals from the seabed or some objects. URASHIMA is also equipped with a side scan sonar. Shown below is a side scan sonar image of a mud volcano.
|Maximum depth capability||3,500 m|
|Cruising distance||More than 100 km|
|Dimensions||10 m (L) × 1.3 m (W) × 1.5 m (H)|
|Weight||Approx. 7 tons|
|Cruising speed||0 – 3.0 knots|
|Power source||Lithium-ion battery|
|Operation||Autonomous or acoustic remote control (wireless, operated from the support ship)|
• Multibeam echo sounder
• Side scan sonar
• Sub-bottom profiler