In this paper, the authors propose a novel exploration system for ocean floor resources using an autonomous underwater glider and OBEM (Ocean bottom electromagnetometer). Studies on the gliding performance and the hydrodynamic characteristics of several kinds of body shape were carried out for the preliminary design of the system. Gliding ratios in a steady flow were estimated based on CFD calculation for different main wing shapes. The effects of the size of the body and the camber of the main wing were examined as well. It was confirmed that designed vehicle had efficient gliding performance to achieve the autonomous OBEM measurement.
Effective use of ocean floor resources such as sea-floor hydrothermal deposit, methane hydrate and manganese nodule are expected and the development of the exploration method for these resources is urged.
Seismic exploration, a multi-beam echo sounder and a sub-bottom profiler equipped with a survey ship or underwater vehicles are used for the ocean floor resources exploration generally. An OBEM (Ocean bottom electromagnetometer) is one of these instruments which measures variation of the electromagnetic field on the sea bed (Kasaya, 2009). It is a promising method which able to meet with the exploration in deep water as well. To achieve the accurate observation by an OBEM, multiple measurements at many survey points are required and it results in high cost of the exploration.
On the other hand, various types of underwater vehicles have developed and they are playing active roles in ocean observations and explorations (Yoshida, 2013). Underwater gliders are one of these vehicles and used for ocean observations, marine surveillance and so on (Eriksen, 2001; Sherman, 2001; Yamaguchi, 2007, Javaid, 2014). The authors propose an autonomous underwater glider which equips an OBEM. The vehicle is composed of a main wing, horizontal and vertical tail wings, a buoyancy controller and a center of gravity shifter in pressure vessels. Some motion sensors and a battery as a power source are also contained in the watertight vessels. This autonomous vehicle has ability to achieve the continuous measurement of ocean floor resources using the OBEM. The buoyancy control mechanism is used for ascent after the measurement at the first survey point and the vehicle moves to the next survey point by gliding. The landing point of the vehicle is precisely controlled by the motion control system using the tail wings. Four long electrode arms of the OBEM are also used as frames of the vehicle body and they function as strength members. Large resistance of the four long electrode arms are expected when the conventional OBEM is moved to the next survey point in water, so the underwater glider needs to reduce the resistance for long time continuous observations. The proposed body shape can reduce the resistance by the main wing which covers the electrode arms and enables autonomous explorations of ocean floor resources using the OBEM.
