Towards real-time control of a double gimbaled acoustic probe for measurement of manganese crusts thickness

Abstract

Manganese crusts are the focus of much attention as a potential mineral resource. The crusts form a thin layer of deposit that is distributed over wide areas on the surface of seamounts. To date, the only way to know the thickness of manganese crusts has been through sampling and drilling, but these methods are expensive and time consuming. We have developed an acoustic device that can determine the thickness of manganese crusts during surveys using an underwater robot. This system has worked successfully during sea trials on the flat top of Takuyo-Daigo seamount, located in the northwest Pacific. However, manganese crusts are known to be distributed not only on flat surfaces but also on the slopes of seamounts, and while accurate measurements are possible on relatively flat areas of the seafloor, the quality of measurements made on steep slopes are degraded by the large angle of incidence of the necessarily narrow beam of the acoustic probe. The aim of this research is to investigate potential methods to improve data return rates of manganese crust thickness measurements on steep slopes. The effects of the angle of incidence on the acoustic reflections measured from a manganese crust sample are determined experimentally and it is found that an angle of incidence larger than 3° has a detrimental effect on the measured reflected signal. High resolution 3D bathymetry data acquired using the 3D visual mapping device `SeaXerocks' was analyzed, so as to determine the actual local inclination of crust covered seafloor. Based on the analysis, it is suggested that a double gimbal system should be introduced to actively control the angle of the acoustic probe so that the acoustic beam enters the seafloor orthogonally. In order to achieve real-time control, a method to determine the relative angle based on sparse measurements of the seafloor is necessary. The control algorithm developed uses two range sensors, consisting of a line projected laser and a single point acoustic range sensor, mounted on front and back of underwater robot so that the relative inclination of seafloor can be determined in real-time. The performance of the developed double gimbal system has been assessed in water tank experiments working on simple sloped models of known geometry. The experiments demonstrate an improvement in the intensity of the acoustic reflections from the slopes as a result of the active control. The setup was used during sea trials at a manganese encrusted seamount to survey steeply sloped surfaces, and the results of the sea trials are presented.