Effective magnetic surveying with multi-thruster underwater vehicles

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Magnetometers are used to record the intensity and direction of local magnetic fields. When mounted on small and inexpensive autonomous underwater vehicles that are programed to traverse a search area, magnetometers provide a means to create magnetic maps that can be used to locate buried minerals. However, to produce useful maps, significant corrections are required to remove the magnetic footprint of the vehicle - the vehicle heading and the vehicle thrusters create fluctuations in the local field that dominate the signal emmitted by local buried features. In 2016, Ocean Floor Geophysics (OFG) successfully developed a Self Compensating Magnetometer (SCM) that can be mounted on Autonomous Underwater Vehicles (AUVs). The compensation system uses knowledge of the state of the AUV's thruster, and a semi-empirical model of the thruster's electromagnetic characteristics to estimate the vehicle's complete magnetic footprint in real time and remove it from the measured signal. In order to adapt the SCM for use in other applications, such as locating shipwrecks, unexploded ordinance, pipes or communication cables, the compensation methodology needs to be enhanced for use on Remotely Operated underwater Vehicles (ROVs); ROVs use an arrangement of several thrusters to move equally well in surge, sway, heave and yaw (unlike an AUV which uses a single thruster to generate surge motion). To modify the OFG SCM correction algorithm so that it can be applied to multi-thruster electric ROVs, a model of the total thruster induced magnetic field must be be derived. This model can be used to determine the structure of the correction equations, and to design a calibration maneuver and optimisation algorithm to automatically determine the coefficients of the compensation equations. In the current project, the modeling, calibration and optimization procedures are being developed and field tested using a small Saab-Seaeye ROV. When completed, the new SCM system will make it possible for archaeologists, police, port authorities, geologists and oceanographers to perform efficient and useful magnetic surveys of submerged terrain.