Compensation of magnetic data for autonomous underwater vehicle mapping surveys

Magnetic surveying is well established in land based mineral exploration. Magnetic data is routinely used to map geology in covered terrains, to identify altered zones, mineralization, bedding attitudes, and fault networks. However, other than during specialized commercial, military, and academic surveys, magnetic data is not normally collected on autonomous underwater vehicles (AUVs) and remotely operated vehicles (ROVs) conducting geological mapping and hydrographic survey operations. One reason for this is the magnetic field produced by the local geology is often overwhelmed by the heading and attitude dependent magnetic fields of the vehicle when the magnetometer is mounted close to or inside the AUV. Magnetometers can be mounted away from the AUV with specialized mounting apparatus e.g. a towed body or pole-mounts, but at the cost of increased complication to operations and risk to vehicle safety. To produce useful data from a magnetometer mounted inside the body of an AUV, it is necessary to compensate not only for the attitude of the AUV in the earth's field, but also for secondary effects related to the strength of the electric currents flowing in the vehicle propulsion and vehicle control circuits. To calculate the compensation terms, both a physical calibration routine and a mathematical treatment of the data are required. Prior to each survey, flying a short calibration maneuver enables the calculation of correction terms to the raw magnetic data. The calibration maneuver consists of two sequential, coincidental squares of four calibration legs per square. These squares are flown in opposite directions with the sides of the squares aligned parallel with the primary survey and tie lines. The AUV is typically flown in terrain following (constant altitude) mode at the nominal altitude of the survey, but is changed during each leg to induce pitch into the flight of the AUV. Correction terms are then calculated from the calibration magnetic field data, AUV attitude and heading and vehicle control data. Applying these correction terms to the raw magnetic data collected during the survey produces very useful magnetic maps for interpreting regional, subsurface geology in the survey area. As shown in this paper, OFG has successfully demonstrated this process in commercial surveys.