Marine Vector Magnetometer on RRS Discovery

The report describes the work undertaken as part of a feasibility study in using an established technique for estimating the absolute strength and direction of the geomagnetic field at sea. Measurements were made on board the RRS Discovery during a survey campaign in the North Atlantic during August 2003. The problem differs from previous marine vector magnetometer tests in that the RRS Discovery is a large steel vessel with a very large associated magnetic signal and, due to cost limitations, the Attitude and Heading Reference System (AHRS) was of lower accuracy than previously used. In a similar procedure to previous surveys, the measuring equipment was calibrated on land prior to being taken to sea. Whilst at sea, the magnetic field of the vessel was estimated by swinging the vessel through 360° before and after the survey. Our main goal was first to check that the magnetometer gave an accurate reading despite the very strong gradient of the vessel’s magnetic field, second to estimate if the AHRS was sufficiently accurate and then to check that our processing technique was valid for data acquired on this type of vessel. After data processing it becomes clear that the magnetometer responds correctly to the variation of the magnetic field onboard the vessel but the AHRS is not accurate enough to adequately determine the magnetometer orientation in normal surveying mode. However, the data are accurate enough to estimate, with reasonable accuracy, the direction of the field at the swing site. The same results holds for our processing technique: it failed for data acquired in normal surveying mode due to anisotropic susceptibility of the vessel, but give results accurate enough at the swing site. Future modification of our acquisition system should include temperature control and an AHRS capable of better representing the magnetometer orientation.