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.