A technique for estimating the absolute vector geomagnetic field from a marine vessel

We have developed a technique to estimate the absolute strength and direction of the geomagnetic field from a marine vessel. This technique will be of value in the study of marine magnetic anomalies, directional drilling or geomagnetic field modelling. One of the main difficulties in this operation is to correct the data for the magnetic field generated by the vessel itself. Assuming the vessel susceptibility is isotropic, we show that by turning the ship through 360° at a place where the strength of the field is known we can estimate the local direction of the magnetic field. Once this is known, the vessel's field at any attitude can be robustly estimated and the measurements of the full magnetic field vector made in a normal surveying mode can be corrected. The ambient magnetic field estimates at the turn locations have proved to be very accurate. In normal surveying mode, these estimates are not as good since they are directly dependent on the accuracy of the vessel attitude measurements. However, when the technique is applied on real data, the total intensity field estimates have a very low level of noise showing that the vessel signal has been well accounted for.