A method for isolating secular geomagnetic variation from shipboard total field measurements: a test case in the NE Atlantic

In this pilot study we describe a method for extracting the secular variation constituent of the Earth's magnetic field from underway shipboard total intensity magnetic measurements. From a large amount of shipboard data obtained in the NE Atlantic, during the period 1956 to 1988, some 29 000 secular variation values were calculated at cross-over points. These results are then compared with those obtained from the International Geomagnetic Reference Fields (IGRF) in this region. Statistical analysis reveals that the ‘shipboard’ values are able to match, and marginally improve upon the IGRF values across the oceans, so long as the determination is made from a sufficiently large data set of observed values. This method thus enables the additional utilization of marine observations in the analysis of secular variation patterns across the oceans, which could lead to a more detailed view of these patterns than has been available before.