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 va riation constituent of the Earth's magnetic field from underway shipbo ard total intensity magnetic measurements. From a large amount of ship board data obtained in the NE Atlantic, during the period 1956 to 1988 , some 29 000 secular variation values were calculated at cross-over p oints. These results are then compared with those obtained from the In ternational Geomagnetic Reference Fields (IGRF) in this region. Statis tical 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 ob served values. This method thus enables the additional utilization of marine observations in the analysis of secular variation patterns acro ss the oceans, which could lead to a more detailed view of these patte rns than has been available before.