As aeromagnetic surveys covered increasingly larger areas and as the need arose for the compilation of adjacent surveys with significant time differences, there has been an increase in the use of geomagnetic field models for regional field definition. Indeed, few aeromagnetic contracts exist today without some provision for removal of the International Geomagnetic Reference Field (IGRF). Yet, as publications indicate (Regan and Cain, 1975a, 1975b; Reford et al, 1976; Regan, 1977, 1978), the IGRF is far from perfect for this application despite the considered attention of the IAGA (International Association of Geomagnetism and Aeronomy) working group charged with its upkeep and maintenance. One reason for this is that the model is meant to serve all areas of geomagnetism and, as such, is not specifically tailored to the needs of the exploration community. The recent summary report of the IAGA committee (Peddie, 1982) could lead to some confusion because there are seven IGRF models published. A rational question for those involved in aeromagnetic surveys is “Which model should be used for regional field definition?”

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