Experience with the use of global reference fields for compilation of aeromagnetic data for Europe

A compilation of aeromagnetic data for Europe is in progress to produce a map showing crustal anomalies of total intensity for the epoch 1980.0 and the altitude 3000m above m.s.l. and to interpret it in terms of large-scale features in the crust. A report is given of our experience with secular variation and with the reference field which has to be subtracted from the total intensity values to obtain crustal anomalies.When aeromagnetic surveys are compiled, data from observatories and repeat stations are necessary in order to estimate secular variation with sufficient precision. Data from repeat stations are more detailed and realistic than computed values from global reference field models. The secular variations computed from three different reference field models differ considerably.The influence of the reference field and its truncation level on the shape of anomalies is examined by calculating differences between the reference fields. The decision to use the DGRF 1980 model for publishing a map showing European crustal anomalies for 1980.0 is based on the fact that this model gives a better approximation of observed values at observatories and repeat stations in Central Europe than two other models. These differences between observed and computed values are partly caused by inhomogeneous input data used for the calculation of reference fields. The input data is inhomogeneous because each observatory publishes their annual mean values based on its own observatory standard and not on the International Magnetic Standard.