THE CAUSE AND TREATMENT OF ANISOTROPIC ERRORS IN NEAR-EARTH GEOMAGNETIC DATA

The data available to constrain models of the geomagnetic field are do minated by measurements made at or near to the Earth's surface, typica lly treated as independent and uncorrelated. Here we examine the effec t on field modelling of error anisotropy in these data, potentially le ading to correlations between different field components measured at t he same location. We consider three examples. First, we examine the ef fect of crustal magnetisation, in general the largest source of error in main field modelling. We demonstrate that under a stochastic treatm ent of the crustal field, the error resulting from this source is larg er by a factor of approximately root 2 in Z than in X and Y, and that correct modelling of this effect reduces the variances of the field mo del coefficients by greater than 10%. The anisotropy leads to error co rrelation between measurements of inclination and field intensity. Sec ond, we consider the calculation of cartesian component vector data fr om measurements of field intensity and direction. When measurement err or is the dominant source of uncertainty, the error estimates for the resulting vector data can be strongly correlated. Third, we examine th e proposed deployment of a set of ocean-bottom magnetic observatories. One important difficulty with this project is the determination of ac curate horizontal orientation. We demonstrate that with correct modell ing, orientation information of limited accuracy is sufficient to allo w determination of high-quality field models. (C) 1997 Elsevier Scienc e B.V.