SUMMARY The method of rectangular polynomial analysis (RPA) is developed and refined to represent a curl‐free potential field of internal origin. It is applied to annual mean values of the geomagnetic field from 42 European observatories. RPA is found to be an efficient means of representing the regional field, though less suitable for modelling the anomaly field.

Various methods that take account of the potential nature of the field have been proposed for modelling geomagnetic data on a regional scale. Several of these have been applied to a standard data set based on annual mean values from observatories in Europe. Here, we examine some of the properties of spherical cap harmonic analysis when applied to this data set, and compare the quality of fit with that of the other models. It is found that, for this data set, rectangular polynomial analysis provides a compact fit to main field data, but that in most other cases, for both main field and anomaly data, spherical cap harmonic analysis provides the better fit. Although relatively insensitive to chosen cap size, spherical cap harmonic analysis deteriorates more rapidly than the other methods when the number of coefficients is reduced.

Alldredge's method of rectangular harmonic analysis has been reexamined. After correction of errors, it is found to give improbable values between the data points and wild values outside them. A much more realistic model has been obtained by (1) determining only the most significant coefficients (those that exceed their standard deviations, obtained by an iterative process), (2) introducing new parameters to allow for a linear trend across the region, and (3) increasing the scaling factors so that the sinusoids start and finish outside the region. Modification 1 is the most important for improving the interpolative qualities of the model. Modifications 2 and 3 reduce, but do not entirely eliminate, the wild values near the edges.

In this paper, we apply for the first time the moving-windows application of the Poisson's theorem to the synthetic gravity and magnetic data, followed by calculations of the correlations of the Bouguer gravity and aeromagnetic data of Western Anatolia. The correlation coefficient, slope and intercept parameters were generated from the internal correlations existing between the gravity and magnetic anomalies. Relative negative correlation values of positive gravity and negative magnetic anomalies were found on the Menderes Massif and in the southern part of the Marmara sea. Higher heat flow values were also obtained from these regions. The negative correlation values can be seen on a profile taken along the 28• E longitude and are sourced from a large graben system which has been generated as a result of lithospheric extension in Western Anatolia since the Early Miocene. The grabens were filled up by approximately 2000-m-thick sediments. The negative correlation coefficients and high heat flow values correspond to relative uplift of the asthenosphere in these regions.

Since 1985, magnetic measurements have been conducted between Dokurcun and Abant pilot area of Turkish-German Earthquake Research Project which is located on the western part of the North Anatolian Fault Zone. A total of 28 magnetic observation stations have been established as repeated measurements points, including a base station in Cakillar. In this study, the time differentials were obtained from the measurements taken during the time periods of April-October 1985, June-October 1986 and June 1987. When the local field variations between the successive periods were compared it is clearly seen that the intensity of the anomaly around the Dokurcun (DOK) decreased from +20 nT to -2 nT . These changes could be related with the earthquake of magnitude 2.2 (June, 1985) occurred north of Dokurcun. The other anomalies were also changed related with the epicenter distribution which are moved toward to the west from