Results from equivalent-source distributions derived jointly from high-altitude (average 4 km) aeromagnetic and Magsat-derived (average 400 km) magnetic anomalies over Canada indicate that long-wavelength components (500–2500 km) in these fields are extremely compatible with one another (with a correlation coefficient of 0.95). The jointly estimated anomaly field at the earth's surface can be used as a long-wavelength adjustment surface for regional near-surface magnetic anomaly compilations and in assessing the performance of other downward-continuation techniques. Because near-surface anomalies are not available over all regions of the. world, we compare the jointly estimated anomaly field to the results of two different downward-continuation techniques: the evaluation of anomalies at the earth's surface from spherical harmonic coefficients derived from satellite-altitude data and the use of downward-continuation methods based on harmonic splines. Numerical and visual comparisons of these downward-continued fields with the jointly estimated anomaly field from the equivalent-source method indicate they are well correlated and could provide a useful method of deriving long-wavelength leveling surfaces for regional and worldwide magnetic anomaly maps.

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