Power spectrum analyses of the geomagnetic field over the earth's surface show that the field fluctuations of wave lengths shorter than about a thousand kilometers are connected mostly with the geological structure of the earth's crust. Possible interpretations of the relation between geological structure and the surface geomagnetic anomalies are demonstrated by referring to fairly detailed maps of geology, geomagnetic anomaly, and Bouger anomaly of the gravity field over Japan Islands as well as magnetic properties of various rocks. Special attention is drawn to the absolute importance of natural remanent magnetization of rocks for this kind of interpretation.

It seems likely that geomagnetic anomalies of wave lengths from several hundred kilometers to several thousand kilometers are related to the distribution of continents, to the variation of thickness of the earth's crust, and to the undulation of isothermal surfaces in the earth's upper mantle. Geomagnetic field fluctuations of larger scale seem to be attributable to the electric eddy currents just beneath the surface of the earth's core.

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