The previous chapter discussed the steps by which gravity measurements are converted into gravity anomalies that reflect geological sources. The present chapter treats magnetic anomalies in a similar vein. Whereas the gravity field of the earth is largely time invariant, except for relatively minor or long-term changes due to redistribution of mass (tides, moving magma, glacial rebound, erosion, mountain building, and so forth), the geomagnetic field varies in both direction and intensity over time scales ranging from milliseconds to millennia. It would seem that this added complexity would make the reduction of magnetic measurements significantly more difficult than that for gravity data, but in practice the calculation of magnetic anomalies is relatively straightforward.

Our intent in this chapter is to characterize the global magnetic field in order to isolate the magnetic field caused by crustal sources. This agenda glosses over a large body of information that ordinarily would be included in a chapter of this title, such topics as the origins of the geomagnetic field (magnetohydrodynamic theories); the behavior of the field in the geologic past (paleomagnetic studies); reversals of geomagnetic polarity; the magnetic properties of the sun, moon, meteorites, and other planets; and the interaction of the earth's magnetic field with solar phenomena.