maps of the earth’s surface on which iso-lines (isodynamic, isogonic, and isoclinal) are used to show the distribution of the intensity of the geomagnetic field or its components.
Magnetic maps of the world and of anomalous magnetic fields are most common. World maps reflect the basic characteristics of the main geomagnetic field (the normal field), the source of which is considered to be the movement of electroconductive matter in the earth’s core. The dimensions of the structural features of the main field approximate the dimensions for the continents, so that the usual scale of the world maps is 1:10,000,000 or less. Deviations caused by irregularities in the structure of the earth’s crust, the occurrence of ore deposits, and other local factors are smoothed out on the world maps, but maps of anomalous magnetic fields reflect local deviations of the geomagnetic field from the main. These deviations are ordinarily observed in areas whose linear dimensions are on the order of dozens of km or less. Therefore, magnetic maps of anomalous fields are on a larger scale than world maps (for example, 1:200,000) and are usually compiled on the basis of aeromagnetic surveying. Magnetic maps are essential in studying the structure of the earth’s interior and in prospecting for minerals. Owing to secular changes in the earth’s magnetic field, magnetic maps go out of date and must be redone periodically (every five or ten years).
REFERENCElanovskii, B. M. Zemnoi magnetizm [3rd ed.], vol 1. Leningrad, 1964.
V. N. LUGOVENKO