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Abstract

Most magnetic surveying is now done from the air, measuring the total intensity of the magnetic field. Only relatively limited and special observations, such as in some mineral prospecting, are done on the ground with mechanical magnetometers measuring the vertical component or electronic magnetometers measuring the total intensity.

For any of the airborne magnetometer systems, the instrument assembly in the airplane carries the necessary electronics and a recorder on which a continuous record of the magnetic field is shown. The airplane also usually is equipped with a radar altimeter and with the necessary positioning equipment. The several items of data (magnetic field intensity, electronic or photographic positioning data, barometric elevation, and height above ground) are correlated with one another so that all can be brought together to provide the necessary information for reducing the observations to a magnetic map. In modern equipment, the data recording is on magnetic tape so that the data reduction can be carried out largely by electronic computers and machine mapping.

The volume of surveying carried out with airborne magnetometers is very large. In petroleum exploration, surveys have been made over the order of ten million miles of flight line traverse. Such surveys have covered most of the sedimentary basins of the world including very substantial offshore areas. The degree of control varies widely; some surveys have line spacing as close as one half mile, or more commonly one to two miles, but there are also broad reconnaissance surveys carried out with single lines or band flying.

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