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Magnetic methods have been used since the beginning of geophysical prospecting. Previous to their application in petroleum exploration, several relatively crude types of magnetometers were used in mining prospecting, particularly in the search for iron ores. Magnetic surveys for petroleum exploration were begun in the early nineteen twenties. Contrary to the gravity and seismic methods, the magnetic method was not applicable to the search for salt domes and its early use was not in the Gulf Coast. Very extensive surveys were made in the middle and late nineteen twenties in West Texas and were applied on an intuitive basis in the selection of large lease blocks. One major oil company established very substantial reserves in the Permian Basin by simply leasing large ranches along linear features indicated by magnetic maps. Later it developed that these magnetic features were expression of the tlanks of the large Central Basin Platform which localizes accumulation in several large West Texas fields.

The magnetic method is also an application of a potential field and in many ways is similar to the gravity method. However, as will be pointed out later, the mathematics of the magnetic field are somewhat more complex than those of the gravity field, because of variations of the direction of the magnetic vector with latitude and because different instruments measure different components of the field.

The geological aspects, however, are considerably simpler since, for most applications in petroleum exploration, the sources of magnetic anomalies are within the basement rocks. Therefore magnetic interpretation usually begins at the base of the sedimentary section whereas gravity effects can have their sources at any depth from the grass roots down.

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