Chap. 8] MAGNETIC METHOD 297 



netic characteristics. Another structural application which is important 

 in oil and mining exploration is the mapping of igneous intrusions, igneous 

 dikes, and contact-metamorphic zones. In mining, magnetic methods are 

 frequently applied to the location of magnetic iron ores, magnetite, and 

 hematite, as well as to nonmagnetic ores if they are associated with mag- 

 netite or pyrrhotite, or are contained in lava flov/s. Concentrates of noble 

 minerals (gold, platinum) may be found if they are associated with mag- 

 netite in placers. In engineering, magnetic prospecting has been used to 

 locate buried magnetic objects, ammunition, pipe lines, and the like. 



Experience obtained in the past ten to fifteen years with magnetic 

 prospecting indicates that it is probably the simplest, least expensive, and 

 fastest geophysical method, that it may yield quantitative results provided 

 conditions are suitable, but that it must be supplemented and even re- 

 placed by other geophysical methods where the structural consistency of 

 the anomalies is doubtful. 



II. MAGNETIC ROCK PROPERTIES 

 A. Definition 



The magnetization of rocks differs from other physical rock properties 

 in one fundamental respect: it is composed of two phases; one remains the 

 same regardless of orientation of a specimen, while the other reverses its 

 sign when the specimen is reversed. The former is called the induced, 

 the latter the remanent magnetization. In rocks the remanent magnetism 

 may be a fraction of the induced magnetization or it may exceed it. Sepa- 

 ration of the two magnetizations does not suggest that different physical 

 phenomena are involved. On the -contrary, they may both be obtained 

 from a single test involving the tracing of the familiar "hysteresis" curve. 

 Although it is impossible to separate the two magnetizations in the field, 

 their approximate proportions must be known for a complete interpretation 

 of magnetic data. 



The strength of the induced magnetization is customarily defined by the 

 permeahility of the material or a related quantity, the magnetic suscepti- 

 bility. Their physical significance is best understood by the familiar 

 analogy of the magnetic and electrical circuits. The magnetomotive 

 force in a magnetic circuit (measured in gilberts) is analogous to the 

 electromotive force in an electrical circuit. If the magnetomotive force is 

 produced by a solenoid of N turns with I amperes, it is OAtNI, and 1 

 gilbert = 0.796 ampere turns. The current in an electric conductor corre- 

 sponds, in a magnetic body, to the flux $, measured in maxwells, or lines 

 of force. The electric resistance is analogous to the magnetic reluctance 



