Subsurface Methods as Applied in Geophysics 1045 



50X10"^ units, and many are diamagnetic, the susceptibility of gypsum and 

 anhydrite, for example, varying from —1 to —10X10"® c.g.s. units. In 

 general it may be stated that two factors are chiefly responsible for the 

 magnetization of rocks: magnetite content and geologic history (meta- 

 morphism, tectonic movements, lightning, etc.) . The presence of magnetite 

 in many of the igneous rocks suggests that the magnetization of the rocks 

 is due to two sources. First, the contribution due to the "permanent mag- 

 netization" or remanent magnetization of the magnetite and, second, the 

 magnetization induced in the rock by the earth's magnetic field. Designat- 

 ing polarization as /, susceptibility as k, the exciting field as H, and the 

 remanent magnetization as Ip, one may write the relationship as follows: 



In most igneous rocks the remanent magnetization is the dominant 

 factor contributing to the total polarization of the rock. The contribution 

 due to remanent magnetization is impossible to determine in rocks lying 

 thousands of feet below the surface, for not only is intensity involved but 

 the direction of the force as well. This fact greatly limits the quantitative 

 interpretation of magnetic results. 



Inasmuch as irregularities in the distribution of magnetic materials 

 give rise to the observed anomalies at the surface, it has become routine 

 practice for many of the larger operators to make determinations of the 

 susceptibility and remanent magnetization in the laboratory. Cuttings, 

 cores, and outcrop samples are carefully studied and the results tabulated. 

 Studies of this nature make more accurate quantitative interpretation 

 possible and aid in the planning of field surveys. 



The EartKs Magnetic Field 



Magnetic measurements made over the surface of the earth would 

 indicate that the central core of the earth is magnetic, and seismic results 

 indicate that the core is metallic. However, the interior of the earth must 

 be at a high temperature, temperatures much in excess of the Curie 

 point, at which materials lose their ferromagnetic properties. The most 

 reasonable theory proposes that the magnetic field is due to electric cur- 

 rents flowing through the central core. This theory is substantiated by 

 recent measurements of telluric currents, an increase in the earth's 

 magnetic field being accompanied by an increase in telluric-current 

 activity. 



The distribution of the magnetic field at the surface of the earth 

 indicates that the earth approximates a polarized sphere with magnetic 

 poles located near the axes of rotation. For sake of illustration, we may 

 imagine the surface distribution of the earth's magnetism to be accounted 

 for by a short bar magnet placed in the center of the earth. (See fig. 

 548.) By determining the total intensity and the direction of the magnetic 

 vector, we may describe the magnetic field at any point on the surface of 

 the earth. The strength of the earth's magnetic field is about 0.5 oersteds 



