1044 Subsurface Geologic Methods 



located at or above the earth's surface. This natural-force field will consist 

 of the field due to subsurface geologic features superimposed on the ter- 

 restial magnetic field. Variations in the distribution of magnetized ma- 

 terial beneath the surface will alter the intensity of the magnetic field at 

 the surface. Since sedimentary rocks are generally nonmagnetic and the 

 basement rocks slightly magnetic, variations observed at the surface are 

 ordinarily attributed to basement relief or to intrabasement variations in 

 distribution of magnetic materials. 



For convenience of illustration any material, solid, liquid or gas, 

 may be considered to contain a large number of elementary magnets or 

 dipoles. In the unmagnetized state, the dipoles may be considered to have 

 random orientation and distribution, and the average field due to the 

 dipoles is zero. If these elementary magnets are placed in a magnetic field, 

 they tend to align themselves in the direction of the field. The magnetic 

 induction within a body due to an external field may be increased in 

 paramagnetic material or reduced in diamagnetic material, depending 

 upon the arrangement of the electrons in the particular atom. A material 

 may be termed nonmagnetic if the paramagnetic effects exactly equal the 

 diamagnetic effects. Paramagnetic substances are probably more common 

 in the materials that go to make up our earth, all of the igneous rocks 

 showing paramagnetic effects. A vacuum is truly nonmagnetic; however, 

 measurements of certain samples of sulphur, calcite, and several other 

 substances frequently show nonmagnetic effects. Quartz, graphite, and 

 rock salt illustrate diamagnetic materials. The stronger the exciting field, 

 the greater is the attempt at alignment of the elementary magnets, and 

 the stronger is the magnetic field produced by the body itself. The meas- 

 ure of the ability of a material to produce a magnetic field of its own is 

 termed "magnetic susceptibility," the susceptibility depending upon the 

 strength of the field in which the measurement is made. A select group 

 of elements (iron, nickel, and cobalt) and certain alloys produce a mag- 

 netic field in the absence of any external field. Commonly called "perma- 

 nent magnets," these substances are properly termed "ferromagnetic." 

 The greater portion of the magnetic effects observed over the surface of 

 the earth is due to the variation in the distribution of magnetite, which is 

 a ferromagnetic substance. 



In the earth's magnetic field, paramagnetic materials (susceptibilities 

 greater than zero) tend to increase the normal field; nonmagnetic materi- 

 als (susceptibilities equal to zero) have no effect on the normal field; and 

 diamagnetic materials (susceptibilities less than zero) tend to decrease the 

 normal field. (See fig. 547.) 



Rock Properties 



As a rule, basic igneous rocks show the largest susceptibilities, rang- 

 ing as high as 16,000 XlO"^ c.g.s. units. With an increase in silica content, 

 the susceptibility decreases, values given in the literature ranging from 8 

 to 2,700X10^ c.g.s. units for granites. Sedimentary rocks seldom exceed 



