656 ELECTRICAL METHODS [Chap. 10 



surface layer is present, or a ground water level exists. These conditions 

 affect not only the ground wave but the surface wave as well. Both are 

 shown schematically in Fig. 10-20.^^ 



G. Resistivities and Dielectric Constants of Minerals, Ores, 

 Rocks, and Formations 



Resistivities of minerals, ores, rocks, and formations vary within much 

 wider limits than their other physical properties. For instance, the 

 density may vary between the limits of 1 and 8 for minerals and between 

 1.5 and 4 for rocks and formations. The range of elastic wave speeds in 

 formations is from about 150 to 7000 meters per second; magnetic suscep- 

 tibilities vary from 2 to 1 X 10~^ Extremes in electrical resistivity are 

 represented by silver with 1 X 10"^ ohm-cm and by sulfur with 10^* 

 ohm-cm. This range is not encountered in practice. It is probably 

 greatest in ore prospecting and lies between 10~^ and 10^ ohms, which 

 corresponds to 10 powers. For comparisons of resistivities it is therefore 

 advantageous to apply a logarithmic scale. 



Minerals and rocks may be divided according to resistivity into three 

 groups, each of which comprises a range of 8 powers: 



1. Minerals of good conductivity, in the range of 10~ to 10 ohm-cm. 



2. Minerals and rocks of intermediate conductivity, covering the range 

 from 10 to lO' ohm-cm. 



3. Minerals and rocks of poor conductivity, in the range of 10^" to lO" 

 ohm-cm. 



In the first group are the metallic elements and graphite, the arsenides, 

 the tellurides, the sulfides with the exception of sphalerite, cinnabar, and 

 stibnite. The group also includes a few of the oxides, such as specularite, 

 magnetite, pyrolusite, and ilmenite, although these are on the border line 

 between the first and the second group. In the second and largest group 

 of intermediate conductors are the oxide minerals except those mentioned 

 above, most ores, virtually all rocks possessing electrolytic conductivity, 

 and anthracite. Most minerals, particularly the rock-forming types, such 

 as all silicate minerals, the phosphates, and the haloids, belong in the 

 third group of poor conductors, as do also the hydrates, borates, nitrates, 

 carbonates, sulfates, chromates, and molybdates. 



In Tables 62 through 70 resistivities are given in the following order: 

 elements, arsenides, tellurides, sulfides, oxides, haloids, various rock- 

 forming minerals, and miscellaneous commercial minerals. Then follow 

 the ores and rocks with impregnations of conductive minerals. Next is a 



tabulation of resistivities of igneous and metamorphic rocks, determined 



(Continued on p. 664.) 



" F. Hack, Ann. Phys., 27, 43-63 (1908). 



