ELECTRICAL METHODS 599 



375. f The speed of propagation will depend upon the conductivity of 

 the medium surrounding the conductor ; in air this speed will be that of 

 light (300,000,000 meters per second), while in a highly conductive 

 material such as sea water, the speed may be only a few meters per second. 

 A direction-finding coil pivoted with its axis of revolution parallel to the 

 conductor will give the maximum signal response when the coil is per- 

 pendicular to a tangent to the wave front. A minimum signal will be 

 obtained when the coil is parallel to the tangent. This is illustrated for 

 three positions in Figure 375. Under such conditions the direction-finding 

 coil would become a simple means of locating a subsurface conductor by 

 triangulation. In practice, it will be found that the field surrounding the 

 conductor is distorted by : ( 1 ) irregular shape or configuration of the ore 

 body or conductive zone; (2) inhomogeneity of the conductive medium 

 surrounding the major zone of current flow; and (3) distortion of the 

 wave front as the electromagnetic wave emerges from the ground. 



Distortion of Wave Front 



Because the velocity of propagation of an electromagnetic wave varies 

 with the dielectric properties of the medium through which it passes, 

 there is a resultant distortion of the wave front as it emerges from the 

 ground. In practice, therefore, the wave front traveling outward from 

 the current-carrying zone is not a true circle, and the conductor cannot 

 be located by a simple triangulation process. 



The velocity of an electromagnetic wave may be expressed in terms of 

 its velocity in vacuum by the relationship : 



V = -^ + K' 



where 



V = velocity in medium of given properties 

 Ve = velocity in vacuum (300,000,000 meters per second) 

 H = magnetic permeability of material 

 e z= specific inductive capacity of material 

 K and A'' =: constants which vary with frequency, etc. 



If this formula is applied to a conductor of infinite conductivity 

 ( o- = 00 ) the wave would have a limiting velocity equal to zero (since K 

 is a function of a) and its energy would be almost completely dissipated by 

 generation of eddy currents. A small portion of the energy will be reradi- 

 ated. This result indicates that a highly conductive ore body will absorb a 

 considerable portion of the energy and reradiate (in the form of a secondary 

 field) a smaller portion. 



t J. J. Jakosky, "Fundamental Factors Underlying Electrical Methods of Geophysical Pros- 

 pecting," Eng. and Min. Journal, Feb. 11-18, 1928. 



