ELECTRICAL METHODS 



601 



Figure 377 is a schematic plot of the wave front for an electromagnetic 

 wave emerging from average dry ground. Immediately above a highly 

 conductive body, the arm connecting the two coils is downward and toward 

 the conductor. As readings are taken to either side of the vertical, how- 

 ever, the direction is not toward the conductor, and empirical correc- 

 tions must be made, as will be described later. 



Contacting Ore Body. — In a modification of the electromagnetic 

 process, developed at the University of Arizona, t one terminal of the 

 current supply is connected with the ore body directly. The chief use 

 of this method is in tracing out the extensions of known ore bodies. One 

 energizing electrode is located to make contact with one portion of an 

 ore body (usually an outcropping or working face) and the other electrode 

 is located at a considerable distance from the first contact. If the ore body 

 is shallow, medium high frequencies may be employed for this work. 



Atria f for 

 High Frequencu 

 worK 



FiG. 378. — Diagram of connections for contacting method. 



A schematic diagram of connections for this apparatus is shown in 

 Figure 378. If radio frequencies are used, the ore body must be very 

 shallow. The "antenna" should be of a proper size to allow efficient opera- 

 tion at or near its fundamental or a harmonic thereof. 



A direction-finding apparatus similar to that illustrated in Figure 376 

 is employed for determining the location of the underground conductive 

 zone. Both the dip (angle measured from the vertical) and the strike 

 (azimuth angle) readings are made in determining the location of the 

 body. When using radio frequencies, it will be found oftentimes that the 

 strike readings are more reliable and give better indications of the presence 

 of a conductive zone than the dip readings. When a vertical antenna is 

 employed the strike readings normally should all be toward the antenna, 

 unless an extension of the ore body exists, in which case there will be 

 very definite deviations of the strike. 



t D. G. Chilson, "Process of and Device for Locating Ore," U. S. Patent 1,491,900, issued 

 April 29, 1924. 



