604 EXPLORATION GEOPHYSICS 



the resistivities of disseminated ores may depend markedly on the fre- 

 quencies employed.)* 



The frequency of the energizing current and the type of overburden 

 largely govern the maximum depth at which an ore body can be detected. 

 The theoretical formula for the E.M.F. induced in a small diameter ore 

 body by an energizing coil is stated to be : t 



Eo = 2iTJMJe ^ P (7) 



where Eq is the induced E.M.F. ; / is the frequency of the current in the 

 energizing coil ; Mi the mutual inductance between the energizing coil and 

 the conductor; / the current in the energizing coil; ju, the permeability 

 and p the resistivity of the overburden.** 



The electromotive force Eq produces an alternating current 7© in the 

 ore body. The alternating magnetic field of the current Iq, in turn, induces 

 in the search coil an electromotive force Es whose magnitude depends 

 on the current Iq, the frequency of the current /o which is the same as the 

 frequency of the energizing circuit, the mutual inductance between the 

 search coil and the ore body, etc. That is, 



-2.dJtt 

 Es = 2^fMJoe ^ P 



Hence, the net result is that the overburden exerts a shielding effect which 

 varies approximately as the square of the frequency in the energizing 

 circuit.*** 



The mutual inductance between the energizing coil and the conductor 

 also is dependent upon the orientation of the coil with respect to the 

 conductor. In methods where direction-finding coils are employed for 

 detection of the subsurface conductive zone, the plane of the energizing 

 coil is usually maintained in a vertical position and oriented so that it will 

 be parallel to the probable direction of the subsurface conductive zone. Ver- 

 tical coils of this type usually are circular or square and as large as may be 

 conveniently handled in the field work, usually sixty to one-hundred square 

 feet in area. Their small area is offset by use of many turns of wire, the num- 

 ber of turns depending upon the power and frequency of the energizing 



* A disseminated ore may be considered as composed of small, electrically conduc- 

 tive particles distributed in a matrix ; as a rule this matrix is calcite, quartz, or similar 

 material and has a high electrical resistance. A small electrostatic capacity exists 

 between the conducting particles constituting the disseminated ore. From an electrical 

 viewpoint, therefore, a disseminated ore may be considered as a resistance shunted by 

 a capacity. 



t A. B. Broughton Edge and Laby, Geophysical Prospecting, p. 293 (Cambr. Univ. Press, 1931). 

 ** In applying this formula to the E.M.F. induced in an ore body, it is assumed 

 that the material surrounding the ore body has the same values of p and /i as the 

 overburden. 



*** From a physical viewpoint, it is readily recognized that the shielding effect is 

 due to the absorption of the magnetic energy by eddy currents in the overburden. 



