790 



ELECTRICAL METHODS 



[Chap. 10 



cient to consider the induced current as a constant parameter as far as 

 applications in mining are concerned. For stratified ground, expressions 

 for the relation of induced and primary current will be given later. 



The type of indication produced by current concentrations in subsurface 

 ore bodies depends primarily on their geometric disposition. The mag- 

 netic fields follow from relations previously given (eq. [10-46]). As shown 

 in Fig. 10-110, relations are identical for electromagnetic and inductive 

 inethods for a single current concentration (thin vertical ore body). In 

 this case the eddy currents flow in a vertical plane around the sheet. The 

 effect of the return circuit at the bottom and at the sides can be dis- 

 regarded, and formulas (10-46a) then apply. 



In a wide ore body the induced current flows in opposite directions on 

 opposite sides. Disregarding the effect of current concentrations on the 



h^ + ^H / 



Thin, vtrtlul 



Wide, verhcal 



HorizonfttI 



Thin, dipping 



Fig. 10-110. Calculation of electromagnetic fields produced by currents induced 

 in various types of ore bodies (current concentrations indicated by dots). 



bottom of the body, the horizontal component due to the upper concen- 

 trations is 



87, yad 



Y = 2Iid(\ - \) = 



(y2 + a2 + d^y- 4a2?/2" 

 The vertical components are additive, so that 



UMa -\-d' - y') 



\ ri ra / 



(10-55a) 



(10-556) 



(?/2 + a2 + d2)2 - 4a2 2/2 



if the zero point of the coordinate system is above the center of the 

 ore body. 



In a thin horizontal bed the current distribution is the same as in the 

 upper surface of a wide vertical ore body. Therefore, the horizontal and 

 vertical components are given by the preceding formulas. In a dipping 

 thin ore body the eddy current will be concentrated along the upper and 



