Chap. 10] 



ELECTRICAL METHODS 



769 



For a current density of 

 1 amp./lOO m , a unit effect 

 (Hi,h: = 1.10"'' Gauss) is produced 

 when cos di+i — cos Oi = 0.1 and 

 Rji+i = 10 m. This applies to 

 both horizontal and vertical com- 

 ponents; for the latter, the dia- 

 gram is rotated 90° (see Fig. 10-91). 



Grounded electrodes cause cur- 

 rents to flow not only by galvanic 

 action, but by inductive action as 

 well. Since the ore body, with 

 the cable or loop, acts as a trans- 

 former with a short-circuited turn, 

 eddy currents are produced along 

 the edges of the ore body, and flow 

 down the dip as well as around its 

 upper edge. The latter cause the 

 greatest portion of the field observed 

 at the surface. The field may be 

 calculated from formula (10-46a) 

 by adding the effects of the two current concentrations, 

 width of the ore body, we have for the components 



Fig. 10-91. Interpretation grating 

 for electromagnetic-galvanic methods 

 (after Belluigi). Each compartment, 

 when it is traversed by current at right 

 angles to its section, produces the same 

 effect on the horizontal component of 

 the electromagnetic field. For calcula- 

 tions of the vertical component, the dia- 

 gram is rotated 90°. 



With 2a as the 



\ ri Ti J 



(10-48) 



where li is the induced current. 



Use of grounded cables results in a superposition of currents produced 

 by galvanic and inductive action. Considerable phase shifts may occur 

 between the currents, so that in the absence of definite phase data only 

 approximate curves can be given for the resulting fields (see Fig. 10-92c). 

 Since one side of an ore body is usually much closer to the primary cable 

 than the other is, the maximum on that side is greater than the minimum 

 on the far side. The latter can be brought out by reversing the position 

 of the primary cable or loop. The curves shown in Fig. 10-92d represent 

 the variation of the X and Y vectors compounded from fields with greatly 

 differing phase angles produced by both galvanic and inductive action. 

 Separate measurement or calculation of in-phase and quadrature compo- 

 nents has the advantage of segregating the induced from the galvanic and 



