Chap. 10] 



ELECTRICAL METHODS 



771 



nonnal fields. A further advantage is gained by using insulated loops 

 whenever possible, because the galvanic effects are then eliminated. Since 

 conditions in the field are not so simple as those assumed in the theory, 

 interpretation is simplified by such applications of primary energy and 

 observation methods as will produce the clearest type of indication. 



In electromagnetic-galvanic methods, corrections are required for: (a) 

 cable leads, (b) topography, and (c) normal field. The field of the cable 

 depends on the position of the point of observation with reference to the 

 cable. It can be calculated for all three intensity components (see 

 page 777). The topographic effect may be of an electrical or geometric 

 nature. The former is due to a 

 distortion of current lines on the 

 ground surface (see page 702) and 

 may be determined by small-scale 

 experiments. The geometric in- 

 fluence is due to a change in rela- 

 tive position of ore body and plane 

 of observation. A correction for 

 the normal field is necessary in 

 order to obtain the best picture 

 of anomafies due to subsurface 

 bodies. The normal field distri- 

 bution may be calculated from 

 the formula previously given (see 

 page 766). A better procedure is 

 to measure actually all field com- 

 ponents in a location known to be 

 free from outstanding anomalies 

 (see Fig. 10-93a). In the interpretation of electromagnetic -survey data, 

 considerable help may be expected from model experiments.^^ 



4. Results. Electromagnetic-galvanic methods have been used for the 

 location of ore bodies and structural investigation. 



Fig. 10-936 shows two sets of curves on parallel bodies of steep dip, 

 observed in Sweden. The horizontal and vertical components are those 

 of a thin sheet as illustrated schematically in Fig. 10-92a. Fig. 10-94 

 shows the variation in the dip of a reception frame for minimum sound 

 (Elbof method) for a profile near the Home mine in Rouyn, Quebec. The 

 range in the angles is 50", undoubtedly because the ore bodies are very 

 near the surface. 



Various electromagnetic methods have been used for structural studies. 



Fig. 10-936. Horizontal and vertical 

 intensity anomalies on parallel ore bodies 

 (after Lundberg and Sundberg). 



«« K. Sundberg, Beitr. angew. Geophys., 1(3), 335 (1931). Heine, op. cit., 141-145. 



