802 



ELECTRICAL METHODS 



[Chap. 10 



Fig. 10-118 shows the effect of a plate on both the amplitude and the 

 phase of the horizontal component measured across the cable. The 

 gradient of both amphtude and phase decreases with the depth of the plate. 

 This example represents one of the earlier applications of the Sundberg 



method; in later surveys, in-phase 

 and quadrature components were 

 measured at definite distances.^"" 



(g) Results obtained by horizontal 

 loop methods. Inductive methods 

 were used at an early date in the 

 Skellefte district in northern Sweden, 

 where the overburden is relatively 

 thin, and fairly ideal conditions 

 exist. The general geology of this 

 district has been previously de- 

 scribed (page 703). A number of 

 typical indications obtained in sev- 

 eral ore fields have been published 

 by Lundberg, Sundberg, and Ek- 

 lund.^"^ The profile shown in Fig. 

 10-119 was surveyed on the ice 

 of Lake Menstrask. Geologic and 

 electrical prospecting was begun in 

 1921 and continued through 1923- 

 1924, but drilling was not started 

 until 1923-1924. The ore bodies 

 occur in two zones, the principal 

 zone passing beneath the lake. 

 The country rocks are quartz-por- 

 phyritic or brecciated halleflintas; 

 layers of slate are conmion. The 

 ore bodies are partly in graphitic 

 slate containing disseminations of pyrite and pyrrhotite and partly on 

 the contacts of slates and halleflinta. They consist chiefly of pyrite, but 

 copper is also present. About 150,000 electrical observations were made 

 in this field, supplemented by torsion-balance observations to differentiate 

 between impregnations and ore bodies. While most of the drilling located 

 ore under the electrical indications, nothing but black slate was encoun- 

 tered in one part of the field. 



OemMmfcrP 



Fig. 10-117. In-phase (subscript P) 

 and quadrature (subscript Q) compo- 

 nents of electromagnetic fields produced 

 by galvanic and inductive action of a 

 long grounded cable on dipping (model) 

 ore body (after Hedstrom). 



"« Sundberg, World Petrol. Congr. Proc, B(I), 107 (1934). 

 *" H. Lundberg, K. Sundberg, and E. Ekiund, "Electrical Prospecting in 

 Sweden," Sveriges Geol. Unders. Arsb., 17(8), 37-73 (1925). 



