734 



ELECTRICAL METHODS 



[Chap. 10 



crystalline, highly metamorphosed rocks of 10,000 ohm-m resistivity. 

 Curves of the type under discussion may likewise be constructed for the 

 three-layer case. These are divided into an upper two-layer portion and a 

 three-layer curve proper, to interpret the lower part of the curve. It is 

 evident that in this case the number of combinations of k values for which 

 such diagrams must be available is much greater. 



The Schlumberger method has the obvious advantage that depth deter- 

 minations can be made directly in the field without any calculation — at 



A 



A 



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Fig. 10-65. Studies of the effect of dip of model formations in a water tank, using a 

 four-terminal contacting arrangement with constant electrode separation of 8 inches. 



the expense, however, of a great amount of office calculation beforehand 

 to meet various conditions. This method increases in value the more 

 field data are obtained and interpreted. 



4. Model experiments. Small-scale experiments are of twofold value in 

 the interpretation of resistivity measurements: (1) for checking theoretical 

 calculations, and (2) for simulating geologic bodies whose effects cannot be 

 calculated. These experiments are usually made in tanks, filled with water 

 to simulate the country rock (or top layer), whereas ore bodies are repre- 

 sented by metal plates, and stratified ground by layers of sand (p large). 



