Chap. 10] 



ELECTRICAL METHODS 



729 



mo 



16000 



I 



I 



moo 



'11000 





wpoo 



mo 



(fiOO 



1.0 



0.8 



0.6 



0.4 



0.2 



(a) 



It is noted that, for the 

 value k = 0.7, the 'depth h 

 remains practically constant; 

 h = 142 may be taken as the 

 depth to the surface of separa- 

 tion between limestone and 

 clay or sand. If the h values 

 are plotted as a function of k 

 (Fig. 10-636), the curves in- 

 tersect at about 140 feet. The 

 most probable value of k is 

 0.702. The true depth in this 

 case ranged from 145 to 150 

 feet. 



Tagg's method of interpre- 

 tation may be extended to 

 the three-layer case, provided 

 the infinite third layer does 

 not influence the first part of 

 the curve too much. This 

 requirement is satisfied when 

 the thickness of the second 

 layer is two to three times the 

 thickness of the top layer. 

 The procedure followed is 

 equivalent to a reduction of 

 the three-layer to a two-layer 

 problem, that is, it is equiva- 

 lent to the construction and 



evaluation of the approximation curves shown in Figs. 10-53 and 10-54. 

 The order to be followed is as follows : (1) Average the surface resistivities for 



SO' 



ioo' 



100' 



JOO' 400' xo 



(ft) 



100' 150' m' 250^ 



Resistivity curve (a) and graphical 



Fig. 10-63 



depth interpretation (6) (after Tagg) 



"See also Sundberg, A.LM.E. Geophys. Pros., 146 (1934); and S. J. Pirsop, 

 A.LM.E. Geophys. Pros., 148-158 (1934). 



