Chap. 10] 



ELECTRICAL METHODS 



733 



as variable. Fig. 10-64 shows a diagram prepared for a two-layer case in 

 which the resistivity of the upper formation is 480 ohm-m while the re- 

 sistivity of the lower formation is infinite. Apparent resistivities are shown 

 for top-layer thickness varying from 5 to 125 m. The former are plotted 

 as ordinates, and the lengths of the electrode arrangement (I = 3a) as 

 abscissas. In the two-layer case several of such diagrams are available 

 for various k values ; and it is not difficult to select approximately the right 

 diagram, since the resistivity of the upper medium may be determined 

 fairly closely. 



£000 



I 



V 



fOfO 



300 m 



Fig. 10-64. Schlumberger two-layer interpretation diagram, with field curves 



(after Poldini). 



Interpretation consists of placing the field curves over the theoretical 

 curves and determining the depth by interpolation. For instance^ in the 

 curve for station 1 the depth obtained from the curve was 8 m while the 

 depth reached by drilling was 8.75 m. In the curve for station 13 the depth 

 read by interpolation was 32 m and the depth found by drilling was 31.05 

 m. In the curve for station 40 the interpolated depth was 51 m and actual 

 depth was 51.05 m. The results under discussion were obtained in a re- 

 sistivity survey on a dam site at Littleton, New Hampshire,' where the 

 upper layer consisted of Quartemary sands, gravel, and clay, underlain by 



