742 



ELECTRICAL METHODS 



[Chap. 10 



(10 per cent of depth); only two wells showed discrepancies. Where bed- 

 rock consists of unaltered crystallines and the overburden is glacial drift, 

 the conductivity contrast is great enough to assume an infinite bedrock 

 resistivity in the interpretation. On the other hand, when altered crystal- 

 lines or sediments are overlain by river gravpis traversed by water, the 

 opposite type of resistivity indication (good conductor below) may be 

 observed. 



Fig. 10-74 shows the results of a resistivity survey made fcr a determina- 

 tion of physical rock characteristics at the site of a proposed aqueduct. 

 It revealed a fault zone 350 feet wide in which the crystalline rocks were 



8 7 6 4 3 s 7 e 



Gnphgsicd Stations • 

 Fig. 10-74. Location of fault zone by resistivity mapping (after Henderson). 



crushed and waterlogged . (reduced resistivities), making it necessary to 

 reject the site for the proposed structure. 



In highway construction, resistivity surveys can be of considerable help 

 by locating construction materials. There are still manj-^ projects where 

 sand and gravel are hauled for considerable distances, while sources of 

 near-by supply might be located by resistivity measurements without much 

 trouble. The curves reproduced in Fig. 10-75 show how effectively a 

 gravel and sand lens in a clay bed can be mapped. Many more examples 

 of this type are given by Kurtenacker in an article dealing especially with 

 the applications of geophysics to highway problems.^* 



"K. S. Kurtenacker, A.I.M.E. Geophys. Pros., 49-59 (1934). 



