740 



ELECTRICAL METHODS 



[Chap. 10 



(clayey overburden, small conductivity contrast, varying water content), 

 the data may become very unreliable, as indicated by Stem's measurements 

 in the Niederlausitz and Hawkins'" observations in Ontario. Extensive 

 resistivity measurements have been made by Ewing and- Crary on anthra- 

 cite coal beds. Further references to geophysical results on coal measures 

 are given in an article by the author.^'^ In the prospecting for salt, potash, 



RtsistMtifDtpfh Caryes 

 5k f ion No. 10 



Conducttrx Micaflons 



m 200 250 

 5hkt Intervti 



Profile along Secfion Mo. 10 



Proposed 

 DOhok 



Fig. 10-71. Apparent resistivity curves on three traverses (15° off strike) above 

 Peach Bottom Vein, Alleghany County, North Carolina (after Griswold). 



Z 4 6 8 10 ji 



6ravtl 



e 4 6 e 



I 4 b S 



Fig. 10-72. Apparent resistivity curves on lignite beds, Ville, Germany (after Stern). 



or sulfur for mining purposes, the geologic problems are the same as in 

 the location of salt domes in oil exploration. 



Determination of overburden thickness, a problem frequently encoun- 

 tered in mining operations in connection with sinking shafts, driving tun- 

 nels, and excavations from surface, has been accomphshed successfully by 



" R. H. Hawkins, A.I.M.E. Geophys. Pros., 76-120 (1934). 

 " "Geophysics in the Non-Metallic Field," A.I.M.E. Geophys. Pros., 546-577 

 (1934). 



