736 



ELECTRICAL METHODS 



[Chap. 10 



ing to the high resistivity of the lime, this anticline appears as a peak of 

 high apparent resistivity. The variations in the curve are probably due 

 to small irregularities in the surface of the glacial till. 



Not all anticlines can be expected to appear as resistivity highs. Fre- 

 quently the overburden is of high resistivity and the anticlines are made 

 up of shale formations with lower resistivity. In such cases an anticline 

 will appear as a resistivity low. The Caucasian foothill zone, extending 

 from the vicinity of the well-known field of Grozny to Vladicavcas, shown 

 in Fig. 10-67, is an example. All anticlines appear as elongated zones of 

 low resistivity. The depth of penetration and electrode separation in this 

 survey was about 820 feet. 



moo 



ispoo 



topoo 



spoo 



SkcitI Till, 



Fig. 10-66. Resistivity profQe across buried anticline, taken vrith an electrode spacing 

 of 200 feet (after Hubbert). 



The survey represented in Fig. 10-68 is of interest because of the ex- 

 cellent correlation possible between resistivity profiles. The section (New 

 Mexico)^ is made up of gypsum, red beds, shales, and limestone with salt 

 underneath, at a depth of 400 to 500 feet. The red bed and shale section 

 (with the exception of the near-surface portion) appears as a good con- 

 ductor. The first downbreak of the curve is readily correlated, the 

 characteristics of the first four curves being similar in all details. The 

 correlation of the troughs of these curves gives a picture closely related 

 to the surface of the salt beds several hundred feet down. It would be 

 difficult, however, to find a direct indication of salt in the lower portion of 

 the curves. This results from the screening effect of the good conductors 

 above. 



