ADVANCES IN OIL PROSPECTING 149 
prevails between the curves of 43, 44, 45, and 46. Particularly, 43 is 
very similar to 44, and 45 very similar to 46. Corresponding with the 
bottom of the first red-bed layer, all the curves mentioned have the 
same appearance, which becomes less pronounced toward the north- 
east, but may yet be recognized very faintly in curve 49. Therefore, 
vA 2 J ¥ Siz e 
Wi -= 
dsl belween /ab 
about % mile 

Structure 
Flevation 
=~ %h 
Fic. 33.—Results of potential-drop-ratio observations and corresponding structure 
at Berthoud dome, Colorado. 
the writer has used that particular part of the curve as horizon 
marker for the resistivity correlation. It is seen that the resistivity 
contour thus obtained is not everywhere in accord with the logs, but 
that it checks very well with the structure correlation made on top 
of the salt beds. The next two figures, 32 and 33, show some results 
which were obtained with the single-probe and the potential-drop-ratio 
479 
