ADVANCES IN OIL PROSPECTING 153 
ment by the Swedish American Prospecting Corporation. As seen from 
Figure 20, in electrical vertical drilling by the Racom method, forma- 
tion boundaries are represented by peaks in the ratio curves. Plotting ° 
the data-as done in Figure 35, that is, ratios higher than one, on the 
right, and ratios lower than one, on the left, a comparison with Figure 
20 shows that a peak on the right means a poor conductor below that 
peak, and a peak on the left means a good conductor below the peak. 
Thus it is readily seen that the sandstones are poorer conductors than 
the shales, which is in accordance with an observation which was 

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Fic. 35.—Results of electrical vertical drilling and corresponding geologic section 
at Spring Coulee, Alberta, Canada. Potential-drop-ratio method was used. (From 
pamphlet of Swedish American Prospecting Corporation.) 
previously made when discussing the results represented in Figure 31. 
It is also seen from Figure 35 how well the electrical results check the 
well data and how the electrical vertical drilling by means of the 
potential-drop-ratio method may be used to advantage in carrying 
the structural data obtained from wells into unknown territory. 
3. RESISTIVITY CORRELATION IN WELLS 
In discussing the results which have been obtained on structure 
with the resistivity and potential-drop-ratio methods the experience 
obtained by measuring resistivities in wells should by no means be 
overlooked. In the first place, the results obtained by determining 
the resistivities 7m situ at the depths reached by the drill, give very 
483 
