150 C. A. HEILAND 
methods on the Berthoud dome, not far from Denver. Some oil and 
gas have been produced from this dome. The geologic section consists 
of Cretaceous shales and sands. The curve shown in Figure 32 was ob- 
tained by using the method No. JJa (Fig. 7), and is plotted together 
with the log of a well next to which the measurements were made. 
Pending further work in this area, the present interpretation is that 
the peaks in the curve are produced by the water sands, and that these 
sands contain water of artesian origin, which is encountered in 
geologic structure of many types in eastern Colorado near the Front 
Range of the Rocky Mountains. If nothing else, this curve shows that 
certain resistivity peaks appear at certain depths which can be cor- 
related with definite geologic horizons, and which can be traced 
throughout the structure. 
This makes it possible to determine structural contours in this 
area as shown in Figure 33. It should be understood that these results 
were obtained with an equipment which was not at all adequate for 
the purpose. Before the regular Racom equipment (already de- 
scribed) became available for field work, we made some preliminary 
field tests with the Gish-Rooney apparatus modified for the deter- 
mination of potential-drop ratios. Hence, the results are masked to 
some extent by the variations in surface resistivities which can be 
eliminated to a much greater degree in the Racom. Nevertheless, the 
results are reliable enough to justify at least a preliminary inter- 
pretation. The potential-drop-ratio curves were turned go° and 
plotted vertically. It is seen that certain peaks in the curves repeat 
themselves in every traverse, and these are indicated in solid black. 
Then the structural contour as known from the well logs was super- 
imposed upon the potential-drop-ratio profiles, and was plotted on a 
scale 3/2 that of the potential profiles, in accordance with what has 
been previously stated about the interpretation of potential-drop- 
ratio measurements. It is seen that the known structural informa- 
tion and the peaks in the potential-drop-ratio profiles correspond 
fairly well. In this connection, it should be remembered that the in- 
dications obtained from a potential-drop-ratio log in electrical verti- 
cal drilling are altogether different from the indications obtained in 
a resistivity log. In the former, formation boundaries are marked by 
a peak in the drop-ratio curve (Fig. 20). 
Although the results just described are not as good as they might 
have been if more adequate equipment had been used they proved 
definitely that the potential-drop-ratio method could be used to ad- 
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