92 C. A. HEILAND 
of formations as already stated are those which in most cases mask 
the effect of the oil. 
On the other hand, it must be admitted that notable advances 
have been made in approaching the solution of the problem, at least 
for favorable conditions. Furthermore, remarkable progress has been 
made in reducing the difficulties which have been outlined in the pre- 
ceding paragraph; that is, attempts have been made to increase the 
depth penetration, and to perfect the technique in such a way as to 
obtain a more interpretable type of indication. 
It is quite logical that the resistivity and surface-potential methods 
offered the greatest possibilities in this respect; for the first necessity 
in attempting an increase in depth penetration is a decrease in work- 
ing frequency. For inductive methods, this would materially reduce 
the current strength induced in subsurface conductors; for electro- 
magnetic methods, it would embody a complication in methods of 
observation (replacement of the telephone by the galvanometric- 
rectifier method, or, if the telephone is to be retained, the use of a 
frequency converter). 
From what has gone before it is obvious that an increase in depth 
penetration should be concomitant, if possible, with a greater pre- 
cision in determining effects from such depths. Therefore, efforts have 
been made to find such electrode arrangements which would give the 
most distinct indication of formation boundaries, as the original 
Wenner 4-terminal, and similar electrode assemblies do not always 
give readily interpretable curves. Probably the most successful ac- 
complishment along this line has been the perfection of the potential- 
drop-ratio method which required an appreciable modification in the 
technique of the resistivity measurement which had been employed 
up to that time. The potential-drop-ratio method permits the de- 
termination of small differences in apparent resistivity with a great 
deal of precision, requires no connection of the ratiometer to the 
power electrodes, and gives well interpretable curves. 
There is a great deal of literature on the resistivity method (see 
reference list at the end of this article), chiefly referring to the appli- 
cation of the method in mining and civil engineering. In the geophysi- 
cal laboratory of the Colorado School of Mines, extensive studies have 
been made on the application of resistivity methods, not only to the 
problems already mentioned, but also to structural oil prospecting. 
The Gish-Rooney equipment was perfected, model experiments were 
made, a Racom outfit was acquired through the courtesy of the 
422 
