100 CA? ELLAND 
stones and sandstone, 20 to 40 for clays and,sands, and 2 to 20 for 
porous sands, clays, and soils. 
C. RESISTIVITY METHODS 
In the two sections that follow, the principles, apparatus, and 
electrode arrangements used in the resistivity and potential-drop- 
ratio measurements are discussed first. After that, some outstanding 
results are described which have been obtained with both methods 
on geologic structure of various types, as well as in direct prospecting 
for oil. 
Though the purposes for which the resistivity and potential-drop- 
ratio methods are used are usually identical, these two methods differ 
essentially in electrode arrangement and technique of measure- 
ment. 
In the resistivity methods proper, current is usually supplied to 
two points, and the potential distribution between these two points 
is investigated by either one or two electrodes. The current is deter- 
mined which flows between the two outside current terminals, and 
the potential difference is observed which exists either between the 
two potential electrodes, or between the one potential electrode and 
one neighboring current electrode. The ratio of voltage and current, 
multiplied by a constant depending on the mutual arrangement of 
the electrodes, gives the resistivity of the surface formation, if only 
one formation exists within the reach of the whole contacting arrange- 
ment. If more formations are present the “‘apparent”’ resistivity is 
measured as a function of the electrode spacing, and thus, as a func- 
tion of the depth to the interfaces of the formations involved. 
In the potential-drop-ratio methods, current is again supplied to 
two points in the area under investigation; but this time, the in- 
vestigation of the potential distribution takes place outside of the 
pole doublet and is usually made in the vicinity of one electrode. The 
drop of the potential in the vicinity of this one electrode is intimately 
related to the existence of resistivity changes with depth. Experience 
has shown that the most interpretable results are obtained by not 
investigating the drop of the potential itself, but by determining the 
ratio of potential drops in two adjacent intervals. That is to say, the 
testing device in this method requires only 3 electrodes, and is in- 
dependent of any connection to the current circuit or to the source of 
energy, which gives these methods a distinct advantage over the 
resistivity methods proper. 
436 
