ADVANCES IN OIL PROSPECTING QI 
also differ in a number of respects. The seismic method is essentially 
a dynamic method (velocity measurements) while the electrical 
method is essentially a static method (determination of potentials 
and electromagnetic fields), disregarding for the moment that the 
element of time enters into the latter through frequency and phase. 
The seismic and electrical methods each have a dual advantage; 
namely, that two types of the energy may be observed; in seismic 
methods, the refracted and reflected energy; in electrical methods, 
the potential distribution and the electromagnetic field of the ground 
currents. As far as seismic methods are concerned, the variety of 
application is very nearly exhausted with the foregoing. In electrical 
methods, added advantage is obtained by varying the type of appli- 
cation of primary energy: first, in regard to frequency; second, by 
employing either galvanic or inductive coupling. 
Notwithstanding such advantages, electrical methods, as a whole, 
have not found such wide application in oil work as seismic prospect- 
ing. One reason for this is their more complicated field technique when 
the horizontal radius of operation is increased. Another reason is the 
limit in depth penetration when the frequency is increased. A third 
reason is the following: in most geophysical methods, the sought for- 
mations must have a thickness comparable with their depth. This 
holds for the seismic refraction method and the electrical-potential- 
methods. Noted exceptions are the seismic reflection-method, in 
which even 2 comparatively thin formation may still be noticed, 
provided it has good reflecting characteristics, and the inductive 
electrical method, in which a thin layer may also be determined, pro- 
vided it excels in conductivity. However, there again, the electrical 
methods are at a disadvantage, as the depth penetration, according 
to practical experience, of the inductive method is not nearly as great 
as that of the seismic reflection method. A fourth reason, finally, for 
the more limited application of electrical methods is purely geological ; 
namely, that the factors which can modify the electrical characteris- 
tics of formations in both horizontal and vertical direction (such as 
pore volume and electrolytic conductivity of moisture) are more 
numerous than similar factors affecting their elastic characteristics. 
Theoretically, all these obstacles could be amply compensated if it. 
should be possible to perfect electrical methods to such a degree as to 
enable the direct location of petroleum. However, the reasons that are 
a great handicap to such effort are again purely geological in nature; 
namely, the variable factors influencing the electrical characteristics 
