ELECTRICAL PROSPECTING 3 
marker bed which is below the unconformity and is conformable to the oil 
bearing horizon. The depth to which it is possible to work with any geo- 
physical scheme is, therefore, an important consideration. 
The depth which can be obtained reliably in electrical prospecting is a 
much debated question. Up to the present time the depth at which structures 
have been reliably mapped by electrical surveys has not exceeded 1500 feet. 
Electrical surveying has gone but a little deeper than the core drill. 
While electrical prospecting is thus a competitor of the core drill, it may 
be used as an aid in making correlations between core drill holes and also 
between wells in cases where geological correlation is difficult. This is ac- 
complished by making resistivity measurements in the holes at a series of 
depths and correlating on a resistivity basis. 
Electrical methods of geophysical exploration may be divided into two 
classes, direct current methods and alternating current methods. The first 
of these also includes the use of very low frequency alternating currents which 
are commonly used to eliminate the effects of polarization. The frequency, 
however, is low enough so that the effect of currents induced by the varying 
magnetic field may be neglected. Induced currents play the major role in the 
second, or alternating current method of prospecting. 
Il. Direct CurrRENT METHODS OF PROSPECTING 
There are a great many direct current methods available for the determin- 
ation of the changes in resistivity with depth, but the same fundamental 
al+ 
Sy = 

Fig. 1. Lines of flow in a homogeneous earth. 
principles apply to all of them. A common practice is to pass the current into 
the earth by means of two electrodes on the surface, as shown in Fig. 1. If the 
earth is homogeneous, the lines of flow are arcs of circles connecting the elec- 
trodes. The potential on the surface of the earth is then called the normal 
147 
