2 L. J. PETERS AND J. BARDEEN 
Systems of electrical prospecting are, therefore, confined to studying the 
manner in which the resistivity or the dielectric constant varies in the earth's 
crust. The relative effect which these two constants have on the measurable 
electromagnetic quantities at the surface of the earth will be discussed later. 
However, it can be stated that up to the present time, the most successful 
systems of prospecting have been based upon a study of the resistivity varia- 
tions in the crust of the earth. 
The problems of any scheme of geophysical prospecting may be stated as 
follows: 
First, from mathematical and physical considerations it is decided what 
data must be taken at the surface of the earth in order to predict the dis- 
tributions of a chosen physical constant or constants in the earth’s crust. 
Second, instruments and field technique are developed to obtain these 
data. 
Third, mathematical tools are developed which translate the quantities 
measured at the surface of the earth into the distribution in the earth’s crust 
of the physical constant or constants under consideration. When these phys- 
ical and mathematical problems are solved there remains the problem of 
finding out what relation (if any) the distributions of the physical constants 
which are determined bear to geology and of interpreting the measurements 
in terms of geology. Furthermore, to be economically justifiable the overall 
cost from field measurements to translation into geology must be com- 
mensurate with the value of the information obtained. 
An ideal system of prospecting for oil would be one which directly in- 
dicated the presence of oil. Many attempts have been made to devise an 
electrical system which would meet this ideal requirement. However, these 
attempts have met with little or no success. In the present state of the art, 
the practical systems of prospecting for petroleum are those which are de- 
signed to give information concerning subsurface geology. 
The basic assumption in mapping geological structures by electrical 
methods is that in general changes of resistivity follow the bedding planes. 
Thus a clay bed or shale bed may have a lower resistivity than surrounding 
limestones or sandstones. These beds usually are parallel with or are only 
slightly inclined to the surface of the earth. In general, the lateral changes in 
resistivity are neglected, and the changes in resistivity with depth are deter- 
mined. If many such determinations are made over an area, the resistivity 
depth curves may be correlated and the subsurface topography may be ob- 
tained. 
Generally one or more beds which have a large resistivity contrast with 
the beds above or below them are used as marker beds in mapping geological 
structures. It is obvious that the bed which is being used as a marker must 
conform to the bed carrying the oil in order to be of any practical use as a 
marker bed in exploring for petroleum. In many localities the relatively shal- 
low beds are more or less parallel to the deeper ones. However, in many other 
places major unconformities occur and the shallow beds are no longer sub- 
stantially parallel to the deeper ones. In such cases it is necessary to follow a 
146 
