170 ODE NGISH 
then more definite conclusions as to the nature of the structure are 
likely to be justified. 
The electrical conductivity, or the resistivity, of earth materials 
is the electrical property upon which those geoelectric methods 
which have shown some promise in the oil field depend. Methods em- 
ploying radio waves would, of course, respond to differences in the 
dielectric constant and some entertain hopes for such methods. Per- 
haps, when the technique for sending directed beams of radio waves 
is sufficiently developed some success will be enjoyed by ‘these 
methods. 
The resistivity of earth materials varies between great extremes. 
Thus of the materials which come into account in the oil field, salt 
water, at one extreme, may have a resistivity as low as 10 ohm- 
centimeters, whereas the value for the igneous bed rocks, the other 
extreme, may run high in the millions of ohm-centimeters. Oil itself 
has very high resistivity, but to utilize this property for direct de- 
tection requires a relation between the extent of the pool and the 
depth from surface which probably occurs only rarely. Although some 
claims of direct detection have been made, the evidence presented is 
not conclusive. 
The diversity in the resistivities of earth materials is far greater 
than that of any other property upon which geophysical methods de- 
pend. This is, however, not the advantage that it may at first thought 
seem. One reason for this is that the condition for detecting structure 
does not continue to improve indefinitely as the contrast in resistivity 
increases, but a limit is soon approached where “‘enough is as good as 
a feast.’’ Most of the benefit is obtained when the resistivities differ 
by a factor of roo. Another reason is that this great diversity increases 
the chance that inhomogeneities within a structure and even small 
local differences in constitution may act as disturbing factors. Of 
course, if the data are obtained in such a way as to constitute an 
adequate sampling, then by suitable treatment in which these dis- 
turbing effects are regarded as accidental errors of observation they 
can be largely smoothed out. Such disturbing effects are doubtless 
met with to some extent in all geophysical measurements. Their 
magnitude is a determining factor both as regards the number and 
the distribution of observations required in a survey in order that 
reliable indications of the more general features may be obtained. 
This is also a factor which plays a part in determining the optimum 
sensitivity of the measuring instruments. Thus, if the accidental 
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