USE OF GEOELECTRIC METHODS IN SEARCH FOR OIL 171 
errors of observation are about one-tenth the magnitude of the dis- 
turbing effects, the measurements are doubtless of adequate precision. 
Greater precision may make the work unduly tedious and lead to 
confusion. This does not apply to systematic errors of measurement. 
Advantage may accrue if these are of an order of lower magnitude 
than that of the accidental errors. 
The various electric methods which have been in use for one or 
another phase of geophysical exploration may be classed as follows: 
radio methods, electromagnetic methods, equipotential methods, re- 
sistivity methods, and miscellaneous electric methods. It appears 
that of these the electromagnetic and the resistivity methods have 
enjoyed a measure of success in the exploration for oil. When this is 
taken along with the fact that, in comparison with gravimetric and 
seismic methods, these electric methods are late comers in this field. 
their present prospects seem good. 
The introduction of electrical methods into the more difficult 
phases of exploratory work has doubtless been hindered by the tardy 
development of a quantitative theoretical basis for the interpretation 
of the electrical survey data. In the case of seismic and gravimetric 
methods such a basis had been fairly well developed through their 
years of use in the basic studies of geophysics. The theoretical prob- 
lems presented by the electric methods are more complicated, but 
these have been attacked in recent years by several investigators and 
the results obtained are of considerable practical value. This is es- 
pecially true of those investigations which apply to resistivity methods. 
The detailed solutions here are restricted to the simpler ideal cases, 
but these provide norms with which survey results may be compared. 
It is to be expected that calculations will be extended to include other 
typical cases and that in the near future methods to facilitate such 
calculations will also be developed. 
From the theoretical considerations it appears that geophysical 
measurements alone will admit of a unique quantitative interpreta- 
tion only in special cases. Of the several interpretations which would 
be consistent with the measurements, some may be eliminated as 
being incompatible with facts known to the geologist. The number of 
possible interpretations is likely to be further reduced if surveys by 
more than one method are made. Thus, by the consideration of other 
information along with the survey data, it seems possible to decide 
upon one as the most probable interpretation. When there is added to 
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