62 W. D. KELLER 
the portions of the circuits on each side of the observer will be un- 
symmetrical or unequal electrically. The difference between the two 
is measured by connecting in a centrally located supplementary po- 
tential electrode placed at the instrument. This partitioning operation 
is important in prospecting where lateral changes in the rocks are 
significant or critical. 
6. When rapid traversing is being done in search for deposits or 
lateral changes at depths less than 100 feet, an electrode spacing of 
40 feet or 60 feet seems to be the optimum. 
7. The location of a bed, deposit, structural change, or water table 
is due primarily to differences in inherent resistivity of the bodies 
rather than to a difference induced by the transition surface of the 
bodies or conditions. Hence, it is difficult to differentiate between two 
unlike adjacent bodies having a similar resistivity. 
8. The routine of making measurements and obtaining resistivity 
data is the lesser part of the work of the resistivity prospector. The 
logical and proper interpretation of the resistivity ‘code’ is by far 
the greater thought-provoking part of his task. For a most efficient 
and successful prosecution of the method, the observer should cor- 
relate the resistivities, so far as possible, as they are obtained in the 
field with the changes in topography, soil, water table, and probable 
changes in geology. For that reason, data which are obtained by an un- 
trained observer in ‘‘cookbook” fashion can not be interpreted in the 
office to their fullest extent. It is likewise most essential that the ob- 
server have an understanding of the fundamentals of geology. 
g. The earth-resistivity method can probably not be advanta- 
geously employed by one going into an area in which the geology is 
entirely unknown. It will probably find its greatest value in supple- 
menting and filling gaps between core-drill prospects, locating the 
position of hidden structural changes thought or known to exist in 
the area, and rapidly, approximately, and cheaply determining depths 
and thicknesses of strata, particularly overburden over solid rock, 
and in use in some types of rapid reconnaissance work. 
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