ADVANCES IN OIL PROSPECTING Log 
In Figure 7, these electrode arrangements are shown. Also the 
formulae are given for the computation of the resistivities of the 
ground if no unhomogeneity is within the reach of the measuring 
arrangement. Otherwise, the resistivity values obtained by the ap- 
plication of the formulae represent apparent resistivities only, from 
which the true resistivities, as well as the depth to the formation 
boundaries, may be obtained by applying methods to be described 
later. 
b. CUSTOMARY ELECTRODE ARRANGEMENTS 
Of the many electrode arrangements previously described, only 
a few are customarily applied and it all depends on the purpose for 
which a resistivity investigation is made, which method is used. 
There are two distinct applications of the resistivity method. 
First is the method of resistivity mapping. The object of this method 
is to obtain a contour map of the area, showing lines of equal re- 
sistivity. These lines represent the ground resistivity only to a definite 
depth, and, to facilitate the interpretation of the results, two or more 
contour maps may be made for the same area, representing the re- 
sistivity down to two depths of investigation. This can be accom- 
plished simply by working with two fixed electrode separations, as 
described hereafter. The second method is the so-called method of 
electrical drilling. The results are to give the vertical variation of the 
resistivity at one point only and the depth at which any changes 
occur. The only way to accomplish this is, of course, to survey along 
a line beginning at the point where the vertical resistivity variation 
is to be tested; and this line is the longer, the greater the desired 
depth penetration. It is thus seen that accurate results in this method 
depend altogether on the question whether or not the vertical differ- 
entiation of the resistivity carries unchanged in a horizontal direction 
below the last point of measurement. 
b I. RESISTIVITY MAPPING 
In this method, usually the 4-terminal electrode arrangement is 
employed and the whole contacting outfit is carried over the area 
to be surveyed with a constant electrode separation. If it is desired 
to obtain two equi-resistivity maps covering two depths of penetra- 
tion, two electrode separations are used at each point of observation. 
The results of some equi-resistivity surveys are shown in Figures 26, 
28, 29, 30. 
1 As a matter of interest it may be mentioned that the equiresistivity method has 
recently been used for oil prospecting in shallow water (ref. list No. III, 41). 
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