ADVANCES IN OIL PROSPECTING 13] 
I. ELECTRODE ARRANGEMENTS 
ad. IN POTENTIAL-DROP AND PHASE-ANOMALY MAPPING 
The arrangement customary in this work is shown in Figure toga. 
Current from an A.C. source, that is, either from a generator or froma 
buzzer, is supplied to the ground at the points Z.and £;. The potential 
drops are measured between the power electrodes, at the points A, B, 
C, D, E, et cetera, and the bridge arrangement occupies first the points 
A, B, and C; then the points B, C, and D; then C, D, and E, et cetera. 
The procedure is accurate but laborious. For the reasons just stated, 
that is, that the potential-drop indications are more pronounced for 
vertical formation boundaries than for horizontal ones, this method 
of potential mapping has not found very much application in oil 
work. Only vertical electrical drilling by means of the potential-drop- 
ratio method has gained importance. 
b. ELECTRODE ARRANGEMENTS FOR ELECTRICAL VERTICAL DRILLING 
The two types which have been used to date are illustrated in 
Figure 19 c and d. The electrode set-up used by Koenigsberger differs 
from the arrangement employed by the Swedish American Prospecting 
Company in that the spacing of the current electrodes is changed, 
while in the Swedish American arrangement the current basis re- 
mains constant. Furthermore, in Koenigsberger’s arrangement the 
current basis is short and only twice as long as the interval of the 
search electrodes A, B, and C while in the Swedish method the ex- 
ploring electrodes are used in the vicinity of one current electrode and 
the other current electrode is practically at infinity. Finally, in the 
Swedish American arrangement the interval between the exploring 
electrodes remains fixed, while in Koenigsberger’s method this interval 
increases with an increase in the length of the current basis. Hence, 
for purposes of interpretation, the depth reached in Koenigsberger’s 
method is a function of the interval a, while in the Swedish method 
the depth is proportional to the distance of the center probe, B, 
from the power electrode E. In Koenigsberger’s method the measure- 
ments are made in the extension of the current basis; in the Swedish 
American method, the traverses may be laid so as to radiate in all 
directions from the near power electrode; to eliminate the influence of 
the far current electrode completely, the most advantageous arrange- 
ment is at right angles to the current basis (strictly on the circum- 
ference of a circle with the radius £,Z, about the far current elec- 
trode). 
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