ADVANCE IN OIL PROSPECTING 129 
D. PoTENTIAL-DRop-RATIO METHOD 
Although this method has been chiefly developed for the purpose 
of obtaining better interpretable curves in electrical vertical drilling, 
it may, like the resistivity method proper, be applied also in what 
may be termed potential-drop mapping, and phase-anomaly mapping. 
As the name implies, the principle of measurement is the deter- 
mination of the ratios of the potential difference between three points 
(Fig. 19, a). The instruments give the voltage difference A—B di- 
vided by the difference B—C. It is therefore possible, by successive 
occupation of the points A, B, and C, and then of B, C, D et cetera, 
to determine the potential drop along a traverse line. This leads to 
the plotting of potential-drop curves and phase-anomaly curves 
(Fig. 19 6). It is seen that the potential decreases and the phase angle 
increases, above conductive bodies and vice versa. However, this 
method of plotting potentials and phases does not furnish nearly as 
much quantitative data in respect to depth of formation boundaries 
as the equi-resistivity map method (for example, Fig. 26). The infor- 
mation obtained by a potential and phase profile is at the present 
stage of interpretative technique, comparable with the information 
conveyed by an equipotential line map. Electrical vertical drilling 
by means of the potential-drop-ratio method, however, furnishes a 
type of curve which is most readily interpreted with reference to the 
depth of formation boundaries (for example, Fig. 35). While the 
mapping of potential drops is chiefly applicable to the detection of 
abrupt changes in conductivity in a horizontal direction’ (steeply 
dipping ore bodies, faults, vertical formation boundaries, and the 
like), the potential-drop-ratio method is to be given preference when 
the depth to horizontal formation boundaries is determined. Figure 
20 shows this advantage of the potential-drop-ratio method by a 
comparison of the indications obtained by it with the type of curves 
furnished by: (@) equipotential line, (6) potential drop, and (d) ap- 
parent resistivity methods. It is seen that the interpretation of the 
potential-drop ratios becomes particularly easy if the curves shown 
under (c) are turned through go°, that is, if the geologic section is 
plotted in the usual manner, with the exception that the vertical scale 
is stretched to 3/2 of the original scale, and the electrical curve is 
superimposed upon it. We shall return to this method again when 
dealing with the interpretation of the potential-drop-ratio indications. 
1 See also Hedstrom (ref. list No. IV;). 
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