118 C. A. HETLAND 
nature of the indications, derived from their qualitative analysis, 
warrants it. 
Due to the nature of resistivity work, qualitative methods are 
usually applied in resistivity mapping. The area is covered with a 
4-terminal contacting outfit, and resistivities are measured; the re- 
sults are plotted and points of equal resistivity are connected by lines 
which resemble contour lines, and which are interpreted accordingly. 
The interpretation assumes a somewhat more quantitative nature if, 
at every point, instead of one electrode spacing, two electrode spacings 
are used, so that two contour maps may be drawn for any one area, 
covering two depths of penetration. Finally, considerations of a 
quantitative nature enter also when the contact plane of two forma- 
tions of different resistivities is considered, and if one, two, or three 
of the contacts are in one, and the remainder in the second formation. 
The theoretical computations for this case have been carried out 
by Tagg (ref. list No. Ilio) and Hedstrom (ref. list No. IV7) and the 
results are shown in Figure 14. The curves illustrate how the apparent 
resistivity is influenced, for various values of resistivity ratios of the 
formations involved, by the distance of the center of the contacting 
arrangement from the boundary. The curves of Figure 14 are com- 
puted for four electrodes at right angles to the formation boundary, 
and show, therefore, four discontinuities. If the contacting arrange- 
ment is parallel with the fault plane, the apparent resistivity changes 
gradually as the boundary is approached. Tagg has also computed 
the apparent resistivity curves for this case (ref. list No. Ili). 
Outside of the conditions occurring in the immediate vicinity of 
a formation boundary, the method of resistivity mapping involves 
essentially only qualitative methods of interpretation, which consist, 
as stated before, of an interpretation of the equi-resistivity lines 
(Figs. 28 and 29), or of a resistivity profile (Figs. 26 and 30). In inter- 
preting a resistivity profile, it is important to take into consideration 
the influence of the electrode separation, and thus of the depth pene- 
tration, for which Figure 26 is an example. Finally, in the interpreta- 
tion of results obtained by resistivity mapping, tank experiments may 
prove to be of great value, inasmuch as they are very easily performed. 
The geologic bodies are usually represented by metallic conductors, 
and these are arranged at various depths or with varying angles of 
dip and strike as the case may require. Then the 4-terminal contact- 
ing arrangement is carried across the body at the surface of the water 
or solution in the tank, and the apparent resistivities are measured 
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