ADVANCES IN OIL PROSPECTING 121 
field data is obtained. In doing so, all available geological data must, 
of course, be considered. It may then happen that the correct inter- 
pretation is the one which does not show as good agreement between 
the theoretical and the field data as another assumption which is less 
probable for geologic reasons. It should again be recalled, in this 
connection, that the possibility of quantitative interpretation in 
electrical vertical drilling depends altogether on whether the forma- 
tions involved retain their resistivities ina horizontal direction. The 
determinations of theoretical effects, such as involved in the indirect 
methods of interpretation, may not only be made by means of com- 
putation, but also by means of model experiments. For this purpose 
tanks may be used, filled with water or weakly electrolytic solutions, 
and the geologic bodies may be represented by metal plates, et cetera 
(for ore bodies, water tables, and faults), or by sand or clay layers. 
When working with metal plates it is convenient to use a horizontal 
tank instead of a vertical tank, that is, a tank in which the model 
ore bodies are moved horizontally away from the electrodes to simu- 
late changes in depth (Fig. 14 C). Instead of using tanks for the model 
experiments, pits may be dug into the ground and filled with such 
alternating layers of clays, sands, et cetera, as the case may require 
(reference list No. III33).! 
Summing up the methods of interpretation applied in resistivity 
work, we have: 
I. Qualitative interpretation: resistivity mapping 
II. Quantitative interpretation: electrical vertical drilling 
a. Indirect methods 
1. Computations 
2. Model experiments 
b. Direct methods 
There exists a very extensive literature on the theory of quantita- 
tive interpretation in resistivity work. The references to this literature 
are given in section II of the bibliography attached to the end of this 
paper. It is believed to be fairly complete. A perusal of this literature 
will show that practically the only electrode arrangement for which 
a complete theoretical treatment is given is the 4-terminal Gish- 
Roeney methed, and the papers which give this theory in probably 
the most complete manner are those by Hummel (ref. list No. Ih), 
Tagg (ref. list No. Iljo,14,16), Roman (ref. list No. IIs), Peters and 
1 If sand and clay layers, et cetera, are used in a small laboratory tank, thelatter has 
to S compensated to eliminate the effects of the walls (see T. A. Manhart, ref. list No. 
TIj7). 
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