ADVANCES IN OIL PROSPECTING 125 
the center of the second medium. It is of interest to compare the 
theoretical curve of Figure 17 6 with the curve obtained in actual 
practice shown in Figure 34. 
One can readily see that already in the 3-layer case the analysis 
of the results obtained by the 4-terminal method presents consider- 
able difficulties, and it is not surprising that on that account the use 
of this method for electrical vertical drilling has been largely aban- 
doned and has been replaced by the various types of the single-probe 
method and by the potential-drop-ratio method. 
Only in the 2-layer case, a rigorous direct interpretation, and a 
determination of the depth to the interface are possible. How this can 
be done in favorable geological cases, has been shown by Tagg (ref. 
list No. TTh0,14,16)- 
Tagg’s method is based on the following consideration. If we 
measure the apparent resistivity as a function of horizontal distance, 
the following quantities are given: the apparent resistivities as a 
function of electrode separation, and the resistivity of the upper 
layer. The ratio of the upper to lower resistivity, and the depth of the 
boundary are unknown. Theoretically, therefore, the equations can 
be solved by determining the apparent resistivities at two electrode 
/ 
: (Fig. 16), and 
hy # the depth to the interface, it follows that from the surface re- 
sistivity, and the apparent resistivity for only one electrode separa- 
tion, # as a function of k can be determined and graphically repre- 
sented. By taking then the apparent resistivity for a second electrode 
separation, a second function of # of k becomes available and can 
be represented by a curve, and the intersection of these two curves 
gives the depth. In practice, the more electrode separations used, 
the greater are the number of curves of 4=/f(k), and consequently 
the greater the accuracy in the determination of the point of inter- 
section of the corresponding curves. 
In order to show how the interpretation is made, a practical ex- 
ample is demonstrated in Figure 18. In the field, the curve shown in 
the upper part of Figure 18D was obtained, indicating immediately 
(by comparison with Figure 16) that a poor conductor is underlain by 
a good conductor and that & is negative. The resistivity of the top 
Stratum is taken from the curve. If the data are not as good as they 
are in the illustrated case, several separate determinations may be 
made with small electrode separation. Then for definite electrode 

separations. Designating by & the resistivity ratio z ; 
455 
