120 C. A. HEILAND 
and plotted. Figure 15 shows, as an example, the results of tank ex- 
periments made at the Colorado School of Mines with the objective 
to study the effect of dip on the equi-resistivity profiles. 
The quantitative methods of interpretation are chiefly used in 
interpreting the results of electrical vertical drilling. The quantitative 
methods may be of a direct and of an indirect nature. In the direct 
methods, we determine the depth to formation boundaries directly 

Fic. 15.—Studies of effect of dip of formation models in water tank, using 4- 
terminal contacting arrangement with constant electrode separation of 8 inches. Solid 
lines: Apparent resistivity when line of electrodes is parallel with strike of formation. 
Dotted lines: Apparent resistivity when electrode line is at right angles to strike. 
from the curves. These direct methods are applicable in simple cases, 
that is, when only one formation boundary is present or possibly two. 
Otherwise, indirect methods are much safer; their principle is the as- 
sumption of certain formations in definite depths and a theoretical 
computation of the results to be expected. These results are then 
compared with the data obtained in the field, and the assumptions 
are modified until a satisfactory agreement between theoretical and 
450 
