ANISOTROPY IN APPARENT RESISTIVITY CURVES 51 
3. The application of Tagg’s method of interpretation to depth 
determination gives a solution which is multiplied by a factor equal 
to the square root of the anisotropy coefficient. 
4. The Gish-Rooney rule of depth determination is approximately 
verified when the anisotropy coefficient a equals 1/2. 
3. TWO STRATA OF DIFFERENT ANISOTROPIES 
This case has been investigated previously and formula IV gives 
the value of the relative apparent resistivity. In order to obtain an 
idea of the amount of deviation in the resistivity curve produced by 
the presence of anisotropy we have computed the apparent re- 
sistivity curves for the following two-layer case. 
h =4 Pw=4 
hy=o0 P2»=O 
and for different combinations of the anisotropy coefficients a; and 
a2 as shown in Figure 4. 
Comparative studies of the two-layer curves in Figures 3 and 4 
indicate that the anisotropy of the surface layer governs the general 
outline of the apparent resistivity curve; however, it appears rather 
impractical to compute a correcting factor for the depth determina- 
tion. Considering the theory and the results obtained in the case of 
different anisotropies for a horizontally stratified ground, it appears 
that a determination of the depth to horizontal discontinuities can not 
be obtained by application of Tagg’s method of interpretation. 
Furthermore, the extension of the method to the solution of 
three-layer problems as proposed by Manhart and Tattam may lead 
to considerable error since in many cases it is not less than imposing 
a solution on the problem. 
In order to minimize the chances of errors in interpreting the 
three-layer resistivity curves, the writer proposes the method out- 
lined in the following paragraphs for the interpretation of homo- 
geneous or equally anisotropic layers. 
INTERPRETATION OF THREE-LAYER CURVES 
For the interpretation of resistivity curves when Tagg’s charts 
are applied to the solution of a three-layer case, the writer of the 
present article proposes the following “Successive Approximation 
method” for which the steps are as follows. 
1. Average the resistivities of the surface for electrode spacings 
from 5 to 20 feet, which will give a value of the resistivity of the top 
layer pi. Very careful measurements must be made in the field and 
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