ANISOTROPY IN APPARENT RESISTIVITY CURVES 49 
( I 
\y/+ | 6 “a(n —— | 
I == nN 
Wie fa ee 


P20 — Pi P3v — P2v 
where K= ee and Kyi = ee 
P2v == Pl» P3v == P20 
h and hy are respectively the thicknesses of the top and intermediate 
layer, the third layer having an infinite thickness. 
The resistivity curves have been calculated for the following 
conditions: 
Pw=I1 h =1 
Pw =3 hy=3 
P31 —-9O hg = 0 
and for the following anisotropy coefficients: 
a=4, 2,1, 0.5 and 0.25 
For large electrode spacings, the average resistivity of the two top 
layers can be calculated by Kirchhoff’s law for two resistances con- 
nected in parallel. This average resistivity is precisely 4 here, and 
for large electrode spacings the three-layer case is reduced to the 
two-layer case previously mvestigated. This facilitates considerably 
the numerical computations since for increasing relative electrode 
separation the three-layer curves and the two-layer curves are 
tangent and finally superimposed. The results obtained are plotted 
in Figure 3. 
By considering the curves in Figure 3, the following conclusions 
for a three-layer curve can be arrived at. 
1. Anisotropies of sedimentary beds larger than one, shift the 
points of maximum curvature toward smaller electrode spacings and 
the greater the anisotropy the larger the amount of displacement. 
2. Anisotropic ground conditions (a<1) reduce the anomaly in 
the resistivity curve and the smaller the anisotropy coefficient, the 
larger the reduction. For anisotropy coefficient a larger than 1, the 
anomaly is increased and the curves are flattened. This is a fairly 
general condition encountered in field practice. 
709 
