BULLETIN OF THE AMERICAN ASSOCIATION OF PETROLEUM GEOLOGISTS 
VOL. 19, NO. 1 (JANUARY, 1935), PP. 37-57, 7 FIGS. 
EFFECT OF ANISOTROPY ON APPARENT 
RESISTIVITY CURVES! 
SYLVAIN J. PIRSON? 
Golden, Colorado 
ABSTRACT 
The anisotropic character of the sedimentary formations is a fact which has been 
so far generally overlooked in the interpretation of resistivity curves. The purpose of the 
present article is to indicate the normal distortion of the resistivity-depth curves for 
non-homogeneous horizontal stratified conditions. Two cases are investigated, namely 
where the anisotropies of the individual layers are the same and where they assume 
different values. The amount of error made in the accepted methods of interpretation 
is also indicated. The writer proposes a method of interpreting three-layer resistivity 
curves when due allowance is made for the anisotropic conditions of the ground. 
_ A practical case illustrates the method. 
INTRODUCTION 
The anisotropic character of the sedimentary formations is a fact 
which has been, so far, generally overlooked in the interpretation of 
resistivity curves. The distortion of the electrode spacing-apparent 
resistivity curves by non-homogeneous conditions of the individual 
layers accounts for the numerous empirical methods of interpretation. 
Gish and Rooney (1)? proposed, as a first approximation, to interpret 
the depth to horizontal discontinuities as being equal to the electrode 
separation at which a point of maximum curvature is obtained in the 
resistivity curve. Schlumberger (2) estimates the depth as being 
three-fourths of the same distance. Lancaster-Jones (3) proposed a 
closer approximation for the depth by taking two-thirds of the elec- 
trode spacing for which an inflection point occurs in the resistivity 
curve. However, by virtue of the nature of such a point which lies 
on a practically straight part of the curve, the depth determination 
may be erroneous. The closest approximation in depth determination 
was obtained by Tagg (4) who really advocated a mathematical 
solution of the resistivity curve by making use of nomographic charts 
accurately determined from the formula of the apparent resistivity 
1 Presented before the Geophysics Division of the Association at the Dallas meet- 
ing, March 23, 1934. 
2 Instructor in geophysics, Colorado School of Mines. Present address (January, 
1935): Seismograph Service Corporation, Kennedy Building, Tulsa, Oklahoma. 
3 Numbers in parentheses indicate references at end of this article. 
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