ANISOTROPY IN APPARENT RESISTIVITY CURVES 309 
gives larger apparent resistivities pa parallel with the bedding surfaces 
than perpendicular to them. The explanation of this paradox is 
beyond the scope of the present paper, but a few practical results are 
given. 
Measurements were made on sedimentary beds 7m situ using one- 
foot electrode separation. 
An outcrop of hard broken clay (10 feet thick), slightly stratified, 
of the Laramie formation (Tertiary) in Golden, Colorado, has given 
the following results. 
Resistivity parallel with the bedding surface: 825 ohm feet 
Resistivity perpendicular to the bedding surface: 388 ohm feet 
A hard laminated shale interbedded with sandstone of the Benton 
formation (Tertiary) in Golden gave the following results. 
Resistivity parallel with the bedding surface: 
Electrodes in sandstone seam: 3,000 ohm feet 
Electrodes in shaly seam: 1,760 ohm feet 
Resistivity perpendicular to the bedding plane: 
204 ohm feet 
Another series of measurements was made at the same location 
on a laminated fine white sandstone (Benton formation) and the 
following results were obtained. 
Resistivity parallel with the bedding plane: 1,770 ohm feet 
Resistivity perpendicular to the bedding plane: 970 ohm feet 
From the preceding results, one should not generalize that the 
large axis of the anisotropy ellipse will always be in the stratification 
plane; actually, jointing in shales may produce an anomalous aniso- 
tropy. It is thus possible for the conductivity to be larger perpendicu- 
lar to, than parallel with, the stratification. The anisotropy co- 
efficient a of sedimentary formations which is defined as the ratio 
of the apparent parallel resistivity to the apparent normal resistivity 
may then assume values ranging from close to zero to values as high 
as ten. The isotropic conditions of a layer are represented by a=1. 
ANISOTROPY REDUCTION THEOREM 
The resistivity method of prospecting being a surface potential 
method, it is necessary to investigate the distribution of potential 
in an anisotropic medium in order to calculate the apparent re- 
sistivity. Let us consider the case of two superimposed layers desig- 
nated by the indexes 1 and 2 and of anisotropy coefficients a1 and a2. 
The resistivities perpendicular to the stratification are designated by 
Pi and p2, and the resistivities parallel with the bedding planes by 
Pir and pe, which latter resistivities may generally be assumed to be 
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