38 SYLVAIN J. PIKRSON 
in the case of two plane boundaries. Tagg, however, stressed the 
point that before his method could be applied, the ground under 
investigation must satisfy the hypothesis at the base of the establish- 
ment of the charts; mainly, the individual layers must be homo- 
geneous and isotropic. A misleading extension of Tagg’s method to 
the interpretation of the three-layer resistivity curves has been pro- 
posed by Manhart (5) and Tattam (6), since they actually overlook 
the ground condition in their method. The author of the present 
article proposes here a “‘successive approximation method”’ for the 
extension of Tagg’s two-layer method to the case of three horizontal 
layers. However, the ground under investigation must be homo- 
geneous and isotropic or at least the individual layers must have the 
same coefficient of anisotropy. 
Consequently, the method is best applicable to the determination 
of depth to bed rock, igneous or vertically stratified, under a cover 
of placer or gravel beds. The method can also be advantageously 
applied to the determination of thickness of limestone beds, lava 
flows, et cetera. An example of application of the writer’s method 
is indicated here, together with the indication of the correction to be 
applied in order to compensate for the anisotropy of the ground. 
ANISOTROPY OF SEDIMENTARY LAYERS 
The effect of anisotropy on the apparent resistivity has been 
mentioned only a few times in the geophysics literature by Maillet 
and Doll (7), Slichter (8), and Miiller (9). 
It is generally accepted that in stratified media the conductivity 
is larger parallel with the bedding plane than perpendicular to the 
stratification, the mobility of the ions being larger parallel with the 
schistosity than at right angles to it. However, if the apparent re- 
sistivity pa of such a stratified ground be measured with the aid of 
four equidistant contacting electrodes, the outside ones being the 
current electrodes, and the inside ones the potential electrodes 
(Wenner-Gish-Rooney set-up), the formula, 
V 
Pa = 270 —») 
: I 
where 
a =electrode spacing 
V =potential at the inside electrodes 
I =current at the outside electrodes, 
698 
