
ANISOTROPY IN APPARENT RESISTIVITY CURVES 53 
It is a typical three-layer curve (Fig. 5) with some irregularities of 
the order of the errors of observation expected with the equipment 
in use. They are then smoothed out before calculations. The deter- 
minations were made in an area underlain by valley fill and at about 
2 miles from two wells which encountered water at 271 feet. Con- 
sequently the cone of exhaustion did not reach the region where the 
measurements were taken. 
; cy 
0 500 63 j /000 (400 ohm «foot 





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Sa 
Fic. 5.—Apparent resistivity-electrode spacing curve. 
— — — — Actual data. 
Graphic approximation curve. 

According to the method developed in the present article the three 
top values of the resistivities are averaged, which gives 1/3(428+611 
+ 932)=p'1=637 ohm feet for the resistivity of the top layer. The 
graphic approximation curve a is then traced, which meets the orig- 
inal curve tangentially. This curve is used to determine the thickness 
of the upper layer by the application of Tagg’s method. The tabula- 
tion then obtained is shown in Table I. 
The curve’s depth-resistivity factors plotted from Table I on 
Figure 6 converge in a rather small area and consequently the depth 
713 
