42 W. D. KELLER 
TYPICAL RESISTIVITY CURVES 
To obtain typical resistivities of the various kinds of rock at 
depths to 100 feet, a set-up was made over a particular kind of rock 
having a minimum thickness of at least 100 feet and readings were 
taken at increasing electrode spacings. The resistivity unit for each 
spacing (or depth) was computed and the results plotted-graphically, 
as shown in the following curves. Depth in feet is shown on the 
abscissa and resistivity on the ordinate. 
It may. be pointed out that the resistivity indicated for the 5o0- 
foot depth, for example, is not for the rock at that depth alone, but 
includes all rock between, adjacent to the electrodes, and to a depth 
of 50 feet. 
ALLUVIUM 
Graph No. 1 shows the range of the resistivity of alluvium in the 
simple resistivity units up to 100 feet in depth. The stations were 
located on the Mississippi River flood plain near Foley, Lincoln 
County, Missouri. The alluvium of curve 51 is underlain by Jefferson 
City dolomite and that of 56 by St. Peter sandstone. The curves are 
essentially the same. The slight upturn of the upper one at less than 
20-foot depths was due to dry soil on the edge of a corn field. 
In all resistivity curves of fine alluvium obtained by the writer 
the range is low, usually under 75 units. The two curves given are 
typical. 
In high or dry gravel banks the resistivity may be considerably 
higher.’ 
The resistivity of sedimentary rocks seems normally to increase 
with depth, and the higher the resistivity, the faster the increase. 
SHALE 
Curve 63 on Graph No. 2 is typical of the resistivity of an ordinary 
shale, the Grassy Creek shale, located about 3.5 miles southwest of 
Foley, Missouri. 
The resistivity of slightly sandy shale is shown on curve VH 450 
W. It was taken in the Cherokee formation, 3 miles east of Linn, 
Osage County, Missouri. The curve is also typical and is similar to 
63 and 65. 
Curve 65, Graph No. 3, shows the resistivity of Maquoketa shale 
measured on a gentle slope above a creek bed a few hundred feet 
north of the location of curve 63. The higher trend of 65 over 63 is 
probably due to the fact that the Maquoketa is much more calcareous 
and sandy than the Grassy Creek. 
564 
