96 C. A. HEILAND 
which is, therefore, of a metallic nature, the conductivity of all the 
other rocks and formations is of a purely electrolytic character. There- 
fore, all these rocks are highly resistant when dry; or, in other words, 
the conductivity of a rock is proportional to the amount of water 
which it contains, and to the quantities of ionized salts dissolved in 
the water. 
I. FACTORS AFFECTING ROCK RESISTIVITY 
From what has been previously stated, it is obvious that the re- 
sistivity of the formations which are of importance in electrical pros- 
pecting is affected primarily by the following factors: (1) the resis- 
tivity of the mineral constituents; (2) the resistivity of the water 
filling the pores; (3) the pore volume; (4) the shape of the pores; (5) 
the temperature; and (6) the pressure at the particular depth of the 
formation under investigation. 
As far as the resistivity of the mineral constituents is concerned, 
which make up most of the rocks encountered in oil resistivity pros- 
pecting, this factor is of comparatively small influence. As has been 
stated before, only rocks with a metallic mineralization have a com- 
paratively low resistivity, while the resistivity of the mineral matter 
in most barren sedimentary and igneous rocks is so high (of the order 
of 10*-10 ohms cm.~*), that they may be considered as insulators 
for the purposes of geophysical prospecting; that is to say, in most 
sedimentary rocks, the influence of the pore volume and of the con- 
ductivity of the waters filling the pores is so much more predominant 
a factor in determining the conductivity of the rock, that the influence 
of the resistivity of the mineral matter may be disregarded in com- 
parison. 
The resistivity of the water filling the pores in such rocks as are 
of importance in resistivity oil prospecting is of paramount importance. 
Following Sundberg (ref. list No. Iy,10), the impregnating waters 
may be divided into the following groups. 
A. Surface waters (waters from the surface of the ground down to and in- 
cluding the ground water). 
(a) Fresh water, such as rain, snow, lake, river, and ground water, and 
the normal moisture in the upper soil. The percentage of dissolved 
substances ranges from o.o1 to o.1 at an average. These dissolved sub- 
stances are ordinarily carbonates and silicates and small amounts of 
chlorides and nitrates of calcium, magnesium, sodium, and potas- 
sium. The resistivity of such surface waters varies at an average be- 
tween 30,000 and several hundred thousand ohms cm-. 
426 
