98 C. A. HEILAND 
porosities of such formations as are ordinarily encountered in oil 
prospecting cover a wide range, approximately between 10 and 45 
per cent. 
The pore volume, of course, is equal to the maximum amount of 
water which is possible in a rock. This condition is generally assumed 
to exist below the ground-water level, except where the water has 
been replaced.by gas or oil. Above the ground-water level, there are 
considerable variations, which account for the varying values of re- 
sistivities that are often encountered near the surface in resistivity 
work. The moisture content of the soil is generally very low near the 
surface on account of evaporation, then increases rapidly with depth, 
then decreases and reaches a maximum in the ground-water level.! At 
the immediate surface, the moisture, and therefore the resistivity, 
varies considerably with the meterological factors. During rainfalls, 
the resistivity will usually be very great on account of the purity of 
waters; during droughts, highly conductive waters migrate upward, 
resulting in a decrease of the resistivity of the formations concerned. 
Due to the fact that the conductive medium, the water, in a rock 
is not everywhere of the same section, but varies from a very thin 
section where the grains are touching each other to a greater section 
where grains are not in contact, the relation between pore volume and 
rock resistivity is not altogether simple; it may, however, be worked 
out for certain simplified assumptions; for instance, for the case that 
the grains are spheres of equal size or for the case that the pores have 
the shape of three mutually perpendicular and parallel systems of 
tubes. Figure 1 a shows the relation between a resistance factor P, 
which is the ratio of the actual water resistivity depending on porosity 
to the water resistivity, without regard to porosity (Chapter B, II, d), 
and the water volume of a rock in percentage. 
Finally, the temperature and the pressure influence the electro- 
lytic resistivity of rocks. Figure 1 c shows the influence of tempera- 
ture; an increase in temperature decreases the resistivity consider- 
ably; the relation shown in the figure holds for any kind of an electro- 
lyte, and indicates that a rise in temperature by only 25° C. from zero 
is sufficient to increase the conductivity of a rock approximately 100 
per cent. 
The effect of pressure is much smaller and is illustrated in Figure 
2. The effect is different on concentrated solutions from what it is on 
solutions of smaller concentrations. At any rate, with the maximum 
1 For details, see ref. list III, 38. 
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