Greinar (Vísindafélag Íslendinga) - 01.01.1976, Blaðsíða 18

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we find a 1000 Qm resistivity in layer 3. The results are sum-
marized in Table 1. The approximate depth to layer 3 from Pálma-
son (3) and the assumed temperature gradient, inferred from
Pálmason (3) are also shown.
4. DISCUSSION
The electrical conductivity of water-saturated rocks depends on
several parameters, in particular the amount of pore fluid (poro-
sity), concentration of the pore fluid, temperature and pressure.
Experiments on the dependence of conductivity of rocks as a func-
tion of the above mentioned factors are scarce. Several compila-
tions on the resistivity of dry rocks as a function of pressure and
temperature, and on water saturated rocks at room temperature
are available (11). Brace (12) and Hermance (13) have calcu-
lated resistivities of crustal rocks at higher temperatures from the
conductivity of the fluid (from Quist and Marshall (14)) and the
porosity of the rocks using Archie’s law. But they have also found
discrepancies between measured and predicted resistivity at room
temperature which may be caused by surface conduction along
pore walls.
An experiment in whioh both pressxrre and temperature are
varied for different porosity and salinity seems only to have been
carried out by Parkhomenko et al. (15). They have found a linear
relationship between log resistivity and log porosity and also be-
tween log resistivity and the reciprocal temperature. Their results
are summarised in Figure 7 for rocks saturated with 0.5 g/1 NaCl
solution, which is equivalent to concentration typical for Icelandic
high temperature water. The data scatter somewhat for different
samples and for different types of rocks but nevertheless it should
be possible to obtain from such curves an estimate of porosity for
given temperature and resistivity and for known salinity. In addi-
tion to the free moving water, rocks in situ may contain consider-
able amount of stationary water bound to the surface of the minerals
and in closed pores. This water does not take part in the flow of
ground water and may have much higher ionic strength than the
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