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Figure 5.1 Variation of water resistivity with concentration of dissolved
NaCl. The uses that can be made of waters of various salinities are also
indicated.
a dry sand and underlying weathered limestone. Bulk resistivities of more
than 10 000 morlessthan1 m are rarely encountered in field surveys.
Most rock-forming minerals are very poor conductors, and subsurface
currents are carried mainly by ions in the pore waters. Pure water ionises
only very slightly, so water in its pure state is almost non-conductive; the
electrical conductivity of pore waters depends on the presence of dissolved
salts, mainly sodium chloride (Figure 5.1). However, clay minerals are
ionically active and clays conduct well if even slightly moist.
In many rocks, resistivity is roughly equal to the resistivity of the pore
fluids divided by the fractional porosity. A closer approximation is provided
by Archie's Law , which states that:
P m
ρ =
a
w /
where ρ is the bulk resistivity of a saturated porous medium, P is the
fractional porosity, ρ w is the pore fluid resistivity, and m and a are empirical
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