ABEM Terrameter SAS 1000 / SAS 4000
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The presence of clay minerals strongly affects the resistivity of sediments and weathered rock.
The clay minerals may be regarded as electrically conductive particles, which can absorb and
release ions and water molecules on its surface through an ion exchange process.
As the variation in temperature of the ground is generally small, the temperature influence is
normally negligible. However, in e.g. geothermal applications the variation can be significant,
as well as in permafrost regions. The mobility of ions increases with increasing temperature,
as the viscosity of water is lowered. Hence a decrease in resistivity with increasing
temperature can be observed for materials where electrolytic conduction dominate.
10.1.2
Measurement Principles
Measurement of the resistivity of the ground is carried out by transmitting a controlled current
(
I
) between two electrodes pushed into the ground, while measuring the potential (
U
) between
two other electrodes. Direct current (DC) or an alternating current (AC) of very low frequency
is used, and the method is often called DC-resistivity. The resistance (
R
) is calculated using
Ohm's law:
R
U
I
=
(A.1)
The material parameter resistivity (
), which is the inverse of electrical conductivity (
), is
related to the resistance via a geometrical factor. It is common, but not necessary, to place the
potential electrodes symmetrically spaced on the line between the current electrodes. The
resistivity of ground can be calculated using:
=
K
U
I
where the geometrical factor is
K
r
r
r
r
=
−
−
+
−
2
1
1
1
1
11
12
21
22
1
(A.2)
for a generalized array, whose value depends on the positions of the electrodes as defined in
Figure 24.
Figure 24. A generalized four-electrode array with potential electrodes P
1
, P
2
and current
electrodes C
1
, C
2
(0<
,
<
)
