ABEM Terrameter SAS 1000 / SAS 4000
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Self-potentials recorded at Lund University - 2000-10-30
Steel electrodes
Pb-PbCl
2
electrodes
50 cm spacing between the electrodes
5.1.3
Important remarks concerning current electrodes
Especially for the current electrodes it is very important with good galvanic contact with the
soil. For small electrode separations (a few meters) it might be enough to stick the steel
electrodes a few centimetres into the ground. For larger separations it is very important with a
good grounding of the current electrodes. In dry conditions it is usually necessary to apply
water around the electrodes. In the figure below is illustrated the relation between electrode
burial and contact resistance, assuming that the ground is homogeneous in terms of resistivity.
It is seen, that theoretically it is very favourable to bury the electrodes some 20-30 cm,
whereas he profit of increasing the depth to, say, 60 cm is very limited. In a real situation,
however, it is possible that the uppermost part of the ground is drier and more high resistive
than underlying horizons, in which case it may pay off well to bury the electrodes deep
enough to reach a moist zone.
In order to decrease the contact resistance it is possible to use more electrodes, connected in
parallel. Provided the separation between the two electrodes to be connected is larger than the
depth of burial, the contact resistance will be almost reduced by a factor of two. Similarly,
three or more electrodes can be connected in parallel.
In dry and permeable soils, like sand, a common problem is that the water used to improve
electrode contact is drained away before measurements are finished. In such cases it is useful
to mix for example a starch compound (e.g. Johnson Revert intended to stabilise wells during
drilling) into the water to make it more viscous, this can serve to keep the water in place for a
sufficiently long time. Mixing salt into the water will further improve the ground contact.
Figure 7. Potentials measured between a pair of Pb-PbCl
2
electrodes (lower curve) and a
pair of steel electrodes (upper curve), showing typical difference in electrode noise.
