ABEM Terrameter SAS 1000 / SAS 4000
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In homogeneous ground the apparent resistivity will equal the true resistivity, but will
normally be a combination of all contributing strata. Thus, the geometrically corrected
quantity is called apparent resistivity (
a
).
Figure 25 shows examples of different collinear electrode configurations in use: Wenner (
,
and
), Schlumberger, dipole-dipole, and pole-pole. It can be noted that the Wenner
configuration is a special case where the four electrodes are equally spaced with a separation
a. For the Schlumberger array the l/L-relation will vary during normal surveying, similarly the
factor n will vary in a dipole-dipole survey. The different electrode configurations offers
advantages and disadvantages compared to each other in terms of logistics and resolution, and
the choice is usually a trade-off between these factors. Furthermore, the reciprocity principle
states that the current and potential electrodes may change places without affecting the
measured quantity. In some applications it may be an advantage to make use of the reciprocity
principle for logistic reasons, or for estimating the measurement accuracy.
Figure 25. Examples of different electrode arrays (A and B represent current electrodes, M
and N potential electrodes). Also a combination of pole-pole and dipole-dipole is
possible. This configuration is denoted pole-dipole.
