The Study of the Geometry of Aquifers - Hydrogeology: Objectives, Methods, Applications - page 104

Ω⋅m for mud or muddy clay, a few tens of Ω⋅m for clay, marl, or composite
terrain, and by several hundreds or thousands of Ω⋅m for limestone, granite,
or dry gravel.
The principle is to inject a direct current of intensity I into the soil
between two terminal electrodes A and B (emission line), and to measure
the difference in potential V created between two intermediate electrodes
M and N (reception line). The intensity I is read from an amperemeter and
the difference in potential V is measured with a potentiometer (Figure 52).
The four electrodes are aligned and set up symmetrically with respect to
the center of the arrangement. The Schlumberger setup is most commonly
used, where the distance MN is small compared to the distance AB. This
procedure enables the determination of the apparent resistivity of the
terrain ρa, calculated using the equation ρa = k⋅V/I, where k is a coeffi cient
dependent on the setup of the electrodes.
Figure 52 Principle of an electric survey.
Electric sensing can measure the apparent resistivity by progressively
increasing the distance between the electrodes A and B and the center, and
therefore increasing the depth of investigation below the measuring station.
By transferring the apparent resistivities calculated based on the length
AB/2 onto a bilogarithmic diagram, one obtains a curve characteristic
of the survey performed (Figure 53). It can be interpreted in order to
deduce the actual resistivity and the thickness of each formation under
examination, by using catalogs of existing curves, in the case of small
emission lines. For larger emission lines, however, more complex methods
(auxiliary point method) or computer programs (allowing the optimization
of interpretations based on the available data) are required.