Environmental Engineering Reference
In-Depth Information
12.3.2.1.
Cone penetrometer with measurement of electrical resistivity
The principle of the analysis of resistivity is generally applied in practice by
using a cone penetrometer equipped with one or two pairs of electrodes in the form
of a ring. The resistivity measurements are made by applying a sinusoidal current
through the ring electrodes and measuring the potential difference between these
electrodes. The resistivity electrodes are 5 to 15 mm wide and made of steel or
brass, spaced 10 to 150 mm apart.
As in the traditional technique, the larger the space between the electrodes, the
greater the depth the electric current penetrates the ground surrounding the
penetrometer. A typical cone penetrometer resistivity uses a source of 1,000 Hz and
can make a resistivity measurement of between 1 and 250 ohm-m with an accuracy
of +/-0.2 ohm-m [BRA 97]. The resistivity sensor operates on the principle that the
voltage drop measured across the electrodes for a given pulse, while the probe
penetrates the soil, is proportional to the electrical resistivity of the soil. A detailed
description of this probe is given by [CAM 90].
Changes in electrical conductivity can be used to clarify the stratigraphy, but can
also be used to pinpoint sources of contamination or to define areas of
contamination. They are particularly useful for detecting organic immiscible liquids
(NAPL: non-aqueous phase liquid), hydrocarbon brines and landfill leachates
[CAM 94a, CAM 94b, GAG 98, STR 98]. The immiscible organic liquids are
generally non-conductive and their resistivities are very high. In contrast, dissolved
inorganic compounds, such as found in brines and leachate, are highly conductive
and thus significantly reduce the resistivity of soils.
Resistivity probes are very effective in measuring specific electrical properties
because they are in direct contact with the ground. Therefore, several problems
associated with investigations using a drilling fluid can be eliminated. Typical
values of resistivity and electrical conductivity measured in a variety of materials
are presented in Table 12.3.
Regardless of the material investigated, resistivity measurements are always
influenced by the specific properties of materials in which they are performed and
do not give an absolute indication of the presence of a contaminant. It is therefore
very important to calibrate a sensor in the environment in which it is going to be
used.
