Environmental Engineering Reference
In-Depth Information
Because of the difficulties of relating measured resistivity values to specific soil or rock
types, subsurface conditions are usually inferred from the vertical and lateral variations in
the measured values. In multilayered systems, interpretations must be confirmed by cor-
relations with test boring data and, in general, electrical resistivity should be considered
as a preliminary exploration method.
Electrical Conductivity (EM) Surveys
Applications
Terrain conductivity meters read directly the apparent conductivity, and interpretation of
profiles is usually qualitative, based on showing anomalies that are then investigated by
other methods. Some applications are:
Mapping nonorganic contamination of groundwater, which usually results in an
increase in conductivity over “clean” groundwater. An example is acid mine
drainage.
Mapping soil and groundwater salinity.
Mapping depth to basement rock.
Locating buried metal tanks and drums.
Locating buried mine adits.
Operational Procedures
Conductivity is compared with resistivity in Figure 2.34. The electrical conductivity meter,
the Geonics EM 34, uses two dipoles which can be used in either the vertical or horizontal
mode. A single dipole, in the horizontal mode, is shown in Figure 2.35. Each mode gives a
significantly different response. One person carries a small transmitter coil, while a second
person carries a second coil that receives the data from the transmitter coil. Electrical con-
tact with the ground is not required and rapid exploration to depths of 60 m are possible.
The electrical conductivity meter, EM 31 (Figure 2.36), is operated by one person.
Exploration is rapid, but the effective depth of exploration is about 6 m.
Underground Mine Study
Old maps indicated that an adit for a lead-zinc mine was located somewhere in an area
to be developed for expensive home construction. The general area had suffered from
Conductive end plates
(area A )
Ammeter (current I )
Battery (voltage V )
Resistivity (
)
Conductivity (
)
= R A
L
= G L
A
m
/m
V
where R =
I
FIGURE 2.34
Resistivity vs. conductivity.
where G =
I
V
 
 
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