Electrical resistivity methods involve the measurement of
the
apparent resistivity of soils and rock as a function of
depth and position. Although one of the more costly
engineering geophysical applications, resistivity surveys
can
provide unparalleled data quality in subsurface
imaging. The most common electrical methods used in
engineering and environmental investigations are vertical
electrical soundings (resistivity soundings) and resistivity
profiling. In resistivity soundings, the distance between
the
current electrodes or the distance between the current
and
potential dipoles is expanded in a regular manner between
readings, thus yielding information of the
electrical properties of soils from deeper and deeper
depths. With resistivity profiling the electrode spacing
is
fixed and measurements are taken at successive
intervals along a profile.
During resistivity surveys, current is injected into the
earth through current electrodes and the potential
difference
is
measured between potential electrodes. The resistivity
of
soils is a function of porosity, permeability, ionic content
of
the pore fluids, and clay mineralization. Apparent
resistivity is the bulk average resistivity of
all
soils and rock influencing the flow of
current. It is calculated by dividing the
measured potential difference with the
input current, and multiplying by a
geometric factor related to the array
being used and electrode spacing.
Data
are generally presented as profiles
or
contour maps and interpreted qualitatively.
