Geology Reference
In-Depth Information
Resistivity meters are designed to measure potential
differences when no current is flowing. Such a null
method is used to overcome the effects of contact resis-
tance of the electrodes with the ground. The potential
between the potential electrodes is balanced by the po-
tential tapped from a variable resistance. No current then
flows in the resistivity circuit so that contact resistance
will not register, and the variable resistance reading
represents the true resistance of the ground (equal to
the ratio
D
V/I
in the relevant equations).
Previous generations of resistivity meters required
the nulling of a displayed voltage by manual manipula-
tion of a resistor bank. Modern instruments have
microprocessor-controlled electronic circuitry which
accomplishes this operation internally and, moreover,
performs checks on the circuitry before display of the
result.
Resistivity surveying for shallow penetration can be
made more efficient by the use of spike electrodes
which are mounted on small wheels and towed along
a profile by the operator. Improvements in instrument
technology have also led to the development of elec-
trodes in the form of antennae which are capacitively
coupled to the ground (Panissod
et al
. 1998), so that there
is no need for spike electrodes to be placed in the ground
and a CST may be accomplished by an operator towing
the array at a walking pace by foot or vehicle. Measure-
ments can be taken automatically and are no longer re-
stricted to areas where electrodes can be in-serted, such
as road metal, ice, permafrost, etc. Such a system allows
the collection, by a single operator, of 500% more data
in the same time as a conventional instrument with a
crew of two. However, the limitations of the physical
dimensions of such equipment considerably restricts
penetration.
Measured
potential
difference
Electrodes + -
XXX
X
X
Telluric
shift
True potential
difference
XXX
X
X
Electrodes - +
Time
Fig. 8.7
The use of alternating current to remove the effects of
telluric currents during a resistivity measurement. Summing the
measured potential difference over several cycles provides the true
potential difference.
levels of resistance commonly encountered in resistivity
surveying.Apparent resistivity values are computed from
the resistance measurements using the formula relevant
to the electrode configuration in use.
Most resistivity meters employ low-frequency alter-
nating current rather than direct current, for two main
reasons. Firstly, if direct current were employed there
would eventually be a build-up of anions around the
negative electrode and cations around the positive elec-
trode; that is, electrolytic polarization would occur, and
this would inhibit the arrival of further ions at the elec-
trodes. Periodic reversal of the current prevents such an
accumulation of ions and thus overcomes electrolytic
polarization. Secondly, the use of alternating current
overcomes the effects of telluric currents (see Chapter 9),
which are natural electric currents in the ground that
flow parallel to the Earth's surface and cause regional po-
tential gradients. The use of alternating current nullifies
their effects since at each current reversal the telluric cur-
rents alternately increase or decrease the measured po-
tential difference by equal amounts. Summing the results
over several cycles thus removes telluric effects (Fig. 8.7).
The frequency of the alternating current used in resistiv-
ity surveying depends upon the required depth of pene-
tration (see equation (9.2)). For penetration of the order
of 10 m, a frequency of 100 Hz is suitable, and this is de-
creased to less than 10 Hz for depths of investigation of
about 100 m. For very deep ground penetration direct
currents must be used, and more complex measures
adopted to overcome electrolytic polarization and tel-
luric current effects. Many modern instruments make
use of a square wave current input to overcome the
polarization.
8.2.6 Interpretation of resistivity data
Electrical surveys are among the most difficult of all the
geophysical methods to interpret quantitatively because
of the complex theoretical basis of the technique. In
resistivity interpretation, mathematical analysis is most
highly developed for VES, less well for CST over two-
dimensional structures and least well for CST over three-
dimensional bodies. The resistivity method utilizes a
potential field and consequently suffers from similar
ambiguity problems to the gravitational and magnetic
methods.
Since a potential field is involved, the apparent resis-
tivity signature of any structure should be computed by
