Electrical and electromagnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 275

5.6.5 Resistivity/IP survey practice

array the gradient of the potential at the midpoint of the
current dipole is required in order to maximise resolution
of horizontal layers, so the potential dipole needs to be
much smaller than the current dipole. The gradient array,
an extension of the Schlumberger array, allows surveying
on lines parallel to the current dipole. Figure 5.40 shows
for each array the nominal location for assigning the
measurement, relative to the electrodes, and the pseudo-
depth (Z) for plotting the data.
The gradient array is commonly used for mapping. All
the other arrays can be used for profiling and soundings,
and maps can be created by combining adjacent traverses.
As described in Section 5.6.2 , resistivity is calculated
from the current and the measured potential, and a geo-
metric factor is applied to compensate for the arrangement
of the electrodes. Geometric factors obtained from Eq.
(5.18) for the arrays described here are shown in Table 5.2 .

The equipment required for field resistivity measurements
is not complex. IP measurements, on the other hand,
require a transmitter capable of generating time-varying
potentials, typically several thousand volts to ensure that
the potential differences produced over the survey area are
stronger than the ambient electrical noise level, and a
receiver that can digitally process a large volume of time-
varying voltage measurements. Modern instruments can
measure several electrical parameters simultaneously and,
monitor and attenuate electrical noise.
In order to obtain reliable measurements, particular
attention must be paid to the preparation of both the
current and potential electrodes. For IP surveys, minimis-
ing contact resistance in order to maximise the current
flow and maximise the amplitude of the received signal is
particularly important for measuring the weaker secondary
time domain IP decay potentials (see Section 5.6.3.1 ) . This
is achieved by using foil in pit electrodes (see Section 5.4.1 ).
Potential electrodes must be non-polarising (see Section
5.4.1 ) . For resistivity surveys, electrochemically inactive
stainless-steel stake electrodes are sufficient.

5.6.5.2 Selecting an array
The key characteristics of an electrode array are its lateral
and vertical resolutions, depth of investigation, ease with
which data can be acquired, ease with which its responses
can be interpreted, signal-to-noise ratio (see Section 2.4 )
and EM-coupling characteristics (see Section 5.6.7.2 ).
Geometrically symmetrical arrays, i.e. the symmetry of
the electrode layout about the midpoint, produce a symmet-
rical response along the pro le over a source with symmet-
rical geometry and are easier to interpret. Maximising what
may be small signal amplitude at the potential dipole is also
an important issue. All of these characteristics can vary
widely between the different arrays, and for each array they
vary according to the electrical structure of the subsurface.
Increasing the current proportionally increases the amp-
litude of the measured potential so currents as high as a
few tens of amperes may be used in IP surveys. The use of
computer modelling during survey design can assist in
evaluating the nature of the responses from the various
arrays and help in selecting the most appropriate array for
the targeted subsurface structures.
Surveying with the pole

5.6.5.1 Electrode arrays
In principle, resistivity/IP measurements can be made with
any configuration of the four electrodes, and with either all
or some of the electrodes on the surface or below the
surface. With the deployment of a large number of surface
electrodes in a grid network over the target area, and the
use of multichannel receivers, a large number of measure-
ments can be made ef ciently. Electrodes do not have to be
repeatedly moved; instead changes to the position of the
array, and the spacing and relative positions of the elec-
trodes, are achieved by linking the various electrodes to the
various recorder channels.
The electrodes are usually arranged in-line, i.e. along the
survey traverse, this being logistically convenient. The
commonly used electrode con gurations for surface
surveying are shown in Fig. 5.40 . Note that to create the
effect of an isolated electrode (a pole) the other electrode in
the dipole is positioned at sufficient distance so as to not
influence the measurement. This is typically greater than
ten times the spacing of the other electrodes. For the pole

dipole arrays is
fast, the latter requiring slightly more effort to move the
third electrode. Both arrays find application in reconnais-
sance work. The dipole

- pole and pole -

dipole array also requires more
complex logistics, but it produces good lateral resolution of
steeply dipping features, although signal amplitude may be
low. The array is commonly used for detailed work with
the dipole spacing parameter (n) ranging typically from 1
to about 8, although larger spacing is sometimes used

-

-

dipole and dipole dipole arrays, the dipole spacing is usu-

ally set as an integer multiple (n) of the dipole length, and
for the dipole

-

dipole array the current and potential
dipoles are usually the same length. In the Schlumberger

-