Geoscience Reference
In-Depth Information
The dipole-dipole array (Figure 5.4c) is normally configured to have a relatively large separa-
tion between the pair of current electrodes on one side of the array and the pair of potential elec-
trodes on the other side of the array. The dipole-dipole array is employed both for mapping lateral
changes in ρ
a
and for assessing the variation of resistivity with depth. When using this array for
areal mapping of ρ
a
, the spacing between electrodes remains constant as the array is moved from
one location to another, but for measuring resistivity changes with depth, the array midpoint stays at
the same location and the current electrode pair and the potential electrode pair are moved further
apart. As deduced from inspection of Figure 5.4, if the overall electrode array length is large, the
dipole-dipole array has a decided advantage over other arrays, due to the lesser amount of electric
cable that is needed for transferring current or measuring voltage. The amount of electric cable
required by the Schlumberger or Wenner arrays, in particular, can become quite unmanageable for
long electrode arrays. An additional advantage of the dipole-dipole array is that the cables for the
current electrodes are more easily kept separate from the cables for the potential electrodes, which
reduces the electric potential noise due to electromagnetic coupling (Sharma, 1997). The apparent
resistivity for the dipole-dipole array (Figure 5.4c) is expressed as follows:
∆
V
I
=
() ( ) ( )
ρ
π
nn
1
n
2
a
(5.15)
a
5.6 ConventIonAl eQUIpMent
Field resistivity surveys carried out with one of the more common electrode arrays (Shlumberger,
Wenner, or dipole-dipole) utilizes equipment that is fairly basic. All that is actually needed is an
electric current power source, a transmitter to regulate and measure the electric current, a receiver
to determine the electric potential difference (essentially a high-impedance voltmeter that draws
very little current), four insulated single-core copper wire cables, and four electrodes. The transmit-
ter and receiver are often but not always combined into a single unit. Iron, steel, or copper stakes are
commonly used for the electrodes that are inserted into the soil at the ground surface.
One problem does occur with the use of metal stakes for electrodes, and it involves the forma-
tion of unwanted electric potentials due to electrode polarization caused by contact between the
metal in the stake and the electrolytic aqueous solution present within the soil. This problem is
alleviated by using low-frequency alternating electric current (AC) to cancel out these spurious elec-
tric potentials. Typically, AC with a frequency less than 10 Hz is used for conventional resistivity
surveys, so that these simple metal stake electrodes can be employed. Lower-frequency AC is used
instead of higher-frequency AC, because higher-frequency AC reduces the electric current density
with depth and, in turn, the depth of investigation (Sharma, 1997). Direct electric current (DC) is
employed for very deep investigations and requires the use of nonpolarizing electrodes. A nonpolar-
izing electrode is typically composed of a copper rod inserted through the lid of a container that is
porous at its base and filled with an aqueous copper sulfate solution (Milsom, 2003).
5.7 CoMpUteR-ContRolled MUltIeleCtRode SySteMS
Conventional resistivity methods can be enhanced by the group installation of twenty-five or more
electrodes at a time, which are inserted at the ground surface along a line or in a grid pattern. These
electrodes are then connected via switching units and multicore cable to a computer-controlled
transmitter and receiver unit (Sharma, 1997). This computer-controlled multielectrode system
allows an electric current to be applied between any two electrodes along a line or within a grid,
while at the same time the electric potential difference is measured with respect to another electrode
pair along the line or within the grid. The system is normally programmed to collect a sequence of
