Geology Reference
In-Depth Information
Figure 6.3
Approximate variation in gradient-array response with voltage
dipole position. Array total length 2
L
, voltage dipole length
a
. The factor
C
is defined in Figure 6.2d. The distances
x
and
y
define the position of
the centre of the voltage dipole, as measured along the centre-line and at
right angles to the centre-line respectively. For
x
=
0
and
y
=
0
,
C
=
2
and the gradient array and Schlumberger array 'ideal dipole' equations
are identical. The approximate equation should be used only where the
curves are roughly horizontal, and it is generally preferable to use the basic
array equation and the exact factors. This is easily done using a computer
spreadsheet program.
Schlumberger array
: The only array to rival the Wenner in availability of
interpretational material, all of which relates to the 'ideal' array with
negligible distance between the inner electrodes. Favoured, along with
the Wenner, for electrical depth-sounding work.
Gradient array
: Widely used for reconnaissance, especially in induced po-
larisation (IP) surveys. Provided that sufficiently powerful generators are
available, large numbers of readings can be taken on parallel traverses
without moving the current electrodes. Figure 6.3 shows how the geomet-
ric factor given in Figure 6.2d varies with the position of the voltage dipole.
Dipole-dipole (Eltran) array
: Popular in IP work because the complete sepa-
ration of current and voltage circuits reduces the vulnerability to inductive
