Geology Reference
In-Depth Information
M and N. There are different electrode arrangements for measuring the
potential difference, which are uniquely used as different purposes in
exploration techniques. The most commonly used arrangements are Wenner
and Schlumberger configurations.
Wenner Array:
Wenner configuration consist a system of earth resistivity
measurements with four collinear equally-spaced point electrodes on the
ground surface. The two current electrodes A and B and two potential
electrodes M and N are placed at surface at equal distance '
a
' as shown here.
Current
I
is passed into the ground through A and B and the potential
difference is measured through M and N.
AMON
B
_____
___.___
______
a
a
a
The geometrical factor (K) for this array is 2
a
and apparent resistivity
is given by
a
= 2
a
(
V
/
I
)
Schlumberger Array: The Schlumberger array also uses four collinear
point electrodes but measure the potential gradient at the mid point. In this
array the current electrodes are spaced much farther apart than the potential
electrodes, usually five times or more as shown here.
A
M
O
N
B
___________
___.___
____________
where
L
5(
l
)
2(
l
)
- - - - - - - - - - 2
L
- - - - - - - - - -
The geometrical factor for this array is
K
= {(AB/2)
2
-(MN/2)
2
}/MN
(
L
2
-
l
2
)/2
l
Or
K
=
V
/
I
), where A and B are the current electrodes,
M and N are the potential electrodes,
L
= half spacing of current electrodes
and
l
= half of spacing of potential electrodes.
Apparent resistivity
a
=
K
(
Resistivity Profiling
Electrical profiling is used to determine the lateral inhomogeneities up to a
particular depth and thereby a particular section of the subsurface is mapped
along a profile. This method is highly useful in mineral exploration where
the detection of isolated bodies of anomalous resistivity is required and in
groundwater exploration to determine the linear features in the subsurface,
which control the movement of ground water. In resistivity profiling, the
electrode system moves as a whole from one station to other along a line,
known as profile or traverse. For a given spacing, the electrode system has
