Geoscience Reference
In-Depth Information
Resistance Meter
Current
electrode
Current
electrode
Potential
electrodes
C
1
P
1
P
2
C
2
a
a
a
fIGURe 2.1
Wenner array electrodes:
C
1
and
C
2
represent the current electrodes,
P
1
and
P
2
represent the
potential electrodes, and
a
represents the interelectrode spacing. (From Rhoades, J.D., and Halvorson, A.D.,
Electrical conductivity methods for detecting and delineating saline seeps and measuring salinity in Northern
Great Plains soils, ARS W-42, USDA-ARS Western Region, Berkeley, CA, pp. 1-45, 1977. With permission.)
=
l
a
ρ
R
(2.5)
The conductance (
C
, ω
−1
or Siemens, S) is the inverse of resistance, and the EC
a
(dS m
−1
) is the
inverse of the resistivity:
a
==
=
11
ρ
l
a
l
a
EC
C
(2.6)
R
When the four electrodes are equidistantly spaced in a straight line at the soil surface, the elec-
trode configuration is referred to as the Wenner array (Figure 2.1). The resistivity measured with the
Wenner array is shown in Equation (2.7):
2
π
aV
i
∆
ρ
=
=
2
π
aR
(2.7)
and the measured EC
a
is as shown in Equation (2.8):
1
2π
EC
a
=
(2.8)
aR
where
a
is the interelectrode spacing (m). The equations for other electrode configurations can be
found in Dobrin (1960), Telford et al. (1990), and Burger (1992).
The volume of measurement of the Wenner array is relatively large and includes all the soil
between the inner pair of electrodes from the soil surface to a depth equal to roughly the inter-
electrode spacing. Figure 2.2 illustrates the volume of measurement. For a homogeneous soil, the
volume of measurement is approximately π
a
3
. The depth of penetration of the electrical current and
the volume of measurement increase as the interelectrode spacing,
a
, increases.
Apparent soil electrical conductivity for a discrete depth interval of soil, EC
x
, can be obtained
with the Wenner array by measuring the EC
a
of successive layers by increasing the interelectrode
spacings (
a
i-
1
and
a
i
) and using Equation (2.9) for parallel resistors (Barnes, 1952):
(
)
EC
a Ca
aa
−
ai
a
−
−
i
1
EC
=− =
EC
EC
i
i
1
(2.9)
x
a
a
i
i
−
1
−
i
i
−
1
