Geoscience Reference
In-Depth Information
Resistance Meter
C
1
P
1
P
2
C
2
a
a
a
a
fIGURe 2.2
The volume of measurement for a Wenner-array electrode configuration. The shaded area rep-
resents measurement volume.
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.)
where
a
i
is the interelectrode spacing, which equals the depth of sampling;
a
i-
1
is the previous inter-
electrode spacing, which equals the depth of previous sampling; and EC
x
is the conductivity for a
specific depth interval. This is often referred to as vertical profiling.
Electrical resistivity is an invasive technique that requires good contact between the soil and
electrodes inserted into the soil; consequently, it produces less reliable measurements in frozen, dry,
or stony soils than noninvasive EMI measurement. Furthermore, depending upon the manner in
which the ER electrodes are mounted onto the mobile fixed-array platform, microtopography, such
as a bed-furrow surface, may cause contact problems between the electrodes and soil. Even so, ER
is widely used in agriculture and has been adapted for commercial field-scale applications primar-
ily because the ease of calibration is appealing and the linear relationship of EC
a
with depth, which
makes the application of Equation (2.9) possible, is simple and readily understood.
2.2.1.2
electromagnetic Induction
In the late 1970s and early 1980s, de Jong et al. (1979), Rhoades and Corwin (1981), and Williams
and Baker (1982) began investigating the use of EMI to measure soil salinity. de Jong et al. (1979)
published the first use of EMI for measuring soil salinity. The early studies with EMI by Rhoades
and Corwin were efforts to profile soil salinity through the root zone (Corwin and Rhoades, 1982;
Rhoades and Corwin, 1981). Unlike ER, vertical profiling with EMI is not a trivial task, because a
relatively simple linear model can be used for low conductivity media, but for higher conductivity
values, a nonlinear model is required. Williams and Baker (1982) sought to use EMI as a means
of surveying soil salinity at landscape scales and larger with the first use of AEM to map geologic
sources of salinity having agricultural impacts.
Through the 1980s and early 1990s, the focus of EMI work in agriculture was on vertical
profiling (Cook and Walker, 1992; Corwin and Rhoades, 1982, 1990; Rhoades and Corwin, 1981;
Rhoades et al., 1989; Slavich, 1990; Wollenhaupt et al., 1986). Vertical profiling of soil salinity with
EMI involves raising the EMI conductivity meter to various heights at or above the soil surface
(i.e., 0, 30, 60, 90, 120, and 150 cm) to measure the EC
a
corresponding to incremental depths below
the soil surface (i.e., 0 to 150, 0 to 120, 0 to 90, 0 to 60, and 0 to 30, respectively). Site-specific
empirical relationships were developed, which were not widely used because they could not be
extrapolated to other sites without calibration. It was not until the work of Borchers and colleagues
(1997) that inverse procedures for the linear and nonlinear models (Hendrickx et al., 2002) were
developed to profile soil salinity with above-ground EMI measurements. Vertical profiling of EC
a
