Geoscience Reference
In-Depth Information
with EMI is mathematically complex and a difficult quantitative undertaking (Borchers et al., 1997;
Hendrickx et al., 2002; McBratney et al., 2000). As a result, qualitative evaluations of EC
a
at shal-
low and deep depths with EMI are generally used by positioning the EMI instrument at the soil
surface in the vertical (EM
v
) and then the horizontal (EM
h
) dipole mode (i.e., receiver and transmit-
ter coils are oriented perpendicular or parallel with the soil surface, respectively), which measures
to depths of 0.75 and 1.5 m, respectively. This provides measurements of EC
a
at shallow and deeper
depths, which enables the qualitative determination of whether an EC
a
profile is uniform with depth
(EM
h
≈ EM
v
), inverted (EM
h
> EM
v
), or normal (EM
h
< EM
v
).
The depth-weighted nonlinear response of EMI is shown in Equation (2.10) and Equation (2.11)
from McNeill (1980) for the vertical and horizontal dipoles, respectively:
1
Rz
v
()
=
1 2
(2.10)
2
)
/
(
4
z
+
1
2
)
/
1 2
Rz
h
()
=+−
(
4
z
1
2
z
(2.11)
where
R
v
(z)
and
R
h
(z)
are the cumulative relative contributions of all soil electrical conductivity with
the vertical and horizontal EMI dipoles, respectively, for a homogeneously conductive media below
a depth of
z
(m).
At low conductivity values (EC
a
< 100 mS m
−1
), McNeill (1980) showed that the measured EC
a
when the EMI instrument is located at the soil surface is given by Equation (2.12):
4
H
H
EC
=
s
(2.12)
a
2
2
πµ
f
0
s
p
where EC
a
is measured in S m
−1
;
H
p
and
H
s
are the intensities of the primary and secondary mag-
netic fields at the receiver coil (A m
−1
), respectively;
f
is the frequency of the current (Hz); μ
0
is the
magnetic permeability of air (4π10
−7
H m
−1
); and
s
is the intercoil spacing (m).
The calibration of EMI equipment (e.g., Geonics EM38
1
), which can be difficult and time con-
suming, is another dissimilarity with ER. However, the DUALEM-2 does not appear to suffer from
the same calibration difficulties as the EM38 due to the increased distance between the transmitter
and receiver coils. Complexity of the EMI measurement and difficulties in calibration are distinct
disadvantages of the EMI approach that have reduced its use in agriculture. These limitations are
the most likely reasons that there are no commercially available EMI mobile platforms. This has
caused the use of EMI in agriculture, even today, to be principally as a research tool.
Following the early vertical profiling efforts, research with EMI, and concomitantly with ER,
drifted away from salinity and concentrated more on observed associations between ER and EMI
measurements of EC
a
and other soil properties. This research trend significantly contributed to the
base of knowledge compiled in Table 2.1.
2.2.2 M
e a s u R e M e n t
of f
w
a t e R
c
o n t e n t
w i t h
ec
a
Several geophysical techniques have been adapted for agriculture to measure θ within the root zone
including TDR, GPR, CP, AM, PM, EMI, neutron thermalization, NMR, gamma ray attenuation,
and ER. Aside from ER and EMI, neutron thermalization, CP, TDR, and GPR have received the
greatest use for laboratory and field-scale agricultural applications. The history of the agricultural
application of CP and neutron thermalization predates all other geophysical-based approaches for
measuring θ except ER. Gamma ray attenuation has been in use in agriculture since the 1950s, but it
