Geoscience Reference
In-Depth Information
pulse is guided along a transmission line embedded in the soil. The time delay between the reflec-
tions of the pulse from the beginning and the end of the transmission line is used to determine the
velocity of propagation through soil, which is controlled by the relative dielectric permittivity or
dielectric constant. Both TDR and GPR are based on the fact that electrical properties of soils are
primarily determined by the water content (θ) in the frequency range from 10 to 1000 MHz (Topp
et al., 1980). For GPR, however, radio frequency signals are radiated from an antenna at the soil
surface into the ground, while a separate antenna receives both reflected and transmitted signals.
Signals arriving at the receiving antenna come from three pathways: (1) through the air, (2) through
the near surface soil, and (3) reflected from objects or layers below the soil surface. Signal velocity
and attenuation are used, like TDR, to infer both θ and soil apparent electrical conductivity (EC
a
),
which is the electrical conductivity through the bulk soil. Capacitance probes for measuring θ are
placed in the soil so that the soil acts like the dielectric of a capacitor in a capacitive-inductive reso-
nant circuit, where the inductance is fixed. Active microwaves or radar scatterometry are similar to
GPR, except that the antennae are located above the soil surface. The signal penetrates to a shallow
depth, generally <100 mm below the soil surface, for the transmitted frequencies used. Analysis of
the reflected signal results in a measure of θ and electrical conductivity at the near surface. Passive
microwaves are unique in that no signal is applied, rather the surface soil is the EM source and a
sensitive receiver located at the soil surface measures temperature and dielectric properties of the
surface soil from which θ and EC
a
are inferred. Finally, EMI, unlike GPR, employs lower-frequency
signals and primarily measures the signal loss to determine EC
a
. The common operating frequency
ranges of instrumentation for these electromagnetic techniques are EMI (0.4 to 40 kHz), CP (38 to
150 MHz), GPR (1 to 2,000 MHz), TDR (50 to 5,000 MHz), AM (0.2 to 300 GHz), and PM (0.3 to
30 GHz).
Of these geophysical techniques, the agricultural application of geospatial measurements of
EC
a
, as measured by EMI, ER, and TDR, has had tremendous impact over the past two decades.
Currently, EC
a
is recognized as the most valuable geophysical measurement in agriculture for char-
acterizing soil spatial variability at field and landscape spatial extents (Corwin, 2005, Corwin and
Lesch, 2003, 2005a). It is the objective of this chapter to present a historical perspective of the
adaptation of geophysical techniques for use in agriculture with a primary focus on trends in the
adaptation of EC
a
to agriculture, as well as the practical and theoretical factors that have forged
these trends.
2.2 hIStoRICAl peRSpeCtIve of AppARent SoIl eleCtRICAl
CondUCtIvIty (eC
a
) teChnIQUeS In AGRICUltURe—the pASt
The adaptation of geophysical EC
a
measurement techniques to agriculture was largely motivated
by the need for reliable, quick, and easy measurements of soil salinity at field and landscape spatial
extents. However, it became quickly apparent that EC
a
was influenced not only by salinity, but also
by a variety of other soil properties that influenced electrical conductivity in the bulk soil, including
θ, clay content and mineralogy, organic matter, bulk density (ρ
b
), and temperature. The EC
a
mea-
surement is a complex physicochemical property resulting from the interrelationship and interaction
of these soil properties. Researchers subsequently realized that geospatial measurements of EC
a
can
potentially provide spatial distributions of any or all of these properties. This realization resulted in
the evolution of EC
a
in agriculture from a tool for measuring, profiling, and mapping soil salinity
into a present-day tool for characterizing the spatial variability of any soil property that correlates
with EC
a
.
The impetus behind the evolution of EC
a
in agriculture stems from several factors that make it
well suited for characterizing spatial variability at field and larger spatial extents. Most importantly,
measurements of EC
a
are reliable, quick, and easy to take. This factor was instrumental in the ini-
tial adaptation of EC
a
for agricultural use. Historically, considerable research was conducted using
