Geoscience Reference
In-Depth Information
2
Past, Present, and Future
Trends of Soil Electrical
Conductivity Measurement
Using Geophysical Methods
Dennis L. Corwin
ContentS
2.1 Introduction ........................................................................................................................... 17
2.2 Historical Perspective of Apparent Soil Electrical Conductivity (EC
a
) Techniques in
Agriculture—The Past .......................................................................................................... 18
2.2.1 Measurement of Soil Salinity with EC
a
..................................................................... 19
2.2.1.1 Electrical Resistivity .................................................................................... 21
2.2.1.2 Electromagnetic Induction ........................................................................... 23
2.2.2 Measurement of Water Content with EC
a
.................................................................. 24
2.2.2.1 Time Domain Reflectometry 26
2.2.3 From Observed Associations to EC
a
-Directed Soil Sampling .................................. 27
2.3 Current State-of-the-Science of EC
a
Applications in Agriculture—The Present................. 28
2.3.1 Factors Driving EC
a
-Directed Soil Sampling............................................................ 28
2.3.2 Characterization of Soil Spatial Variability with EC
a
............................................... 30
2.3.3 Agricultural Applications of EC
a
-Directed Soil Sampling........................................ 32
2.4 Prognosis of Geophysical Techniques in Agriculture—Future Trends and Needs .............. 34
References ........................................................................................................................................ 36
2.1 IntRodUCtIon
Arguably, the beginnings of geophysics can be traced to Gilbert's discovery that the world behaves
like a massive magnet and Newton's theory of gravitation. Since that time, researchers in geophysics
have developed a broad array of measurement tools involving magnetic, seismic, electromagnetic,
resistivity, induced polarization, radioactivity, and gravity methods. Although at times a formida-
ble technological feat, the adaptation of geophysical techniques from the measurement of geologic
strata to the measurement of surface and near-surface soils for agricultural applications was the next
logical step.
Geophysical techniques currently used in agricultural research include electrical resistivity
(ER), time domain reflectometry (TDR), ground-penetrating radar (GPR), capacitance probes (CPs),
radar scatterometry or active microwaves (AM), passive microwaves (PM), electromagnetic induc-
tion (EMI), neutron thermalization, nuclear magnetic resonance (NMR), gamma ray attenuation,
and near-surface seismic reflection. Several of the geophysical techniques fall into the category of
electromagnetic (EM) methods because they rely on an EM source, including TDR, GPR, CP, AM,
PM, and EMI. Each varies from the other in a subtle way. For TDR, the applied electromagnetic
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