Glossary - Agricultural Geophysics - page 383

Glossary
Agricultural Geophysics Agricultural application of geophysical methods to investigate the shal-
low subsurface (within a typical depth range of 0 to 2 m).
Agrigeophysics Contraction of the term “agricultural geophysics.”
Apparent Soil Electrical Conductivity (Inverse of apparent soil electrical resistivity. ) The elec-
trical conductivity measured for a bulk volume of soil using resistivity and electromagnetic
induction geophysical methods. The measured electrical conductivity represents the true
value for the entire bulk soil volume only if soil electrical conductivity is homogeneous.
However, soil electrical conductivity is more likely to be heterogeneous; hence, the use of
the term “apparent.”
Apparent Soil Electrical Resistivity (Inverse of apparent soil electrical conductivity .) The elec-
trical resistivity measured for a bulk volume of soil using resistivity and electromagnetic
induction geophysical methods. The measured electrical resistivity represents the true value
for the entire bulk soil volume only if soil electrical resistivity is homogeneous. However,
soil electrical resistivity is more likely to be heterogeneous; hence, the use of the term
“apparent.”
Attenuation Reduction in the amplitude or energy of a wave as it propagates through a medium.
(a) Seismic wave amplitude reduction results from geometric spreading, frictional dissipa-
tion of elastic energy into heat, and mode conversion. (b) Generally, the reduction in ampli-
tude of a plane electromagnetic wave traveling through soil or rock depends on the magnetic
permeability, dielectric constant, and electrical conductivity of the soil or rock along with
the frequency of the electromagnetic wave.
Azimuthal Rotation Resistivity Measurement A field data collection mode for the resistivity
method employed to quantify the horizontal directional components of resistivity (horizon-
tal resistivity anisotropy), which may provide useful information on the presence and trends
of aligned features in the subsurface. Azimuthal rotation resistivity data collection is com-
monly carried out with either the electrode array midpoint or one of the array's end points
kept stationary, while successive resistivity measurements are made as a linear electrode
array is pivoted about this stationary point in increments of 10° to 20° through a complete
sweep of 180° or 360°. The spacing distances between electrodes remain constant as the
array is pivoted about a point.
Bulk Modulus A measured constant that defines the stress-strain behavior of an elastic material
due to pressure change. The bulk modulus along with the rigidity modulus and density deter-
mine the seismic P-wave velocity in soil or rock materials.
Capacitively Coupled Resistivity Methods Resistivity geophysical methods that utilize a capaci-
tive coupling approach to introduce electric current into the subsurface. This approach typi-
cally employs coaxial cables that are laid out on the ground surface. The coaxial cable and
the soil surface essentially form a large capacitor. The metal shield of the coaxial cable is
one of the capacitor plates, and the soil surface is the other capacitor plate, with the outer
insulation of the coaxial cable acting as the dielectric material separating the two plates. The
system transmitter applies an alternating current to the coaxial cable side of the capacitor, in
turn generating alternating current in the soil on the other side of the capacitor.
Cation Exchange Capacity (CEC) The amount of cations per unit dry soil weight electrostatically
adsorbed at soil particle surfaces. The measured value is commonly reported in meq/100 g.
Clay Soil particles with an equivalent diameter less than 0.002 mm. The clay size fraction of a soil
is composed primarily of clay minerals and organic matter.
383