Geoscience Reference
In-Depth Information
Clay-pan
A dense, compact, high clay content layer within the soil profile. The presence of a clay-
pan will tend to impede root growth and the vertical movement of water.
Common Midpoint GPR Data Collection
A field data collection mode for ground-penetrating
radar normally used for determining subsurface radar velocities. With this mode of data col-
lection, the midpoint between the transmitting and receiving antennas is kept stationary, and
the transmitting and receiving antennas are successively moved farther and farther apart.
Common Offset GPR Data Collection
The field data collection mode typically employed with
ground-penetrating radar. Transmitting and receiving antennas for this mode have a rela-
tively narrow, but constant, separation distance, and measurements are collected along sin-
gle transects or a grid set of transects that cover the study area.
Constant Separation Traversing Resistivity Measurement
A field data collection mode for the
resistivity method in which the spacing distance between electrodes remains constant as
the electrode array is moved along a transect. Constant separation traversing measurements
are usually collected along a series of transects forming a grid that covers a study area,
thereby making this data collection mode ideal for highlighting horizontal variations in
apparent soil resistivity (or apparent soil electrical conductivity).
Critical Angle
Typically refers to the incident angle (referenced to an imaginary line perpendicular
to the interface) for a seismic wave that upon encountering a subsurface interface produces
a refracted seismic wave (or head wave) that travels along the interface.
Crop Yield
The amount of crop production per unit farm field area. The spatial variation in crop
yield across a field is measured using devices called yield monitors.
Degree of Saturation
The fraction of the soil or rock porosity that is filled with water.
Density
The mass per unit volume of a material.
Dielectric Constant
(Also called
relative permittivity.
) A dimensionless constant indicative of a
material's ability to store charge in the presence of an electric field, or stated in a different
manner, a material's ability to become polarized within an electric field (e.g., the alignment
of polar molecules with the vector orientation of the electric field). Dielectric constant values
for Earth materials typically range from 1 to 80, with 1 being the value for air and 80 being
the value for water. The dielectric constant is an important material property for geophysics,
especially with regard to ground-penetrating radar methods. The velocity,
v
m
, with which a
radar wave travels through a particular soil or rock Earth material is governed by the dielec-
tric constant of that material and can be calculated with the following equation:
v
v
=
0
ε
m
r
where
v
0
is the speed of light in air, and ε
r
is the material's dielectric constant (or relative
permittivity). Given a radar wave normally incident at a subsurface interface between two
soil or rock layers, the reflected and transmitted radar wave amplitude or energy is likewise
controlled by dielectric constant values, in this case, those for the materials on both sides of
the interface. The equations for the radar wave reflection coefficient,
R
, and the transmission
coefficient,
T
(see Glossary definitions) are provided below, and as shown, these equations
are solely functions of dielectric constant.
ε
−
+
ε
r
2
r
1
R
=
ε
ε
r
2
r
1
and
