Geoscience Reference
In-Depth Information
tion, tillage effort, and so forth. Geospatial data sets can include soil property maps derived
from geophysical surveys and crop production maps generated by yield monitors. The ben-
efits of precision agriculture to farmers are maximized crop yields and reduced input costs.
Precision agriculture can additionally provide important environmental benefits, because
with optimal amounts of fertilizer, soil amendments, pesticides, irrigation, and tillage effort
applied on different parts of the field, there are potentially less agrochemicals and sediment
released offsite via subsurface drainage and surface runoff. Reducing the offsite discharge
of agrochemicals and sediment, in turn, diminishes adverse environmental impacts on local
waterways. In essence, precision agriculture techniques allow a farm field to be divided into
different management zones for the overall purpose of optimizing economic benefits and
environmental protection.
P-Wave (Also called a primary wave , a compressional wave , or a longitudinal wave. ) A type of
seismic body wave having an elastic back-and-forth particle motion orientation that coin-
cides with the direction of wave propagation. P-waves can be transmitted through solid, liq-
uid, and gas materials. P-waves are the fastest seismic waves, and their velocity, V P , within
a soil or rock material is given by the following equation:
4
3 µ
k
+
V
P =
ρ
where k is the bulk modulus, μ is the rigidity modulus (or shear modulus), and ρ is density.
As indicated by the equation directly above, the P-wave velocity in soil or rock depends only
on elastic moduli and density of these Earth materials.
Rayleigh Wave A type of seismic wave that travels along the surface. Rayleigh wave particle motion
is elliptical retrograde in a vertical plane oriented coincident with the direction of wave prop-
agation. The amplitude of a Rayleigh wave decreases exponentially with depth.
Relection Coeficient Given a subsurface interface separating two materials having different prop-
erties, the reflection coefficient is the ratio of the reflected radar or seismic wave amplitude at
the interface to the incident radar or seismic wave amplitude at the interface.
Remanent Magnetism Magnetism that exists in a material regardless of whether an external mag-
netic field is present.
Resistivity Methods Geophysical investigation methods used to measure subsurface electrical
resistivity (or its inverse, electrical conductivity). With the conventional resistivity method,
an electric current is supplied between two electrodes inserted at the ground surface, and
voltage is concurrently measured between a separate pair of electrodes also inserted at the
surface. The current, voltage, electrode spacing, and electrode configuration are then used
to calculate a bulk soil electrical resistivity (or conductivity) value. There are a variety of
electrode configurations, called electrode arrays, most of which are linear, with the Wenner,
Schlumberger, and dipole-dipole arrays the ones employed the majority of the time (see
Chapter 5). The development of continuous galvanic contact and capacitively coupled resis-
tivity measurement techniques in the late 1980s and early 1990s has transformed the con-
ventional resistivity method into an effective and efficient tool to assess soil conditions and
properties in large agricultural fields.
Rigidity Modulus (Also called shear modulus. ) A measured constant that defines the stress-strain
behavior of an elastic material due to the application of shear stress. The rigidity modulus
plays an important role in determining seismic wave velocities in soil and rock materials.
Runoff Water flow over the soil surface due to a rainfall or irrigation event.
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