where k is the bulk modulus, ^ is the rigidity modulus (or shear modulus), and p 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 propagation. The amplitude of a Rayleigh wave decreases exponentially with depth.

Reflection Coefficient Given a subsurface interface separating two materials having different properties, 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 magnetic 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 resistivity measurement techniques in the late 1980s and early 1990s has transformed the conventional 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.

Salinity The total soluble salt (dissolved electrolyte) concentration in a soil. High salinity levels in soil can adversely affect plant growth and land use. Salinity is typically determined by the electrical conductivity measurement of a solution extracted from a saturated soil paste.

Sand Soil particles with an equivalent diameter ranging from 0.05 to 2.0 mm.

Self-Potential Methods From an operational standpoint, self-potential methods are probably the simplest geophysical methods, requiring only the measurement of a naturally occurring electric potential difference between two locations on the ground surface. Essentially, a voltmeter is used to measure the electric potential difference between two nonpolarizing electrodes placed at the ground surface that are connected to the voltmeter via electric cable. Naturally occurring electric potential differences in the subsurface are often caused by electrokinetic, electrochemical, or ore body oxidation and reduction processes.

Seismic Adsorption Coefficient A constant value used in the calculation of seismic wave attenuation that accounts for elastic energy loss due to frictional dissipation.

Seismic Methods Geophysical subsurface investigation methods employing explosive, impact, vibratory, or acoustic energy sources to introduce elastic (or seismic) waves into the ground. The seismic waves are timed as they travel via direct, reflected, or refracted paths through the subsurface from the source to the sensors, which are called geophones. Information on the timed arrivals and amplitudes of the seismic waves measured by the geophones is then used to determine below-ground conditions or to locate and characterize subsurface features.

Seismic Waves Elastic vibrations capable of propagating through Earth materials such as soil and rock.

Silt Soil particles with an equivalent diameter ranging from 0.002 to 0.05 mm.

Site-Specific Management The use of spatially governed agricultural practices to manage crops, soil, pests, and so forth, based on information obtained regarding variable conditions within a farm field.

Sodium Adsorption Ratio (SAR) A quantity used as an indicator of the exchangeable sodium percentage (ESP), which is the percentage of the soil cation exchange capacity occupied by sodium ions (Na+). Both SAR and ESP are used to gauge soil sodicity that reflects not only the amount of sodium present but also its impact on plant growth. The value of SAR is commonly determined from the calcium ion (Ca2+), magnesium ion (Mg2+), and sodium ion (Na+) concentrations in a solution extracted from a saturated soil paste, and it is calculated using the following equation:


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