Seismic refraction is a geophysical principle governed by
Snell's Law of
refraction
In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenome ...
. The seismic refraction method utilizes the refraction of seismic waves by rock or soil layers to characterize the subsurface geologic conditions and
geologic structure.
Seismic refraction is exploited in
engineering geology
Engineering geology is the application of geology to engineering study for the purpose of assuring that the geological factors regarding the location, design, construction, operation and maintenance of engineering works are recognized and accou ...
,
geotechnical engineering and
exploration geophysics
Exploration geophysics is an applied branch of geophysics and economic geology, which uses physical methods, such as seismic, gravitational, magnetic, electrical and electromagnetic at the surface of the Earth to measure the physical properties of ...
.
Seismic refraction traverses (
seismic lines) are performed using an array of
seismographs or
geophone
A geophone is a device that converts ground movement (velocity) into voltage, which may be recorded at a recording station. The deviation of this measured voltage from the base line is called the seismic response and is analyzed for structure of ...
s and an energy source.
The methods depend on the fact that seismic waves have differing velocities in different types of soil or rock. The waves are refracted when they cross the boundary between different types (or conditions) of soil or rock. The methods enable the general soil types and the approximate depth to strata boundaries, or to
bedrock, to be determined.
P-wave refraction
P-wave refraction evaluates the
compression wave generated by the seismic source located at a known distance from the array. The wave is generated by vertically striking a striker plate with a
sledgehammer, shooting a seismic shotgun into the ground, or detonating an explosive charge in the ground. Since the compression wave is the fastest of the
seismic waves
A seismic wave is a wave of acoustic energy that travels through the Earth. It can result from an earthquake, volcanic eruption, magma movement, a large landslide, and a large man-made explosion that produces low-frequency acoustic energy. ...
, it is sometimes referred to as the
primary wave and is usually more-readily identifiable within the seismic recording as compared to the other seismic waves.
S-wave refraction
S-wave refraction evaluates the
shear wave
__NOTOC__
In seismology and other areas involving elastic waves, S waves, secondary waves, or shear waves (sometimes called elastic S waves) are a type of elastic wave and are one of the two main types of elastic body waves, so named because th ...
generated by the seismic source located at a known distance from the array. The wave is generated by horizontally striking an object on the ground surface to induce the shear wave. Since the shear wave is the second fastest wave, it is sometimes referred to as the
secondary wave. When compared to the compression wave, the shear wave is approximately one-half (but may vary significantly from this estimate) the velocity depending on the medium.
Two horizontal layers
i
c0 - critical angle
V
0 - velocity of the first layer
V
1 - velocity of the second layer
h
0 - thickness of the first layer
T0
1 - intercept
:
:
:
:
Several horizontal layers
:
Inversion methods
* The General Reciprocal method
* The
Plus minus method
* Refraction inversion modeling (refraction
tomography
Tomography is imaging by sections or sectioning that uses any kind of penetrating wave. The method is used in radiology, archaeology, biology, atmospheric science, geophysics, oceanography, plasma physics, materials science, astrophysics, ...
)
*
Monte Carlo
Monte Carlo (; ; french: Monte-Carlo , or colloquially ''Monte-Carl'' ; lij, Munte Carlu ; ) is officially an administrative area of the Principality of Monaco, specifically the ward of Monte Carlo/Spélugues, where the Monte Carlo Casino is ...
simulation
*
Genetic algorithms
Applications
Seismic refraction has been successfully applied to
tailings
In mining, tailings are the materials left over after the process of separating the valuable fraction from the uneconomic fraction (gangue) of an ore. Tailings are different to overburden, which is the waste rock or other material that overli ...
characterisation through P- and S-wave travel time tomographic inversions.
See also
*
Reflection seismology
References
{{Commons category, Seismic refraction
US Army Corps of Engineers EM 1110-1-1802Central Federal Lands Highway Division
Exploration geophysics
Geophysics
Seismology