Editing Seismic refraction

{{Short description|Geophysical principle}}
[[File:Refracted wave.png|thumb|Propagating seismic waves (bottom) and related travel time diagram (top) of the direct (blue) and the first refracted phase (green)]]
'''Seismic refraction''' is a geophysical principle governed by [[Snell's Law]] of [[refraction]]. The seismic refraction method utilizes the refraction of seismic waves by rock or soil layers to characterize the subsurface geologic conditions and [[Structural geology|geologic structure]].

Seismic refraction is exploited in [[engineering geology]], [[geotechnical engineering]] and [[exploration geophysics]]. [[Seismic refraction traverse]]s ([[seismic lines]]) are performed using an array of [[seismograph]]s or [[geophone]]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 [[Body wave (seismology)|seismic waves]], 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]] 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==
[[File:refraction 2layers.png|800px|center|Two layers model.]]

i<sub>c<sub>0</sub></sub> - critical angle<br />
V<sub>0</sub> - velocity of the first layer<br />
V<sub>1</sub> - velocity of the second layer<br />
h<sub>0</sub> - thickness of the first layer<br />
T0<sub>1</sub> - intercept<br />
<br />
:<math>i_{c_{0}} = asin \left( {V_{0} \over V_{1}} \right) </math>
:<math> T = {2h_{0}cos(i_{c_{0}}) \over V_{0}} + {X \over V_{1}} = T0_{1} + {X \over V_{1}}</math>
:<math> h_{0}= {T0_{1}V_{0} \over 2cos(i_{c}) }</math>
:<math> h_{0} = {X_{cross_{1}} \over 2} \sqrt{{V_{1}-V_{0} \over V_{1} + V_{0}}}</math>

== Several horizontal layers==
<br />
:<math> h_{n} = {V_{n} \over cos(i_{n})} \left( {T0_{n+1} \over 2} - \sum_{j=0}^{n-1}{h_{j}\sqrt{{1 \over V_{j}^2} - {1 \over V_{j+1}^2}}} \right) </math>

==Inversion methods==

* The General Reciprocal method
* The [[Plus minus method]]
* Refraction inversion modeling (refraction [[tomography]])
* [[Monte Carlo method|Monte Carlo]] simulation
* [[Genetic algorithm]]s

== Applications ==
Seismic refraction has been successfully applied to [[tailings]] characterisation through P- and S-wave travel time tomographic inversions.<ref>{{Cite book|last=Alofe|first=Emmanuel|url=http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-452482|title=Reflection Seismic Survey for Characterising Historical Tailings and Deep Targeting at the Blötberget Mine, Central Sweden|date=2021}}</ref>

==See also==
* [[Reflection seismology]]
* [[Seismic wide-angle reflection and refraction]]

==References==
{{Reflist}}
{{More footnotes|date=April 2009}}
{{Commons category|Seismic refraction}}
* [http://www.publications.usace.army.mil/Portals/76/Publications/EngineerManuals/EM_1110-1-1802.pdf US Army Corps of Engineers EM 1110-1-1802]
* [http://www.cflhd.gov/geoTechnical/ Central Federal Lands Highway Division]

[[Category:Exploration geophysics]]
[[Category:Geophysics]]
[[Category:Seismology]]