Seismic refraction

Seismic refraction is a geophysical principle governed by

.

Seismic refraction is exploited in

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

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

. 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
V₀ - velocity of the first layer
V₁ - velocity of the second layer
h₀ - thickness of the first layer
T0₁ - intercept

${\displaystyle i_{c_{0}}=asin\left({V_{0} \over V_{1}}\right)}$

${\displaystyle T={2h_{0}cos(i_{c_{0}}) \over V_{0}}+{X \over V_{1}}=T0_{1}+{X \over V_{1}}}$

${\displaystyle h_{0}={T0_{1}V_{0} \over 2cos(i_{c})}}$

${\displaystyle h_{0}={X_{cross_{1}} \over 2}{\sqrt {V_{1}-V_{0} \over V_{1}+V_{0}}}}$

Several horizontal layers

${\displaystyle h_{n}={V_{n} \over cos(i_{n})}\left({T0_{n+1} \over 2}-\sum _{j=0}^{n-1}{h_{j}{\sqrt {{1 \over V_{j}^{2}}-{1 \over V_{j+1}^{2}}}}}\right)}$

Inversion methods

• The General Reciprocal method
• The
  Plus minus method
• Refraction inversion modeling (refraction tomography)
• Monte Carlo simulation
• Genetic algorithms

Applications

Seismic refraction has been successfully applied to tailings characterisation through P- and S-wave travel time tomographic inversions.^[1]

See also

• Reflection seismology

References

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• US Army Corps of Engineers EM 1110-1-1802
• Central Federal Lands Highway Division