Seismic refraction


Seismic refraction is a geophysical principle governed by Snell's Law. Used in the fields of engineering geology, geotechnical engineering and exploration geophysics, seismic refraction traverses are performed using a seismograph and/or geophone, in an array and an energy source. The seismic refraction method utilizes the refraction of seismic waves on geologic layers and rock/soil units in order to characterize the subsurface geologic conditions and geologic structure.
The methods depend on the fact that seismic waves have differing velocities in different types of soil : in addition, the waves are refracted when they cross the boundary between different types 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 (a.k.a. Compression 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, 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 (a.k.a. Shear 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 the velocity depending on the medium.

Two Horizontal Layers.

ic0 - critical angle

V0 - velocity of the first layer

V1 - velocity of the second layer

h0 - thickness of the first layer

T01 - intercept


Several Horizontal Layers.

Inversion methods