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The seismic refraction test involves the measurement of travel times of p-waves from an impulse source to a linearly array of receiver points along the ground surface. The test uses the travel times of the first arriving wave component only regardless of its travel path. Depending on the source-receiver distance the first arriving component is either a direct wave or a refracted wave (refracted at the interface of two soil layers). Based on the position of the point where this qualitative change of the signal occurs the layer thickness and velocity can be calculated.
The success of the refraction test depends highly on the properties of the layer interfaces (refractors). An interface can only be found if the layer below the interface shows a significantly higher wave speed than the layer above the boundary.
If a numerical inversion algorithm is applied on the gathered arrival times the 2D p-wave velocity distribution can be obtained (refraction tomography).
The seismic reflection test follows the principle of echo-sounding. An impulse source is placed at the surface and a receiver signal is captured in some distance. The arrival of wave components reflected at different deep layer interfaces is identified and converted into a velocity-depth profile.
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