Seismic refraction

Seismic studies the propagation of seismic waves in the ground. Seismic waves produced by impact or vibration, propagate in the soil and, when they meet geological

layers characterized by different acoustic impedance, give rise to secondary seismic waves that are recorded on the surface. In measurements, the first arrivals (firstbreak) of longitudinal waves to the receivers will be represented, up to a certain distance, by the arrival of the direct waves (which propagate along the surfaceof the ground), while beyond a certain "critical " distance, refracted waves will be recorded first, as they propagate with the underlying layer speed (it is assumed a layered medium with materials characterized by propagation velocity increasing with depth). 

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