A refraction survey uses refracted (or head) waves to deduce velocities of the layered-earth model. So-called first arrival information is used for the analysis (Fig. 1). More generalized methods based on the turning waves from an arbitrary velocity model have also been used in recent days. This is called seismic refraction tomography. Historically the refraction method has been commonly used to map depth and velocity of bedrock.
Fig. 1. A field record showing that refraction waves constitute the first arrivals.