Near-surface gradients of rock quality, deformation modulus, Vp and Qp to 1 km depth

In hard rock areas, the uppermost 50 m of the ground may consist of soil, weathered jointed rock, and increasingly sound, more massive rock as depth increases. From experience with seismic refraction work, it is well known that there are extreme seismic velocity gradients in this zone. This is so even if we discount the step increase in P-wave velocity, Vp, at the water table. There are many reasons for the rapid increases in velocity with depth. These include increased stresses, increased rock quality because less weathering has occurred, fewer open joints, less clay, and usually a reduced frequency of jointing.