Vertical increase in seismic velocity with depth in shallow crystalline bedrock

There are numerous examples of the increase in seismic velocity with the depth of crystalline bedrock due to the lithological confining stress that compresses the open fractures in the shallow bedrock.
The resistivity logs obtained in 500–1000 m deep boreholes correspond well with the open fractures observed along the core samples down to a depth of 150–200 m. In the corresponding sonic logs, however, the seismic velocity is constant below the casing at depths greater than 40 m.

The reflection seismic soundings suggest an increase in the velocity at the very surface of the crystalline bedrock. Reflections from shallow depths of less than 100 m show higher velocities than the direct refracted P-wave at the outcropped surface. In the full waveform sonic log there is an increase in the P-wave velocity within the uppermost 15–20 m.
Very slow tube waves are also typical for this depth interval. The change in mechanical rock properties is also verified in laboratory tests.