The propagation properties of sound waves in rocks are an important aid for interpreting the rock behaviour. If suitable frequencies are used, the sound waves react in a very sensitive manner to loosening, hollow spaces and cracks in the ultrasonically tested material and are thus used for crack detection or for the depiction of the development of micro-cracks under changing stress conditions. In the rock laboratory Gesteinslabor Dr. Eberhard Jahns, the velocities of compression waves and shear waves (transversal waves) can be determined in a laboratory experiment.

Fig.: This figure shows the velocities determined in 18 different directions (P- and S-waves) of a sandstone from a depth of 4139 m under a stage-wise increased hydrostatic confining pressure up to 200 MPa (about 29,000 PSI). The diagram unifies the results of measurements on six neighbouring cube-shaped specimens, which are removed from the drilling core in different orientations.

The results are of particular value when they are simultaneously determined e.g. during a tension deformation experiment, and thus, an exact assignment to the relevant stress condition is possible (example). The direction-dependent determination of ultrasonic running times under increasing ambient pressure additionally allows conclusions to be drawn about the numerous storage location-relevant parameters such as e.g. the Poisson′s ratio, the modulus of elasticity, pore pressure effectiveness as well as the in situ rock stresses.