Monitoring the state of microfracturing in rock salt during deformation by combined measurements of permeability and P- and S- wave velocities

Rock salt formations are prime candidates for underground cavities or radioactive waste disposal sites, primarily because of their extremely low permeabilities. Combined gas-permeability and P- and S-wave velocity measurements were carried out on natural rock salt samples in order to investigate the transport properties of rock salt under mechanical stresses. Experiments were done at temperatures up to 60 °C under conditions of hydrostatic compaction and triaxial compressive and extensional strain. The crack-sensitivity of P- and S-wave velocities is used for monitoring the in situ state of the microcracking during deformation. Triaxial deformation of the compacted rock salt samples is accompanied by the onset of dilatancy, that is, the opening of microcracks. The orientation of cracks is controlled by the symmetry of the applied stress field (compressive or extensional). Cracks are mostly oriented parallel to the maximum principal stress direction leading to an anisotropic crack array within the samples. A marked permeability increase is observed under compressive strain because in this case an interconnecting permeability network is generated parallel to the deformation and measuring axis. The inversions of P- and S-wave velocities are used to define the boundary between the dilatant and compressive domains (dilatancy boundary). The results confirm the equation for the dilatancy boundary given by Cristescu & Hunsche (1998).