Seismic tomography on a pillar in a potash mine

Seismic tomographic measurements were performed in a salt mine on a pillar with a cross-section of approximately 14x14 m(2) in a potash seam. Signals generated by a piezoceramic ultrasonic probe were transmitted through the pillar in all directions, thus obtaining 975 different ray paths. There were used signals with center frequencies between 10 kHz and 80 kHz as well as wide band signals. The local distributions of compressional wave and shear wave velocities and of the attenuation were determined by reconstruction of the travel times and amplitudes. Both sorts of images reconstructed by tomography indicated zones of higher dilatancy in a width up to 2.5 m around the pillar surface. In the center of the pillar, a quality factor of approximately 90 could be determined.