AN EXPERIMENTAL STUDY OF THE VARIATION OF P-WAVE VELOCITY IN A MARBLE SLAB LOADED TO FAILURE AT THE CENTER

The P-wave travel time variation in a dry marble slab of size 60060020 mm loaded at its center to failure has been experimentally studied, the results of which are described in this paper.For the wave paths crossing the forthcoming fracture and the portion nearby the center of loading, the travel time increases first and then decreases as the loading increases, while for the portion farther away from the center, the travel time merely increases. For those wave paths nearby and parallel to the fracture, the travel time remains unchanged. However, for the wave paths not crossing the fracture and passing the center of loading, the travel time remains unchanged if the deformation is small in the vicinity of the wave emmitting source but generally decreases if the deformation there is large. For those paths passing through the center of loading, some of the travel times increase first and then decrease while others decrease first and then increase. These results may indicate that before fracturing there exist positive and negative anomalous and normal regions of P-wave velocity around the fracture during the process of its development.Meanwhile, it is also observed that the regions of negative anomalous P-wave velocity are enlarged by steps as the central loading is increased. The regions of normal P-wave velocity as observed from the wave emmitting source located on one side of the fracture is usually the same region of negative anomalous P-wave velocity as obtained from a wave emmitting source located on the other side.