Anomalous P-Wave and S-Wave velocities in a blocky natural sandstone specimen

Abstract

The article studies velocities of P-waves and S-waves in a natural sandstone specimen exposed to axial thrust. The cylindrical core specimen 1 m long and 0.08 m in diameter is composed of different size blocks. Anomalous velocities of the P-waves and S-waves (4-6 kHz) have been revealed by measurements taken in the whole specimen, in its separate blocks and at contacts surfaces between the blocks. The velocities in the whole specimen are much smaller than in its separate blocks. The interblock contacts detain and decelerate the waves. With smaller spacing at the contacts between the blocks, the wave velocities linearly drop to anomalously low vales (hundreds m/s). With higher pressure on the specimen, the wave velocities nonlinearly grow and the time delay reduces. The curve V P / V S -pressure has a peak (1 MPa) that appears for separate blocks and for the whole specimen. The anomalous velocities V P and V S are alleged to appear due to inelastic processes at microcontact surfaces under acoustic wave propagation in the blocky medium.