Spectral acoustic techniques for joint and fracture characterization

Abstract

An ultrasonic pulse transmission method was used on a large block of gabbro (18 ´ 18 ´ 30 cm), at low normal stresses (0.01?15 MPa) and low frequencies (0.02?1 Mhz) in order to determine the transmission coefficient of a simulated fracture. The contact stiffness was measured directly and spectral ratios of the received signals calculated for comparison with predictions from the displacement discontinuity model. The agreement between the measured and derived spectral ratios was significantly better for the rough surface, indicating that the model provides a better approximation for natural fractures than for smooth interfaces. The model predictions also improved at lower frequencies, suggesting that spectral analysis of seismic data might yield useful information about the presence and stress state of fractures in the field.