Experimental Study of Dislocation Damping Using a Rock Analogue

Abstract

In order to explore the effects of dislocations on seismic velocity and attenuation, we conducted a series of forced oscillation and ultrasonic tests on rock analogue samples (polycrystalline borneol) that were predeformed under various differential stress Δσ. Additionally, creep experiments were conducted to determine the steady-state flow law for borneol. The dominant deformation mechanism of polycrystalline borneol changes from diffusion to dislocation creep at about Δσ=2 MPa. At high stresses, power law creep with a stress exponent of ∼4 was measured. Microstructure of the deformed samples showed wavy grain boundaries due to dislocation-induced migration and the occasional existence of microcracks. A borneol sample deformed in the dislocation creep regime showed a significant reduction in Young's modulus E and a slight increase in attenuation Q−1 at frequencies lower than 100 Hz, whereas E at ultrasonic frequency (106 Hz) did not reduce. Therefore, a major part of the dislocation creep-induced anelastic relaxation is a peak with a characteristic frequency between 100 and 106 Hz, which is much higher than the range of grain boundary-induced anelasticity of this material. Further experiments under higher confining pressure are needed to assess the relative contribution from dislocations and microcracks to this peak.