Giant strain-sensitivity of local acoustic dissipation near inner wavy contacts in dry and fluid-saturated cracks

Abstract

Presently, experimental evidence for extremely high strain-sensitivity of dissipation in rocks and similar microstructured materials is obtained both in laboratory and field conditions, in particular observations of pronounced amplitude modulation of the radiation of high-stability seismo-acoustic sources by tidal deformations of rocks with typical strains ∼ 10 −8 . Such data indicate the presence of some thresholdless in amplitude and very efficient mechanism of strain-dependent dissipation. Conventionally, its origin is discussed in the context of frictional or adhesion-hysteretic loss at cracks in rocks. However, such dissipation mechanisms are not relevant to weak perturbations with displacements smaller than atomic size. Here, we revise thresholdless thermoelastic loss in dry cracks and viscous loss in saturated cracks taking into account wavy asperities typical of real cracks, which can create elongated (strip-like) contacts or almost closed "waists" in cracks. Thermoelastic loss at these contacts can be very efficient. Besides, the state of such contacts can already be strongly perturbed by the average strain which yet practically does not change the mean opening of the entire crack. Thus the dissipation localized at such contacts can be significantly affected by quite small average strain (e.g., 10 −8 ), which is usually believed to be unable to produce any appreciable effect on the dissipation. Next, for liquid-saturated cracks, the presence of inner elongated asperities also drastically changes the character of squirt-type viscous dissipation. Velocity gradients and consequently the dissipation are localized in the vicinity of the nearly-closed waists which almost harness the liquid flow in the crack. This dissipation can be comparable in magnitude with viscous dissipation at the entire crack with smooth interface, but the decrement maximum is strongly shifted downwards on the frequency axis. Since near the waist the gap is much smaller than the average crack opening, this local dissipation can also exhibit giant strain sensitivity. These mechanisms of near-contact loss suggest interpretation for observations of pronounced amplitude modulation of seismo-acoustic waves by tidal strains.