Nonlinear vibroacoustic properties of rocks

This is a summary of recent theoretical and experimental work performed by the Nonlinear Acoustic Group (L. Ostrovsky, A. Sutin, V. Nazarov, I. Belyaeva, Yu. Zaitsev) at the Institute of Applied Physics, N. Novgorod, Russia. Experiments show that Earth materials can have anomalously strong mechanical nonlinearity. However, in many cases there is a lack of clear understanding of specific mechanisms responsible for these nonlinear effects. Some theoretical models of structurally inhomogeneous media have been constructed and experimentally verified. These models include: (1) porous waterlike media characterized by small (but finite) shear modulus; (2) grainy media where nonlinearity is caused by the intergrain contacts (Hertz model); (3) some nonlinear models of cracks. Laboratory experiments have demonstrated a good agreement with theoretical results, and in all three cases the values of the nonlinearity parameter (defined similarly to that used in nonlinear acoustics for gases and liquids) could achieve the values of the order of 103–104 and even more. Some estimates for real rocks are also given. They show that nonlinear parameters may prove to be much more sensitive to the details of the material structure than the usual linear ones (e.g., sound velocity). This can serve as a base for developing new methods of seismic surveillance.