### ASA 130th Meeting - St. Louis, MO - 1995 Nov 27 .. Dec 01

## 2aPA5. Nonlinear vibroacoustic properties of rocks.

**Lev A. Ostrovsky
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*Univ. of Colorado, CIRES/NOAA Environ. Technol. Lab., 325 Broadway,
Boulder, CO 80303
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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 10[sup
3]--10[sup 4] 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.