Neutron Scattering and the Nonlinear Acoustic Properties of Rocks

Although geomaterials (rocks) have been studied by macroscopic mechanical experiments for a very long time, the complexity, inhomogeneities, multiple phases, fluid content, interfaces, and lack of transparency have defied most experiments to resolve the corresponding microscopic behavior associated with a particular macroscopic observation. The microscopic origin of nonlinearity and hysteresis (end-point memory) evident in macroscopic stress-strain curves of rocks is still unknown after some 30 years of research by many groups, not to mention more recent low strain results like slow dynamics [see, Physics Today, 52, 30-35 (1999)] or hysteretic temperature dependence [Geophys. Res. Lett., 28, 2293-2296 (2001)]. Although models of the P-M space variety have developed to a degree where some good qualitative agreement is reached, the link to the actual physical microscopic or atomic mechanism for hysteresis is still missing. Neutron diffraction is one of a few experiments that can lead to an understanding of the atomic or microscopic effects that govern the properties of complex geomaterials. Neutrons can easily penetrate rocks [Geophys. Res. Lett., 28, 2105-2108, (2001)] and reveal properties of the bulk interior material (rather than the near-surface regions measurable, say, by X-rays). We are performing experiments with rocks on two beamlines at the Lujan neutron facility at Los Alamos where the scattering data will be correlated with some of the large body of nonlinear acoustic data which exists, to determine which atomic-plane level constituents of the rocks are active in nonlinear processes. We will describe the neutron scattering and acoustic experiments we are carrying out on marble and sandstone samples and will present our first results from the LANSCE neutron facility at Los Alamos. [Work supported by Office of Basic Energy Sciences, DOE, with Los Alamos National Laboratory Institutional Support.]