James A. TenCate, Koen E-A Van Den Abeele,a Thomas J. Shankland, and Paul Johnsonb
Earth and Environmental Sciences Division, Mail Stop D443, Los Alamos National Laboratory, Los Alamos, New Mexico 87545
a)also at K.U. Leuven Campus Kortrijk, Belgium, as Post-doctoral Fellow of the Belgian National Foundation for Scientific Research
b)also at Department de Recherches Physiques, Universite Pierre et Marie Curie (Paris 6), Tour 22-4, Place Jussieu, 75252 Paris Ceder 05, France
We performed linear and nonlinear elastic wave pulse propagation experiments in sandstone rods, both at ambient conditions and in vacuum. The purpose of these experiments was to obtain a quantitative measure of the extremely large nonlinear response found in microcracked (i.e., micro-inhomogeneous) media like rock. Two rods were used, (1)a 2 m long 5 cm diameter rod of Berea sandstone (with embedded detectors) used in previously published experiments and (2) a some what smaller 1.8 m long 3.8 cm diameter rod. In the earlier experiments, wave scattering from the embedded detectors was a critical problem. In most of the experiments reported here, this problem was avoided by mounting accelerometers directly to the outside surface of the smallest rod. Linear experimental results show out of vacuum attenuations varied from 1.7 Np/mat 15 kHz (Q=10) for the large rod to 0.4 Np/mat 15 kHz (Q=55) for the small rod; attenuations for the small rod in vacuum were much less, typically about 0.15 Np/mat 15 kHz (Q=150). Wave velocities ranged from 1900 m/s to 2600 m/s. The nonlinear results illustrate growth of the second and third harmonics and accompanying decay of the fundamental. These nonlinear results compare well with a numerical model. Although the results here were performed at peak strain amplitudes as low as 5 * 10-7, they still show the pronounced nonlinearity characteristic of rock, in agreement with static and resonance studies using the same rock type.