OBSERVATIONS OF NONLINEAR ELASTIC-WAVE BEHAVIOR IN SANDSTONE

An experimental investigation of nonlinear elastic wave behavior was c onducted using a 2-m-long cylindrical rod of Berea sandstone in order to study the strong elastic nonlinearity that is characteristic of mic rocracked materials. Measurements of the displacement field at distanc e x from the source show rich harmonic content with harmonic amplitude s depending on x, source frequency, and source amplitude. The amplitud e of the 2omega harmonic is found to grow linearly with x and as the s quare of both the source frequency omega and the source amplitude U. T his behavior is in agreement with the predictions of nonlinear elastic ity theory for a system with cubic anharmonicity. From the measured am plitude of the 2omega harmonic the parameter \beta\, a measure of the strength of the cubic anharmonicity, is found to be of order 10(4) (7. 0 X 10(3) +/- 25%). This value is orders of magnitude greater than tha t found in ordinary uncracked materials. These results suggest that wa ve distortion effects due to nonlinear elasticity can be large in seis mic wave propagation and significantly influence the relationship of s eismic signal to seismic source.