Observations of nonlinearity with slow dynamics in rocks

Abstract

A typical resonance curve—measured acceleration versus drive frequency—made on a thin bar of rock shows peak bending with a softening (nonlinear) modulus as drive levels are increased. Previous work showed the shapes of these nonlinear resonance curves depend on sweep rate, i.e., the ‘‘slow dynamics.’’ Slow dynamics in a 0.3-m-long, 50-mm-diam bar of Berea sandstone under ambient conditions have been documented for the first time. Peak strain levels during the experiments ranged from 10−11 to 10−5 at a fundamental bar resonance frequency near 4 kHz. Slow dynamics begin to appear at strain amplitudes above 10−6 at ambient conditions and at the onset of nonlinear peak bending. Higher strains condition the rock, altering its response for minutes to hours after the drive has been turned off. Other rocks show similar results. Physical origins of the slow dynamics lie in nonlinear effects at the microstructural level of cracks, pores, and interstitial clays. Further work examines environmental effects on conditioning and recovery as a means of relating them to physical properties and microtexture of the rock. [Work supported by OBES/DOE through the University of California.]