INFLUENCE OF CHANGE IN PHYSICAL STATE ON ELASTIC NONLINEAR RESPONSE IN ROCK - SIGNIFICANCE OF EFFECTIVE PRESSURE AND WATER SATURATION

We describe Young's mode resonant bar results obtained under effective pressure at two saturation states: dry and water saturated. We monito r primary manifestations of nonlinear response in these experiments: t he harmonic content, the source extinction intensity, and fundamental resonant frequency shift. In addition, we describe the hysteretic beha vior of the static pressure response, the linear modulus, and Q. Becau se we currently lack a complete theoretical description Of nonlinear b ehavior under resonance at pressure, we provide relative measures of n onlinear response rather than absolute values. The rocks include Fonta inebleau and Meule sandstones and Lavoux limestone. Dynamic strain lev els range from 10(-8) to 10(-5) and frequencies range from 1 to 10 kHz . The elastic nonlinear response of each of the rocks is markedly diff erent over the range of physical property states explored. The differe nt responses are related to differences in mechanical response resulti ng from rock type, grain cement type, etc. In all of the samples studi ed, the change in resonant frequency as a function of excitation inten sity is not measurable above approximately 10 MPa; however, harmonics are observed at larger effective pressure levels. Hysteresis in veloci ty and Q versus pressure vary considerably between the rocks. The effe ct of Q on the experiments is marked. When Q is low (<10) as for some saturated samples, relative excitations must be large in order to indu ce equivalent dry sample strains.