Through transmission measurement of the nonlinear viscoelastic memory of rocks by co-propagating longitudinal ultrasonic pulses

Because conventional geophysical measurements cannot directly measure the microstructure or pore fluids of rocks, new measurement approaches are needed as well as one that can target regions of interest. The aim of this study is to utilize the nonlinear interaction of two co-propagating longitudinal waves to characterize induced viscoelastic changes occurring in the rock microstructure. Our experimental technique utilizes a strong longitudinal wave pump that changes (minutely) the viscoelastic properties of the sample with dynamically applied strain and a weaker longitudinal wave probe that senses those changes. Particle velocities are measured on the surface of a Crab Orchard Sandstone with a laser vibrometer to calibrate the experiment. Our method allows us to evaluate time dependent nonlinear effects and we find strong evidence that rocks have a short period nonlinear viscoelastic memory that is a function of the time history of the loaded strain. The material softens under extensional strain and remains soft even under compressional strain. Our empirical model shows that the short period nonlinear viscoelastic memory is controlled by both the traditional nonlinear elastic coefficients and a memory strength parameter. Our new acoustic findings may be useful for quantifying changes in microstructure and pore fluids in rocks.