Effects of humidity and temperature on the non-linear elasticity of rocks
Xuan Feng, Michael Fehler, Daniel R. Burns, Stephen Brown, Thomas L. Szabo
Abstract
SUMMARY Small velocity changes, associated with the non-linear response of rocks within the Earth, have been measured from dense arrays of seismometers to observe crustal deformation, the influence of solar heating and in situ pore pressure changes. Characterization of the non-linear elastic response of rocks provides new insight into rock microstructure and possibly new ways to determine the presence and type of fluid in the pore space. We conducted a long-term experiment to understand the influence of humidity and temperature on the non-linear elastic properties (NEPs) of rocks. The relative velocity change (dv/v), a NEP, is computed from the time of flight delay caused by the presence of a superposed high amplitude ultrasonic signal in a sandstone sample within a controlled environmental chamber. Our phenomenological model for diffusion of humidity into the pore space fits the dv/v data exceptionally well, showing the dv/v is a linear function of humidity. We subtract the dv/v predicted by the humidity diffusion model from the measured dv/v to get a residual dv/v. This residual dv/v can be well fit by a temperature diffusion model in which the dv/v is a linear function of temperature. We suggest that dv/v is a sensitive indicator of moisture in pore spaces of rock and the temperature of the rock.