An apparatus to measure elastic dispersion and attenuation using hydrostatic- and axial-stress oscillations under undrained conditions.

ABSTRACT

An experimental apparatus is described for the investigation of frequency dispersion, and related attenuation, of fluid-saturated rocks under confining pressure and undrained boundary conditions. The forced-oscillation method is performed on cylindrical samples. The measurement of stress and strain under hydrostatic oscillations allows the dynamic bulk modulus to be inferred, while axial oscillations give access to dynamic Young's modulus and Poisson's ratio. We present calibration measurements for dispersion and attenuation on standard materials such as glass, plexiglass, and gypsum. Results show that for strain amplitudes below 10-5, robust measurements can be achieved up to 1 kHz and 1.3 Hz, respectively, for axial and hydrostatic oscillations. A new experimental design of the endplatens (sample holders) allows control of drained or undrained boundary conditions using microvalves. The microvalves were tested on a porous Vosgian sandstone. In addition, numerical modeling confirms that the resonances of the apparatus only affect frequencies above 1 kHz, with little sensitivity to the sample's stiffness.