INFLUENCE OF FREQUENCY AND FLUID DISTRIBUTION ON ELASTIC-WAVE VELOCITIES IN PARTIALLY SATURATED LIMESTONES

Elastic waves velocities were measured in the laboratory on homogeneous limestones in three different frequency ranges with varying water saturations (S-w). Measurements at frequencies of approximately 1 kHz were obtained using large (1-m length) resonant bars. The conventional pulse transmission technique was used to measure the samples at 50 kHz and 500 kHz. In order to study the influence of the fluid distribution we used two different saturation methods: drying and depressurization. Using a computerized tomography scan to image fluid distribution, we found that during depressurization, saturation was highly homogeneous at the millimetric scale at all saturations. In contrast, during the drying, heterogeneous saturation was observed at high water saturation levels. Results from elastic wave propagation show that compressional and extensional velocities are sensitive to the saturation technique at high water saturation level. Furthermore, the dependence of velocity upon saturation technique is frequency dependent, and dispersion is greatest at high saturations. We attribute this behavior to a scale effect that varies with the size of heterogeneities of the rock and with the wavelength. In addition to global and local flow mechanisms it seems necessary to include a ''path dispersion'' effect to explain the amount of velocity dispersion observed. For shear waves the experimental results show little variation of the velocity with frequency and very little with fluid distribution.