Jan V. M. Borgomano

Dispersion and Attenuation Measurements on a Bimodal-porosity-oolitic Limestone

Dispersion and attenuation measurements have been performed on an oolitic limestone from Lavoux. This limestone comes from a Dogger outcrop, is pure calcite, and exhibits a bimodal porosity with intragranular microporosity and intergranular macroporosity. Total porosity and permeability of the sample were measured to be respectively around 23% and 10 mD. To measure the elastic-moduli dispersion and attenuation over a wide-frequency range, we used two stress-strain methods: hydrostatic and axial oscillations. The triaxial cell used allows for low-frequency measurements (0.004 Hz to 100 Hz), and ultrasonic-velocity measurements at high frequency (1 MHz). The sample was measured under three-different-saturating conditions: dry, water and glycerine. The results show no dependence to effective pressure, which is related to the absence of cracks in the limestone. Moreover, the transition from drained to undrained regimes has been successfully measured under glycerine-saturated conditions (around 1 Hz). It corresponds to a strong dispersion (~50%). The undrained-elastic moduli are consistent with Biot-Gassmann’s theory. A weak dispersion is observed (< 12%) between the undrained-elastic moduli and the ultrasonic results that remains to be further investigated.