Résumé
Numerous experimental studies show significant differences between the static and dynamic elastic moduli of rocks. These differences are commonly attributed to the presence of cracks and uncemented grain contacts that make rocks non-linear elastic materials with a sensitivity to strain amplitude. To investigate the impact of strain amplitude on the bulk modulus of different rock types, we performed hydrostatic oscillations to induce strain amplitudes from 10–6 to 10– 4 on the samples at frequencies ≤ 0.04 Hz. In addition, we measured the dynamic bulk modulus from ultrasonic wave velocities and the static bulk modulus from hydrostatic pressure ramps. In five out of the six samples, we observe a transition from the dynamic to static modulus with increasing strain amplitude. With increasing effective pressure, the bulk moduli increase, and the strain amplitude dependence is reduced. In the oolitic Chauvingy limestone, in contrast, we observed neither a significant strain nor effective pressure dependence on the bulk modulus, which is interpreted as the absence of pre-existing cracks. In the reservoir carbonate samples, for strain amplitudes > 5 × 10–5 the stress–strain loops become non-linear hysteretic and the measured attenuation plateaus. The presence of additional harmonics in the strain and the absence of harmonics in the stress, indicate that the non-linearity is inherent to the samples. The transition from dynamic to static bulk modulus is explained at first order by slip at grain contacts.