Absorption and modulus measurements in the seismic frequency and strain range on partially saturated sedimentary rocks

The absorptions 1/ Q E and 1/ Q S of partially water‐saturated sedimentary rocks were determined from phase differences between stress and strain for longitudinal deformation frequencies between 0.03 and 300 Hz and torsional frequencies between 0.03 and 100 Hz. Both longitudinal and shear strain amplitudes were of the order of 10 −6 . For water saturation between 0 and 50%, Young's moduli and shear moduli were shown to decrease with increasing saturation. In this saturation range both 1/ Q E and 1/ Q S increased, but no distinguishable absorption maxima were observed. For saturation percentages greater than 50%, the moduli of the samples appeared independent of the water content. 1/ Q E increased until full saturation and showed a strong frequency dependence. 1/ Q S is much less affected by increasing water saturation and has no absorption maxima in general. Since the absorption and the moduli reduction show different saturation dependencies, two different mechanisms for these effects are proposed. The addition of water changes the interaction force between the molecules on opposite walls of thin cracks. This provides a mechanism for the modulus reduction. Calculations based on the Biot‐Gardner theory (Gardner, 1962) indicate that part of the observed absorption may be caused by fluid flow due to the limited sample size as proposed by White (1986). Fluid‐supported thermorelaxation is proposed, as a possible intrinsic absorption mechanism.