Comparison of stress-associated coda attenuation and intrinsic attenuation from ultrasonic measurements

Abstract

S U M M A R YSeismic wave attenuation has been proved to be an indicator of stress changes in rocks. Seismic coda, as a superposition of incoherent scattered waves, is known to reflect small-scale random heterogeneities in the Earth medium. It contains information on stress changes of the Earth's interior, as a result of changes in the physical state of materials. In this paper, we measure ultrasonic properties of rocks under different effective stresses to study the effect of pore-pressure induced stress changes on coda attenuation as a combination of intrinsic attenuation and scattering attenuation. We investigate the stress-associated coda attenuation quality factors Q PC and Q SC as a function of frequencies and characterize its scale dependence on stress variations in rocks by comparing with the intrinsic attenuation quality factors Q P and Q S , calculated from ultrasonic measurements. Comparisons of the P-and S-coda attenuations versus frequencies under different effective stresses demonstrate that the scattering of the S-coda is much stronger because of its shorter wavelength. The intrinsic and coda attenuations versus stress variations present quite different non-linear features, where Q P , Q S, Q PC and Q SC increase with increasing effective stress, but Q PC and Q SC show a greater sensitivity to pore pressure than Q P and Q S .

Nonlinear Acoustic Waves in Fluid-Saturated Porous Rocks – Poro-Acoustoelasticity Theory

“…We perform four numerical tests on this virtual rock sample with different considerations: openpore jacketed sample under hydrostatic loading, open-pore jacketed sample under uniaxial loading, closed-pore jacketed sample under hydrostatic loading and closed-pore jacketed sample under uniaxial loading. Numerical results show that fast P wave's inverse quality factor is both sensitive to the confining pressure and wave centre frequency, which agrees with the laboratory observations of our former studies [47] in the aspect that P wave's inverse quality factors are more sensitive to pressure than S wave's. For the virtual rock sample, velocities of fast P waves and S waves seem to be more sensitive to pressure change than to changes in wave centre frequency, while slow P wave shows an opposite feature.…”

“…As has been shown in figure 1b and figure 3b, the predicted attenuation increases with confining pressure in average, which disagrees with the experimental results in [47]. Local fluid flow, which dominates the wave attenuation and dissipation phenomena in the rocks under low confining pressure, is depressed since the flat throats and soft pores tend to close under higher confining pressure, so that the attenuation in actual experiment generally decreases with effective stress.…”

Experimental identification of the transition from elasticity to inelasticity from ultrasonic attenuation analyses

“…Open microcracks present prior to the experiment cause relatively high attenuation compared with a medium without any microcracks. Progressive closure of microcracks causes a decrease in attenuation up to a level at which a further increase in stress results in no additional attenuation reduction of the ultrasonic waves (Guo et al, 2009;Zhubayev et al, 2016). The stress-dependent attenuation reported in these studies occurs within the elastic deformation field, i.e., below the yield stress levels, and thus no new cracks/microfractures have been formed in these experiments.…”

“…For S-waves traveling through the Whitby shale sample (Figure 4d), the attenuation (Q −1 ) decreases (the wave amplitude increases) in the elastic part of the deformation. This is commonly observed in stressed samples (e.g., Mavko et al, 1995;Guo et al, 2009;Zhubayev et al, 2016), and it is generally attributed to the closure of microcracks. At the transition to inelastic deformation, attenuation is increasing again and continues to increase up to peak stress conditions (the wave amplitude decreases).…”

“…Such experimental studies generally show a decrease in attenuation with increasing stress applied to the rock samples (Toksoz et al, 1979;Winkler and Nur, 1979;Guo et al, 2009;Wei and Fu, 2014;Zhubayev et al, 2016). This attenuation decrease is generally attributed to pore or crack closure (Winkler and Nur, 1979).…”