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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 .

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