  Seismic attenuation in granular porous media is of paramount importance in
rock physics and seismology. Unlike sandstones, shales are mixtures of sand
grains and clays with extremely low porosity and permeability. Swelling of
clays upon wetting induce micro-cracks at grain-clay interfaces and results in
the strong elastic wave scattering. Such scattering prevents adequate
measurements of the absorption from ballistic wave attenuations. Here we infer
this intrinsic attenuation from multiply scattered waves as in seismology and
ultrasonics. We find that increasing confining pressure reduces the scattering
attenuation by micro-crack closure but increases surprisingly the absorption,
likely due to the viscous dissipation involved with more liquids adsorbed in
clays and at grain surfaces. Also, we observe that cyclic heating and cooling
causes the shrinkage of clays and the growth of microcracks as well as the
nucleation of macro-fractures. This leads to a predominant chaotic
reverberation in this fractured shale. Numerical simulations based on X-ray
tomography of the fractured sample confirm the multiple scattering behavior and
reveal the increase of a characteristic length from an initial intact to a
finally fractured shale. This study helps to improve acoustic techniques for
multiscale exploration of gas and oil in shales and other fractured rocks.
