Rocks, dead or alive? Theory prompts new understanding of fluid-rock interaction and shear-wave splitting



For over 40 years, our understanding of fluid-rock interactions in porous rocks has been largely based on the theory of Biot [1956] for the behavior of liquid-filled pores in permeable porous rocks, where Biot assumes that the pore geometry only changes elastically. Such rocks are “dead” in the sense that their only response to changing conditions is elastic oscillations or catastrophic failure. One obvious manifestation that rock is not like this is the pervasive splitting of seismic shear waves in almost all rocks, where the polarization of the faster split shear wave is typically within ±20° of the direction of maximum horizontal compressional stress.