Experimental results on the combined effects of frequency, pressure and pore fluid on the dispersion of elastic waves in porous rock

When cracks and pores are connected within a rock, stress can induce fluid flow from one inclusion to another one. Because cracks are more compliant than equant pores, a stress wave builds up a higher fluid pressure within a crack than within a pore and, consequently, fluid flows from crack to pore at a local scale. At low frequencies, fluid pressure has time to reach equilibrium. The elastic moduli are in that case relaxed moduli, those moduli are defined within the framework of poroelastic theory (drained or undrained moduli). Such a situation can be expected at seismic frequencies. However, in laboratory conditions, most of elastic waves data are obtained in the MHz range. Clearly, in such a high frequency range, there is no time for fluid pressure equilibrium to take place (even at local scale). The elastic moduli are unrelaxed moduli in that case. They are not those accounted for by poroelasticity, but they may be calculated from effective medium theory.