ABSORPTION OF STRAIN WAVES IN POROUS-MEDIA AT SEISMIC FREQUENCIES

An understanding of strain wave propagation in fluid containing porous rocks is important in reservoir geophysics and in the monitoring in u nderground water in the vicinity of nuclear and toxic waste sites, ear thquake prediction, etc. Both experimental and theoretical research ar e far from providing a complete explanation of dissipation mechanisms, especially the observation of an unexpectedly strong dependence of at tenuation Q(-1) on the chemistry of the solid and liquid phases involv ed. Traditional theories of poroelasticity do not take these effects i nto account. In this paper the bulk of existing experimental data and theoretical models is reviewed briefly in order to elucidate the effec t of environmental factors on the attenuation of seismic waves. Low fl uid concentrations are emphasized. Thermodynamical analysis shows that changes in surface energy caused by weak mechanical disturbances can explain observed Values of attenuation in real rocks. Experimental dis sipation isotherms are interpreted in terms of monolayered surface ads orption of liquid films as described by Langmuir's equation. In order to describe surface dissipation in consolidated rocks, a surface tensi on term is added to the pore pressure term in the O'Connell-Budiansky poroelastic equation for effective moduli of porous and fractured rock s. Theoretical calculations by this modified model, using reasonable v alues for elastic parameters, surface energy, crack density and their geometry, lead to results which qualitatively agree with experimental data obtained at low fluid contents.