An exploratory study of the seismic properties of thermally cracked, fluid-saturated aggregates of sintered glass beads

A synthetic rock analogue with simple microstructure was used to advance our understanding of the influence of cracks and pore fluids on seismic properties. The glass beads with ~ 300 μm diameter were sintered near the glass transition with average 1~2% porosity and subsequently quenched from high temperature into water at room temperature to introduce cracks with uniformly low aspect ratio α ~ 0.0007. Jackson-Paterson attenuation apparatus, with independently controlled confining and pore-fluid pressure systems, was used for both torsional and flexural mode forced oscillations at seismic frequencies to extract shear and Youngs modulus respectively, with or without the presence of pore fluids (e.g. argon, water) of varying viscosities. By perturbing the pore pressure at either end of the cracked specimen by ~5 MPa, permeability was obtained by analysing the evolution curve for pore fluid re-equilibration. Shear modulus is found lower with longer oscillation periods for the cracked and argon pore fluid saturated material possibly indicating the transition from the saturated isolated to saturated isobaric regime, with minimal strain-energy dissipation 1/Q < 0.003. The averaged elastic moduli for different oscillation periods and permeability are discovered to be extremely sensitive to variation of effective pressure. The crack closure effects can be observed easily at the effective pressure level at ~ Eα, consistent with the theoretical prediction.