Print this pageBaechle, Gregor T.1, Ralf J. Weger1, Gregor P. Eberli1
(1) University of Miami / RSMAS, Miami, FL
ABSTRACT: Gassmann Revisited: Variations in Dynamic Shear Modulus of Carbonate 
Rocks
Understanding the effects of saturation on acoustic properties of carbonates is
paramount for using the amplitude versus offset (AVO) technique and 4-D seismic surveys in
carbonates. Laboratory experiments on Cretaceous and Miocene limestones display variations
in the shear modulus due to saturation. This variation of the shear modulus questions the
basic assumption of constant shear modulus with saturation in Gassmann's theory.
Ultra-sonic velocities of 30 Cretaceous and Miocene limestone samples with porosities from
5% to over 30% were measured under dry and saturated conditions at variable confining
pressures. Two groups are observed based on the calculated shear moduli: group one shows a
decrease in shear modulus of the rock by up to 2 GPa, group two shows an increase by up to
3 GPa. Furthermore, velocities predicted with the Gassmann equation for the group with
shear weakening overestimate the measured values by as much as 400 m/s; in the group with
shear strengthening the values are underestimated by as much 600 m/s. In addition, the
Vp/Vs ratio shows an overall increase with saturation. In particular, rocks displaying
shear weakening have distinct higher Vp/Vs ratios. Samples with shear weakening and high
Vp/Vs ratio consist mostly of rocks with interparticle/intercrystalline porosity. In
contrast, carbonate rocks with dominant microporosity and moldic pore types tend to show
less effects of fluid saturation on velocity. These data suggest that a) shear moduli do
not remain constant during saturation, b) the rock-fluid interaction causes errors in
Gassmann-predicted velocities
, and c) pore types influence elastic moduli.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.