Influence of fluids on VP/VS ratio: increase or decrease?

SUMMARY The evolution of the ratio between P- and S-wave velocities (VP/VS) with increasing fluid-saturated porosity is computed for isotropic rocks containing spheroidal pores. The ratio VP/VS is shown to either decrease or increase with increasing porosity, depending on the aspect ratio α of the pores, fluid to solid bulk modulus ratio ζ and Poisson’s ratio ν0 of the solid constituents of the rock. A critical initial Poisson’s ratio ν0, crit is computed, separating cases where VP/VS increases (if ν0 < ν0, crit) or decreases (if ν0 > ν0, crit) with increasing porosity. For thin cracks and highly compressible fluids, ν0, crit is approximated by 0.157\, ζ /α, whereas for spherical pores ν0, crit is given by 0.2 + 0.8ζ. When ν0 is close to ν0, crit, the evolution of VP/VS with increasing fluid-saturated porosity is near neutral and depends on subtle changes in pore shape and fluid properties. This regime is found to be relevant to partially dehydrated serpentinites in subduction zones (porosity of aspect ratio near 0.1 and ζ in the range 0.01–0.1), and makes detection of these rocks and possibly elevated fluid pressures difficult from VP/VS only.