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

The evolution of the ratio between P- and S-wave velocities (V_P/V_S) with increasing fluid-saturated porosity is computed for isotropic rocks containing spheroidal pores. The ratio V_P/V_S 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 ν_{0, crit} is computed, separating cases where V_P/V_S increases (if ν₀ < ν_{0, crit}) or decreases (if ν₀ > ν_{0, crit}) with increasing porosity. For thin cracks and highly compressible fluids, ν_{0, crit} is approximated by 0.157 ζ /α, whereas for spherical pores ν_{0, crit} is given by 0.2 + 0.8ζ. When ν₀ is close to ν_{0, crit}, the evolution of V_P/V_S 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 V_P/V_S only.