Influence of fluids on $V_\mathrm{P}/V_\mathrm{S}$ ratio: Increase or decrease?

TL;DR

This study models how the $V_P/V_S$ seismic velocity ratio changes with increasing fluid-filled porosity in rocks, showing it can either increase or decrease depending on pore shape, fluid properties, and initial rock elasticity.

Contribution

It introduces a critical initial Poisson's ratio that determines whether $V_P/V_S$ increases or decreases with porosity, providing a new framework for interpreting seismic data in fluid-rich rocks.

Findings

$V_P/V_S$ can increase or decrease with porosity depending on pore shape and fluid properties.

A critical initial Poisson's ratio $ u_{0,crit}$ predicts the trend of $V_P/V_S$ evolution.

Near $ u_{0,crit}$, $V_P/V_S$ evolution is nearly neutral, complicating fluid detection.

Abstract

The evolution of $V_\mathrm{P}/V_\mathrm{S}$ with increasing fluid-saturated porosity is computed for isotropic rocks containing spheroidal pores. $V_\mathrm{P}/V_\mathrm{S}$ is shown to either decrease or increase with increasing porosity, depending on the aspect ratio $\alpha$ of the pores, fluid to solid bulk modulus ratio $\zeta$, and initial Poisson's ratio $\nu_0$ of the solid. A critical initial Poisson's ratio $\nu_\mathrm{0,crit}$ is computed, separating cases where $V_\mathrm{P}/V_\mathrm{S}$ increases (if $\nu_0<\nu_\mathrm{0,crit}$) or \emph{decreases} (if $\nu_0>\nu_\mathrm{0,crit}$) with increasing porosity. For thin cracks and highly compressible fluids, $\nu_\mathrm{0,crit}$ is approximated by $0.157\,\zeta/\alpha$, whereas for spherical pores $\nu_\mathrm{0,crit}$ is given by $0.2 + 0.8\zeta$. If $\nu_0$ is close to $\nu_\mathrm{0,crit}$, the evolution of $V_\mathrm{P}/V_\mathrm{S}$ 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 zone conditions (porosity of aspect ratio near 0.1 and $\zeta$ in the range 0.01--0.1), and makes detection of these rocks and possibly elevated fluid pressures difficult from $V_\mathrm{P}/V_\mathrm{S}$ only.