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

Brantut, N; David, EC; (2019) Influence of fluids on VP/VS ratio: increase or decrease? Geophysical Journal International , 216 (3) pp. 2037-2043. 10.1093/gji/ggy518. Green open access

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Abstract

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 0.157ζ/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.

Type: Article
Title: Influence of fluids on VP/VS ratio: increase or decrease?
Open access status: An open access version is available from UCL Discovery
DOI: 10.1093/gji/ggy518
Publisher version: http://dx.doi.org/10.1093/gji/ggy518
Language: English
Additional information: This is the published version of record. For information on re-use, please refer to the publisher’s terms and conditions.
Keywords: Wave propagation, Elasticity and anelasticity, Microstructure
UCL classification: UCL
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Earth Sciences
URI: https://discovery.ucl.ac.uk/id/eprint/10071473
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