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Laboratory acousto-mechanical study into moisture-induced changes of elastic properties in intact granite
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  • Rui Wu,
  • Paul Selvadurai,
  • Ying Li,
  • Kerry Leith,
  • Simon Loew
Rui Wu
ETH Zurich, ETH Zurich

Corresponding Author:zilongzhiqi@gmail.com

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Paul Selvadurai
ETH Zürich, ETH Zürich
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Ying Li
ETH Zurich, ETH Zurich
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Curtin University, Curtin University
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Kerry Leith
GNS Science, GNS Science
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Simon Loew
ETH Zurich, ETH Zurich
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The water adsorption into pore spaces in brittle rocks affects wave velocity, transmitted energy and corner frequency of transmitted elastic waves. Experimental and theoretical studies have been performed to characterize moisture-induced elastodynamic variations due to macroporous effects; however, little attention has been paid to the manner in which wetting of nanopores affect elastic wave transmission. In this work, we extend our understanding of moisture-induced elastic changes in a microcracked nanopore-dominated medium (80 \% of the surface area exhibits pore diameters below 10 nm). We studied acousto-mechanical response resulting from a gradual wetting on a freestanding intact Herrnholz granite specimen over 98 hours using time-lapse ultrasonic and digital imaging techniques. Linkages between acoustic attributes and adsorption-induced stress/strain are established during the approach of wetting front. We found that Gassmann theory, previously validated in channel-like nanoporous media, breaks down in predicting P-wave velocity dispersion of microcracked nanopore-dominated media. However, squirt flow – a theory recognized to characterize wave dispersion and attenuation in microcracked macropore-dominated media at pore scale – also accounts for the observed dispersion of P-wave velocity in microcracked nanopore-dominated media spanning stress regimes in both contraction and extension. The transmitted energy change and the corner frequency shift in direct P waves are explained and predicted by the elastic wave propagation within P-wave first Fresnel zone and reflection on the wetting front.