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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

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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 and transmitted amplitude of 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 ultrasonic 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 increase of microcracked nanopore-dominated media. However, squirt flow – a theory recognized to characterize wave velocity increase and attenuation in microcracked macropore-dominated media at pore scale – also accounts for the observed increase of P-wave velocity in microcracked nanopore-dominated media. The transmitted amplitude change in direct P waves are explained and predicted by the elastic wave propagation within P-wave first Fresnel zone and reflection/refraction on the wetting front.