Acoustic velocity changes during shear enhanced compaction of sandstone

Scott, T.E.; Ma, Qingyu; Roegiers, J.-C.

doi:10.1016/0148-9062(93)90020-e

Cited by 43 publications

(59 citation statements)

References 8 publications

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“…The densities of vertical and horizontal cracks may have distinct sensitivities to stress, perhaps due to the closure of horizontal cracks (oriented normal to the compressional axis) during deformation (e.g., Fortin et al, ; Nicolas et al, ; Scott et al, ). However, the limited elastic wave components measured during our experiments prohibit an investigation into crack density anisotropy.…”

Section: Discussionmentioning

confidence: 99%

Evolution of Elastic Wave Velocities and Amplitudes During Triaxial Deformation of Aji Granite Under Dry and Water‐Saturated Conditions

Zaima

Katayama

2018

JGR Solid Earth

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Elastic wave velocities and amplitudes in fine‐grained Aji granite are measured during a series of deformation experiments under both dry and fluid‐saturated conditions, where compressional and shear waves traveled in a direction normal to the compressional axis, and shear wave was polarized normal to the compressional axis. In the early stages of deformation, both VP and VS increase slightly due to closure of preexisting cracks and then decrease due to nucleation and crack growth as the sample approaches failure. During deformation, the different sensitivities of compressional and shear waves to fluid‐filled cracks mean that the ratio VP/VS tends to increase for wet experiments and decrease for dry experiments prior to failure. The relative changes in elastic wave amplitudes are also related to deformation and fluid saturation: amplitude decreases at high differential stresses due to the opening of cracks oriented subparallel to the axis of maximum compression, whereas a relatively minor change in amplitude was observed under fluid‐saturated conditions. Thus, the changes in velocity and amplitude during the deformation experiments are linked to crack development and geometry. However, this sensitivity differs between dry and fluid‐saturated conditions. The monitoring of fluid injection volumes during fluid‐saturated experiments shows that decreasing elastic wave velocities and amplitudes coincide with increasing fluid injection. These systematic relationships among elastic wave velocity, amplitude, crack development, and fluid saturation suggest that understanding changes in seismic wave properties can offer significant insights into the temporal changes in the structure of fracture zones.

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Section: Discussionmentioning

confidence: 99%

Evolution of Elastic Wave Velocities and Amplitudes During Triaxial Deformation of Aji Granite Under Dry and Water‐Saturated Conditions

Zaima

Katayama

2018

JGR Solid Earth

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“…Comparison with dry triaxial experiments made on Berea sandstone by Scott et al [1993]. Confining pressure was equal to 20 MPa in the first experiment (Figures 12a and 12b), 60 MPa in the second (Figures 12c and 12d) and 138 MPa in the third one (Figures 12e and 12f).…”

Section: Comparison With Experimental Data and Other Studiesmentioning

confidence: 94%

Dispersion and anisotropy of elastic waves in cracked rocks

2003

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[1] Rocks in the Earth's crust contain variable amount of cracks, depending on the deviatoric and confining stress levels, pore pressure, and temperature conditions. Crack damage results in effects that have been investigated for a long time, in particular, a decrease in elastic wave velocity and the development of anisotropy. In this paper, we focus on cracked rocks and develop a method to calculate both the elastic wave anisotropy and the dispersion in a fluid-saturated cracked rock. We show that analytic expressions can be derived for both properties in terms of the crack density tensor components. Two fundamental theoretical tools are used in this purpose. The first one is the effective medium theory and the second one is the anisotropic poroelastic theory. Using both, it is possible from first principles to predict anisotropy and dispersion. Results are shown to be valid up to a total crack density of 0.5. The failure threshold appears close to a total crack density of 1. Between 0.5 and 1, the model still gives reasonably good results. Anisotropy can increase up to 60% and dispersion up to 30%, depending on the crack distribution. These results point out important differences that could exist in fluidsaturated rocks between laboratory (high frequency) data and field (low frequency) results.

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“…In the first case, it is well known that elastic wave velocities may be reduced substantially during triaxial compression experiments in the presence of cracks. This is observed in crystalline rocks [e.g., Hadley , 1976], and also in porous rocks [e.g., Scott et al , 1993]. Under hydrostatic tests, Zhang et al [1990] and Wong et al [1997] demonstrate from postmortem microstructural observations that grain crushing in sandstone is generally characterized by extensive microcracking as cracks nucleate and propagate when pressure reaches the critical pressure P *.…”

Section: Introductionmentioning

confidence: 99%

Effects of pore collapse and grain crushing on ultrasonic velocities and V_p/V_s

2007

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[1] Compressional, shear wave velocities and their ratio, V p /V s , were measured along with porosity variations during wet and dry hydrostatic compaction of Bleurswiller sandstone, a 25% porosity Vosgian sandstone. At first, increase in hydrostatic pressure was accompanied by a simultaneous increase of both V p and V s as expected. At a critical effective confining pressure P*, a large mechanical decrease of porosity was observed that was due to pore collapse and grain crushing. Theoretically, two different processes are affecting the elastic wave velocities in counteracting ways during cataclastic compaction: cracking and porosity decrease. Our experimental results show that cracking is the dominant effect, so that grain crushing and porosity reduction were accompanied by a large decrease in velocities. The ratio V p /V s was also observed to change during our experiments: In the wet specimen, V p /V s value increased from 1.72 to 1.84, while in the dry specimen, it increased from 1.59 below P* to 1.67 beyond P*, respectively. To quantitatively interpret these results, an isotropic effective medium model (EM) was used that considered the sandstone as a mixture of spheroidal pores and penny-shaped cracks. In particular, the increase in V p /V s , in the wet case, is well reproduced and shows the important role played by the mechanical coupling of fluid with low aspect ratio cracks (<10 À2 ). In the dry case, however, our experimental results highlight an increase of V p /V s ratio during cataclastic compaction, in apparent contradiction with the predictions of the EM model. Indeed, increases in V p /V s ratio, and hence in Poisson's ratio, are, in general, attributed to fluid saturation. A closer look to the microstructure may provide a possible interpretation: Beyond P*, grains are no longer cemented. Using Digby's granular model as an alternative model, we were able to reach a quantitative agreement with the experimental results. The possible implication is that in both dry and wet conditions, cataclastic compaction due to grain crushing induces an increase in V p /V s ratio.Citation: Fortin, J., Y. Guéguen, and A. Schubnel (2007), Effects of pore collapse and grain crushing on ultrasonic velocities and V p /V s ,

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Acoustic velocity changes during shear enhanced compaction of sandstone

Cited by 43 publications

References 8 publications

Evolution of Elastic Wave Velocities and Amplitudes During Triaxial Deformation of Aji Granite Under Dry and Water‐Saturated Conditions

Evolution of Elastic Wave Velocities and Amplitudes During Triaxial Deformation of Aji Granite Under Dry and Water‐Saturated Conditions

Dispersion and anisotropy of elastic waves in cracked rocks

Effects of pore collapse and grain crushing on ultrasonic velocities and V_p/V_s

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Acoustic velocity changes during shear enhanced compaction of sandstone

Cited by 43 publications

References 8 publications

Evolution of Elastic Wave Velocities and Amplitudes During Triaxial Deformation of Aji Granite Under Dry and Water‐Saturated Conditions

Evolution of Elastic Wave Velocities and Amplitudes During Triaxial Deformation of Aji Granite Under Dry and Water‐Saturated Conditions

Dispersion and anisotropy of elastic waves in cracked rocks

Effects of pore collapse and grain crushing on ultrasonic velocities and Vp/Vs

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Effects of pore collapse and grain crushing on ultrasonic velocities and V_p/V_s