Velocity Measurements and Crack Density Determination During Wet Triaxial Experiments on Oshima and Toki Granites

Schubnel, Alexandre; Nishizawa, Osamu; Masuda, Koji; Lei, Xinglin; Xue, Ziqiu; Guéguen, Yves

doi:10.1007/978-3-0348-8083-1_5

Cited by 30 publications

(42 citation statements)

References 16 publications

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“…Fresh microcracks are opened due to the large deviatoric stress and the sample is stressed up to rupture. Crack densities were inferred from velocity measurements using Kachanov's model [Sayers and Kachanov, 1995;Schubnel et al, 2002], and rupture occurred for a total crack density approximately equal to $1, 5, which is consistent with 3D percolation theory [Guéguen et al, 1997]. Figure Figure 9.…”

Section: Comparison With Experimental Data and Other Studiessupporting

confidence: 62%

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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Section: Comparison With Experimental Data and Other Studiessupporting

confidence: 62%

Dispersion and anisotropy of elastic waves in cracked rocks

2003

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“…Heap & Faulkner (2008) showed that increasing axial stress and increasing level of crack damage induce opposite effects in the elastic moduli, at least for uniaxial loading. We posit that, during triaxial loading to failure in a single cycle (Brace et al 1966;Schubnel et al 2003;Katz & Reches 2004), the increase in differential stress is the dominant factor controlling the elastic moduli, rather than the increase in crack damage.…”

Section: Applicability Of the Modelling Approachmentioning

confidence: 99%

“…However, this comparison was made with data from experiments in which samples were monotonically loaded to failure in a single loading cycle. In this situation, the evolution of elastic moduli is a function not only of increasing crack damage but also of increasing differential stress (Brace et al 1966;Schubnel et al 2003;Katz & Reches 2004) and can result in masking of the changes in elastic moduli caused by the increase in crack damage. Heap & Faulkner (2008) showed that increasing axial stress and increasing level of crack damage induce opposite effects in the elastic moduli, at least for uniaxial loading.…”

Section: Applicability Of the Modelling Approachmentioning

confidence: 99%

Elastic moduli evolution and accompanying stress changes with increasing crack damage: implications for stress changes around fault zones and volcanoes during deformation

Heap

Faulkner

Meredith

et al. 2010

Geophysical Journal International

156

105

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S U M M A R YThe elastic moduli of rock in areas susceptible to crack damage, such as within fault zones or volcanic edifices, can be subject to large modifications. Knowledge of how elastic moduli may vary in such situations is important for both the reliable modelling of volcano deformation and stability and for linear and non-linear elastic crack models for earthquake rupture. Furthermore, it has previously been shown that changes in elastic moduli can induce changes in the stress field surrounding faults. Here we report both uniaxial experimental measurements of changes in elastic moduli during increasing-amplitude cyclic stressing experiments on a range of different rock types (basalts, sandstones and granite) and the results of modelled stress modifications. The trend in elastic moduli evolution with increasing damage was remarkably similar for each rock type, with the exception of essentially crack-free intrusive basalt that exhibited very minor changes. In general, Young's modulus decreased by between 11 and 32 per cent and Poisson's ratio increased by between 72 and 600 per cent over the total sequence of loading cycles. These changes are attributed to an increasing level of anisotropic crack damage within the samples. Our results also show that acoustic emission (AE) output during any loading cycle only commenced when new crack damage was generated. This corresponded to the level of stress where AE ceased during the unloading portion of the previous cycle. Using the multilayer elastic model of Faulkner et al. we demonstrate that the damage-induced changes in elastic moduli also result in significant decreases in differential stress, increases in mean stress and rotation of the applied greatest principal stress relative to the orientation of the mechanical layering. The similar trend in the evolution of the elastic moduli of all the rocks tested suggests that stress modification in the damage zone of faults might take the same form, regardless of the lithology through which the fault runs. These observations are discussed in terms of their applicability to both fault zones and deformation at volcanoes.

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“…Under hydrostatic compression, these cracks are elastically closed, causing acoustic velocities to increase isotropically, provided that the rock is initially isotropic [Hadley, 1976]. By contrast, where differential stresses are applied, elastic crack deformation (closure, opening, or by shear displacement) becomes anisotropic, and the related changes in acoustic wave velocities are therefore also anisotropic [Nur and Simmons, 1969;Soga et al, 1978;Sayers et al, 1990;Sayers and Van Munster, 1991;Stuart et al, 1993;Crawford et al, 1995;Schubnel et al, 2003;Ghaffari et al, 2014;Nasseri et al, 2014]. Such anisotropy will be generated under conventional triaxial stress (σ 1 > σ 2 = σ 3 ), as applied in the vast majority of experimental studies, but will be further enhanced under true triaxial stress (σ 1 > σ 2 > σ 3 ), as is the general case in the crust [Zoback and Zoback, 2002].…”

Section: Introductionmentioning

confidence: 99%

Acoustic characterization of crack damage evolution in sandstone deformed under conventional and true triaxial loading

et al. 2017

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We present a comparative study of crack damage evolution in dry sandstone under both conventional (σ1 > σ2 = σ3), and true triaxial (σ1 > σ2 > σ3) stress conditions using results from measurements made on cubic samples deformed in three orthogonal directions with independently controlled stress paths. To characterize crack damage, we measured the changes in ultrasonic compressional and shear wave velocities in the three principal directions, together with the bulk acoustic emission (AE) output contemporaneously with stress and strain. We use acoustic wave velocities to model comparative crack densities and orientations. In essence, we create two end‐member crack distributions; one displaying cylindrical transverse isotropy (conventional triaxial) and the other planar transverse isotropy (true triaxial). Under the stress conditions in our experiments we observed an approximately fivefold decrease in the number of AE events between the conventional and true triaxial cases. When taken together, the AE data, the velocities, and the crack density data indicate that the intermediate principal stress suppresses the total number of cracks and restricts their growth to orientations subnormal to the minimum principal stress. However, the size of individual cracks remains essentially constant, controlled by the material grain size. Crack damage is only generated when the differential stress exceeds some threshold value. Cyclic loading experiments show that further damage commences only when that previous maximum differential stress is exceeded, regardless of the mean stress, whether this is achieved by increasing the maximum principal stress or by decreasing the minimum principal stress.

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Velocity Measurements and Crack Density Determination During Wet Triaxial Experiments on Oshima and Toki Granites

Cited by 30 publications

References 16 publications

Dispersion and anisotropy of elastic waves in cracked rocks

Dispersion and anisotropy of elastic waves in cracked rocks

Elastic moduli evolution and accompanying stress changes with increasing crack damage: implications for stress changes around fault zones and volcanoes during deformation

Acoustic characterization of crack damage evolution in sandstone deformed under conventional and true triaxial loading

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