Broadband laboratory measurements of dispersion in thermally cracked and fluid-saturated quartzite and a synthetic analogue

Li, Yang; Olin, Melissa; David, Emmanuel C.; Jackson, Ian; Schijns, Heather; Schmitt, Douglas R.

doi:10.1190/tle33060624.1

Cited by 10 publications

(12 citation statements)

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“…Broadly similar results were obtained by Li et al (), using a more elaborate differential effective medium model for material containing both cracks and unclosable pores. The authors inferred a crack density for the same cracked Cape Sorell quartzite of ~ 0.5 for P eff = 20 MPa decreasing to <0.1 for P eff > 80 MPa.…”

Section: Modelingsupporting

confidence: 86%

“…Using the previously mentioned shear modulus for quartz and the Poisson ratio, 0.078, calculated from the Hashin-Shtrikman average bulk modulus value of 37.7 GPa (McSkimin et al, 1965) yields a zero-pressure aspect ratio of 0.001. As crack closure occurs over a range of pressures both lower and higher than 80 MPa, there must in fact be a distribution of aspect ratios, centered around 0.001 as inferred by Li et al (2014). An aspect ratio of 0.001, combined with throat sizes of Figure 4, corresponds to crack diameters of 0.4 mm for the Cape Sorell quartzite and 0.7 mm for the Canadian quartzite, comparable with the grain sizes (both 0.5 mm) of both which can be expected to form an upper limit on crack size.…”

Section: Pressure-induced Crack Closure and Crack Aspect Ratiomentioning

confidence: 95%

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Shear Modulus Dispersion in Cracked and Fluid‐Saturated Quartzites: Experimental Observations and Modeling

2018

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The effect of pore fluids on acoustic wave dispersion in rocks with low aspect ratio crack porosity is important for the interpretation of laboratory and field observations in hard rock mineral exploration environments. Here we make laboratory measurements of shear modulus dispersion at frequencies 0.01–1 Hz and at 1 MHz with different saturating fluids (dry, argon, and water saturated) in two thermally cracked quartzite samples with ~2% total porosity. Measurements are made across a range of effective pressures (10–150 MPa), with the resulting very low permeabilities of the samples varying from 1–300 × 10−21 m2. Moduli across the 0.01–1 Hz band were typically independent of frequency. The shear moduli measured at sub‐Hz frequencies are unaffected by fluid saturation, as expected for the saturated isobaric (Gassmann) regime. In marked contrast, water saturation of the cracked rocks results in very large increases in the shear moduli measured at 1 MHz and low effective pressures, indicative of saturated isolated conditions. Thus, at an effective pressure of 20 MPa, the shear moduli for the two water‐saturated quartzites increase by 74% and 98% from 1 Hz to 1 MHz. The contrast in elastic moduli between dry and water‐saturated conditions is well represented by the theoretical model developed by Walsh and others. The observed dispersion highlights the need for care in seismological application of results obtained at MHz frequencies from laboratory ultrasonic measurements.

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Section: Modelingsupporting

confidence: 86%

Section: Pressure-induced Crack Closure and Crack Aspect Ratiomentioning

confidence: 95%

Shear Modulus Dispersion in Cracked and Fluid‐Saturated Quartzites: Experimental Observations and Modeling

2018

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“…Subsequently, the results of all measurements are presented in graphical format. The full set of tabulated data is accessible in Li (), and a preliminary account of part of this work was presented by Li et al (). Finally, the results obtained in this paper, along with related observations, are used to test the theoretical predictions of Figure concerning fluid flow regimes and related dispersion and attenuation in fractured media.…”

Section: Introductionmentioning

confidence: 99%

A Broadband Laboratory Study of the Seismic Properties of Cracked and Fluid‐Saturated Synthetic Glass Media

David

Nakagawa

et al. 2018

JGR Solid Earth

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For better understanding of frequency dependence (dispersion) of seismic wave velocities caused by stress-induced fluid flow, broadband laboratory measurements were performed on a suite of synthetic glass media containing both equant pores and thermal cracks. Complementary forced oscillation, resonant bar, and ultrasonic techniques provided access to millihertz-hertz frequencies,~1 kHz frequency, and~1 MHz frequency, respectively. The wave speeds or effective elastic moduli and associated dissipation were measured on samples under dry, argon-or nitrogen-saturated, and water-saturated conditions in sequence. The elastic moduli, in situ permeability, and crack porosity inferred from in situ X-ray computed tomography all attest to strong pressure-induced crack closure for differential (confining-minus-pore) pressures <30 MPa, consistent with zero-pressure crack aspect ratios <4 × 10 À4 . The low permeabilities of these materials allow access to undrained conditions, even at subhertz frequencies. The ultrasonically measured elastic moduli reveal consistently higher shear and bulk moduli upon fluid saturation-diagnostic of the saturated-isolated regime. For a glass rod specimen, containing cracks but no pores, saturated-isolated conditions apparently persist to subhertz frequencies-requiring in situ aspect ratios (minimum/maximum dimension) <10 À5 . In marked contrast, the shear modulus measured at subhertz frequencies on a cracked glass bead specimen of 5% porosity, is insensitive to fluid saturation, consistent with the Biot-Gassmann model for the saturated-isobaric regime. The measured dispersion of the shear modulus approaches 10% over the millihertz-megahertz frequency range for the cracked and fluid-saturated media-implying that laboratory ultrasonic data should be used with care in the interpretation of field data. Gurevich et al. (2010) incorporated the influence of "squirt" flow, as originally proposed by Mavko and Nur (1975), for a unified model of Biot and squirt-flow dispersion. In contrast, in effective medium theory, individual inclusions with assigned geometries are embedded within a solid to estimate the overall elastic properties of the composite from the elastic properties of individual constituents (Eshelby, 1957;Kachanov, 1993;Kuster & Toksöz, 1974;Wu, 1966;Zimmerman, 1991). Consideration of frequency-dependent fluid communication between the isolated fluid phases of conventional effective medium models provides insight into the dispersion of fluid-saturated medium and the influence of microstructure on dispersion (Adelinet et al.

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“…Dense soda‐lime glass rods, provided by Nadège Desgenétez of the ANU School of Art, were cut and machine ground to obtain cylindrical specimens (see figure in Li et al, ). Two groups of specimens (Table ) were obtained depending on the required dimensions for the applicable testing techniques; large diameter (38 mm) for experiments performed at ENS Paris (FDL2) and small diameter (15 mm) for experiments performed at ANU (SAGR1; SAGR2 and FDSL1).…”

Section: Experimental Methodsmentioning

confidence: 99%

“…Dense soda-lime glass rods, provided by Nadège Desgenétez of the ANU School of Art, were cut and machine ground to obtain cylindrical specimens (see figure in Li et al, 2014). Two groups of specimens (Table 1) were obtained depending on the required dimensions for the applicable testing techniques; Note.…”

Section: Sample Characterizationmentioning

confidence: 99%

Poroelastic Relaxation in Thermally Cracked and Fluid‐Saturated Glass

et al. 2020

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To test theoretical models of modulus dispersion and dissipation in fluid‐saturated rocks, we have investigated the broadband mechanical properties of four thermally cracked glass specimens of simple microstructure with complementary forced‐oscillation (0.004–100 Hz) and ultrasonic techniques (~1 MHz). Strong pressure dependence of moduli (bulk, Young's, and shear), axial strain, and ultrasonic wave speeds for dry conditions attests to essentially complete crack closure at a confining pressure of 15 MPa—consistent with ambient‐pressure crack aspect ratios ≤2 × 10−4. Oscillation of the confining pressure reveals bulk modulus dispersion and a corresponding dissipation peak, near 0.002 Hz only at the lowest effective pressure (2.5 MPa)—attributed to the transition with increasing frequency from the drained to saturated‐isobaric regime. The observations are consistent with Biot‐Gassmann's theory, with dispersion and dissipation adequately represented by Zener model. Above the draining frequency, axial forced‐oscillation tests show dispersion relatively low differential press's modulus and Poisson's ratio, and an associated broad dissipation peak centered near 0.3 Hz, thought to reflect local “squirt” flow and adequately modeled with a continuous distribution of relaxation times over two decades. Observations of Young's and shear moduli dispersion and dissipation from complementary flexural and torsional oscillation measurements for differential pressure ≤10 MPa provide supporting evidence of the transition with increasing frequency from the saturated‐isobaric to the saturated‐isolated regime—also probed by ultrasonic technique. These findings validate predictions from theoretical models of dispersion in cracked media and emphasize need for caution in the seismological application of laboratory ultrasonic data for cracked media.

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Broadband laboratory measurements of dispersion in thermally cracked and fluid-saturated quartzite and a synthetic analogue

Cited by 10 publications

References 10 publications

Shear Modulus Dispersion in Cracked and Fluid‐Saturated Quartzites: Experimental Observations and Modeling

Shear Modulus Dispersion in Cracked and Fluid‐Saturated Quartzites: Experimental Observations and Modeling

A Broadband Laboratory Study of the Seismic Properties of Cracked and Fluid‐Saturated Synthetic Glass Media

Poroelastic Relaxation in Thermally Cracked and Fluid‐Saturated Glass

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