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  • 1
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    A robust experimental determination of Thomsen's Formula parameter (2015)
    Society of Exploration Geophysicists (SEG)
    Details
    Publication Date: 2015-01-21
    Description: A novel inversion method for the laboratory determination of Thomsen’s $$\delta $$ anisotropy parameter on cylindrical rock specimens from ultrasonic data has been recently reported in the literature. We further assessed this method through a direct comparison of the results of the traditional method (involving a single off-axis P-wave velocity measurement at 45°) and the new method (involving 65 P-wave velocity measurements at several angles to the symmetry axis). We prepared and characterized two vertical shale specimens from the same preserved vertical core to assess their similarity in terms of structure, mineralogy, porosity, and density. The shale was assumed to be transversely isotropic in view of the observed (horizontal) bedding. We subjected both specimens to the same brine saturation and effective stress state. Using the two methods, we obtained similar results for Thomsen’s $$\alpha $$ (vertical P-wave) and $$\varepsilon $$ (P-wave anisotropy) parameters. However, a significant discrepancy was observed for Thomsen’s $$\delta $$ parameter: We obtained results of 0.13 using the new method and 0.39 using the traditional method. As a result of the overdetermined nature of the P-wave velocity measurements used in the new method, we believe that the corresponding $$\delta $$ value is more reliable. Also, the value derived with the new testing method seems to match more closely the reported field data.
    Print ISSN: 0016-8033
    Electronic ISSN: 1942-2156
    Topics: Geosciences , Physics
    Published by Society of Exploration Geophysicists (SEG)
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  • 2
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    The relation between viscosity and acoustic emissions as a laboratory analogue for volcano seismicity (2019)
    Geological Society of America (GSA)
    In: Geology
    Details
    Publication Date: 2019
    Description: 〈span〉〈div〉Abstract〈/div〉Volcano seismicity is an important tool used to monitor volcanic hazards, as seismic signals are commonly associated with fracturing and the movement of volcanic fluids. In investigating the relation between fluids and seismic signals, we record acoustic emissions (AEs) in the laboratory that result from fracturing and fluid depressurization through the fractured rock. We vary the fluid viscosity to create field analogues of hydrothermal and magmatic fluids. While fracturing causes high-frequency volcano-tectonic (VT)–type AE signals, fluid venting results in VT and long-period AEs with variable but overall lower dominant frequencies. The viscosity of the vented fluid correlates (1) inversely with the peak dominant frequency, (2) inversely with the number of induced AEs, (3) proportionally to the onset time of AEs, and (4) inversely with the initial rate of AE generation. Tremor was observed only in the early stages of low-viscosity fluid venting and thus may be more associated with gases and hydrothermal fluids than with magma.〈/span〉
    Print ISSN: 0091-7613
    Electronic ISSN: 1943-2682
    Topics: Geosciences
    Published by Geological Society of America (GSA)
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  • 3
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    Frequency-dependent seismic attenuation in shales: experimental results and theoretical analysis (2014)
    Oxford University Press
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    Publication Date: 2014-06-21
    Description: Samples of shales from the Ordovician Bongabinni and Goldwyer source rock formations were recovered from the Canning Basin (Western Australia). Attenuation was experimentally measured on preserved plugs from these formations in the frequency range between 10 –2 and 10 2 Hz. Samples cored with different orientations with respect to the sedimentary bedding were prepared and tested in their native saturated state and after drying in the oven at 105 °C for 24 hr to assess the effect of fluids and of the sediment anisotropy on attenuation. To aid the interpretation of the experimental results, the clay-rich samples were characterized in terms of mineralogy, water content, porosity, permeability and microstructure. The two shales have significantly different quality factors; and this is seen to be dependent on both the saturation state of the samples and the propagation direction of the oscillatory signal. The attenuation coefficient for compression/extension parallel to bedding is less than that vertical to bedding in both the preserved and partially dehydrated situations. No frequency dependency is observed in the preserved samples within the range of frequencies explored in this study. On the other hand partially saturated samples show peaks in attenuation at around 40 Hz when the stress perturbation is transmitted normal to the macroscopic bedding. The interpretation of the attenuation measurements in terms of well-established theoretical models is discussed in view of the physical characteristics and microstructure of the tested rocks.
    Keywords: Seismology
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 4
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    Damage initiation and propagation assessed from stress-induced microseismic events during a mine-by test in the Opalinus Clay (2014)
    Oxford University Press
    Details
    Publication Date: 2014-06-21
    Description: We report in this paper an original analysis of microseismic events (MSEs) induced by an excavation operation in the clay environment of the Mont Terri underground rock laboratory. In order to identify the MSEs with confidence, we develop a restrictive but efficient multistep method for filtering the recorded events. We deduce the spatial distribution and processes associated with the excavation-induced damage from the spatial location and focal mechanisms of the MSEs. We observe an asymmetric geometry of the excavation damaged zone around the excavated gallery, without notable microseismic activity in the sandy facies sidewall, in contrast with the shaly facies sidewall where a first burst of events is recorded, followed by two smaller bursts: one locates ahead of the excavation front and is associated with a dominant double-couple component, suggesting bedding plane reworking, that is, shear fracture mode, and the MSEs of the other cluster inside the shaly sidewall of the gallery, with a dominant compensated linear vector dipole component, suggesting extensive cracking. We identify and discuss four major factors that seem to control the MSEs source mechanisms: lithology, geometry of the geological features, gallery orientation and direction of the main compressive stress.
    Keywords: Seismology
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 5
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    Prediction of rocks thermal conductivity from elastic wave velocities, mineralogy and microstructure (2014)
    Oxford University Press
    Details
    Publication Date: 2014-04-16
    Description: While knowledge on Thermal Conductivity (TC) of rocks is of interest in many fields, determining this property remains challenging. In this paper, a modelling approach for TC prediction from Elastic Wave Velocity (EWV) measurements is reported. To this end, a new effective TC model for a typical sedimentary rock is introduced that explicitly accounts for the presence of pores, pressure-sensitive microcracks (or grain contacts) and formation fluids. A model of effective elasticity is also devised for this same rock that links its microstructural characteristics to the velocity of elastic waves. The two models are based on the same effective medium approach and involve the same microstructural parameters. A workflow based on this explicit modelling approach is devised that allows for the prediction of the TC of a reservoir rock using (i) the elastic waves velocities, (ii) the dominant mineral content and (iii) the bulk porosity. This workflow is validated using experimental data reported in the literature for dry and water-saturated Fontainebleau and Berea sandstones. The datasets include measurements of TC and EWV as a function of effective pressure. In addition, it is shown that the dependence of TC on the rock microstructure is formally and practically similar to that of EWV. It is also demonstrated that the accuracy of TC predictions from EWV increases with effective pressure (burial depth). The underlying assumptions and limitations of the present approach together with the effect of burial are discussed.
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 6
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    Elastic anisotropy of Opalinus Clay under variable saturation and triaxial stress (2014)
    Oxford University Press
    Details
    Publication Date: 2014-07-20
    Description: A novel experimental method is introduced to estimate the Thomsen's elastic anisotropy parameters and of a transversely isotropic shale under variable stress and saturation conditions. The method consists in recording P -wave velocities along numerous paths on a cylindrical specimen using miniature ultrasonic transducers. Such an overdetermined set of measurements is specifically designed to reduce the uncertainty associated with the determination of Thomsen's parameter compared to the classical method for which a single off-axis measurement is used (usually at 45° to the specimen's axis). This method is applied to a specimen of Opalinus Clay recovered from the Mont-Terri Underground Research Laboratory in Switzerland. The specimen is first saturated with brine at low effective pressure and then subjected to an effective pressure cycle up to 40 MPa, followed by a triaxial loading up to failure. During saturation and deformation, the evolution of P -wave velocities along a maximum of 240 ray paths is monitored and Thomsen's parameters α , and are computed by fitting Thomsen's weak anisotropy model to the data. The values of and obtained at the highest confining pressures reached during the experiment are comparable with those predicted from X-ray diffraction texture analysis and modelling for Opalinus Clay reported in the literature. These models neglect the effect of soft-porosity on elastic properties, but become relevant when soft porosity is closed at high effective pressure.
    Keywords: Marine Geosciences and Applied Geophysics
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 7
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    The relation between viscosity and acoustic emissions as a laboratory analogue for volcano seismicity (2019)
    Geological Society of America (GSA)
    In: Geology
    Details
    Publication Date: 2019
    Description: 〈span〉Volcano seismicity is an important tool used to monitor volcanic hazards, as seismic signals are commonly associated with fracturing and the movement of volcanic fluids. In investigating the relation between fluids and seismic signals, we record acoustic emissions (AEs) in the laboratory that result from fracturing and fluid depressurization through the fractured rock. We vary the fluid viscosity to create field analogues of hydrothermal and magmatic fluids. While fracturing causes high-frequency volcano-tectonic (VT)–type AE signals, fluid venting results in VT and long-period AEs with variable but overall lower dominant frequencies. The viscosity of the vented fluid correlates (1) inversely with the peak dominant frequency, (2) inversely with the number of induced AEs, (3) proportionally to the onset time of AEs, and (4) inversely with the initial rate of AE generation. Tremor was observed only in the early stages of low-viscosity fluid venting and thus may be more associated with gases and hydrothermal fluids than with magma.〈/span〉
    Print ISSN: 0091-7613
    Electronic ISSN: 1943-2682
    Topics: Geosciences
    Published by Geological Society of America (GSA)
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  • 8
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    Ultrasonic monitoring of spontaneous imbibition experiments: acoustic signature of fluid migration (2017)
    Wiley
    Details
    Publication Date: 2017-06-30
    Description: Capillary rise experiments (spontaneous imbibition tests) were conducted in the laboratory with ultrasonic and X-ray monitoring on the Sherwood sandstone and the Majella grainstone. The aim was to provide a direct comparison between the variation in seismic attributes (amplitude, velocity, spectral content, energy) and the actual fluid distribution in the rock . Two pairs of ultrasonic P-wave sensors located at different heights on a cylindrical rock specimen recorded every five seconds the waveforms when capillary forces make water rise up into the rock from the bottom in contact with a water tank. Simultaneously CT-scan images of a vertical cross-section were also recorded. Two important results were found. (i) The amplitude of the first P-wave arrival is impacted by the upward moving fluid front before the P-wave velocity is, while the fluid front has not yet reached the sensors level. In contrast the P-wave velocity decreases when the fluid front reaches the Fresnel clearance zone. The spectral analysis of the waveforms shows that the peak frequency amplitude is continuously decreasing without noticeable frequency shift. (ii) A methodology based on the calculation of the analytical signal and instantaneous phase was designed to decompose each waveform into discrete wavelets associated with direct or reflected waves. The energy carried by the wavelets is very sensitive to the fluid substitution process: the coda wavelets related to reflections on the bottom end face of the specimen are impacted as soon as imbibition starts and can be used as a precursor for the arriving fluid.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 9
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    KG²B, a collaborative benchmarking exercise for estimating the permeability of the Grimsel granodiorite—Part 2: modelling, microstructures and complementary data (2018)
    Oxford University Press
    Details
    Publication Date: 2018-07-25
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Royal Astronomical Society.
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  • 10
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    KG²B, a collaborative benchmarking exercise for estimating the permeability of the Grimsel granodiorite – Part 1: measurements, pressure dependence and pore-fluid effects (2018)
    Oxford University Press
    Details
    Publication Date: 2018-07-25
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Royal Astronomical Society.
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