53 research outputs found
Influence of thermal and mechanical cracks on permeability and elastic wave velocities in a basalt from Mt. Etna volcano subjected to elevated pressure
We report simultaneous laboratory measurements of seismic velocities and fluid permeability on lava flow
basalt from Etna (Italy). Results were obtained for dry and saturated samples deformed under triaxial
compression. During each test, the effective pressure was first increased up to 190 MPa to investigate the
effect of pre-existing crack closure on seismic properties. Then, the effective pressure was unloaded down to
20 MPa, a pressure which mirrors the stress field acting under a lava pile of approximately 1.5â2 km thick, and
deviatoric stress was increased until failure of the specimens.
Using an effective medium model, the measured elastic wave velocities were inverted in terms of two crack
densities: Ïi the crack density of the pre-existing thermal cracks and Ïv the crack density of the stress-induced
cracks. In addition a link was established between elastic properties (elastic wave velocities Vp and Vs) and
permeability using a statistical permeability model.
Our results show that the velocities increase with increasing hydrostatic pressure up to 190 MPa, due to the
closure of the pre-existing thermal cracks. This is interpreted by a decrease of the crack density Ïi from ~1 to
0.2. The effect of pre-existing cracks closure is also highlighted by the permeability evolution which decreases
of more than two orders of magnitude.
Under deviatoric loading, the velocities signature is interpreted, in the first stage of the loading, by the closure
of the pre-existing thermal cracks. However, with increasing deviatoric loading newly-formed vertical cracks
nucleate and propagate. This is clearly seen from the velocity signature and its interpretation in term of crack
density, the location of the acoustic emission sources, and from microstructural observations. This
competition between pre-existing cracks closure and propagation of vertical cracks is also seen from the
permeability evolution, and our study shows that mechanically-induced cracks has lesser influence on
permeability change than pre-existing thermal cracks
A Pre-Landing Assessment of Regolith Properties at the InSight Landing Site
This article discusses relevant physical properties of the regolith at the Mars InSight landing site as understood prior to landing of the spacecraft. InSight will land in the northern lowland plains of Mars, close to the equator, where the regolith is estimated to be â„3--5 m thick. These investigations of physical properties have relied on data collected from Mars orbital measurements, previously collected lander and rover data, results of studies of data and samples from Apollo lunar missions, laboratory measurements on regolith simulants, and theoretical studies. The investigations include changes in properties with depth and temperature. Mechanical properties investigated include density, grain-size distribution, cohesion, and angle of internal friction. Thermophysical properties include thermal inertia, surface emissivity and albedo, thermal conductivity and diffusivity, and specific heat. Regolith elastic properties not only include parameters that control seismic wave velocities in the immediate vicinity of the Insight lander but also coupling of the lander and other potential noise sources to the InSight broadband seismometer. The related properties include Poissonâs ratio, P- and S-wave velocities, Youngâs modulus, and seismic attenuation. Finally, mass diffusivity was investigated to estimate gas movements in the regolith driven by atmospheric pressure changes. Physical properties presented here are all to some degree speculative. However, they form a basis for interpretation of the early data to be returned from the InSight mission.Additional co-authors: Nick Teanby and Sharon Keda
New Experimental Equipment Recreating Geo-Reservoir Conditions in Large, Fractured, Porous Samples to Investigate Coupled Thermal, Hydraulic and Polyaxial Stress Processes
Abstract Use of the subsurface for energy resources (enhanced geothermal systems, conventional and unconventional hydrocarbons), or for storage of waste (CO2, radioactive), requires the prediction of how fluids and the fractured porous rock mass interact. The GREAT cell (Geo-Reservoir Experimental Analogue Technology) is designed to recreate subsurface conditions in the laboratory to a depth of 3.5âkm on 200âmm diameter rock samples containing fracture networks, thereby enabling these predictions to be validated. The cell represents an important new development in experimental technology, uniquely creating a truly polyaxial rotatable stress field, facilitating fluid flow through samples, and employing state of the art fibre optic strain sensing, capable of thousands of detailed measurements per hour. The cellâs mechanical and hydraulic operation is demonstrated by applying multiple continuous orientations of principal stress to a homogeneous benchmark sample, and to a fractured sample with a dipole borehole fluid fracture flow experiment, with backpressure. Sample strain for multiple stress orientations is compared to numerical simulations validating the operation of the cell. Fracture permeability as a function of the direction and magnitude of the stress field is presented. Such experiments were not possible to date using current state of the art geotechnical equipment
Advanced acoustic emission analysis of brittle and porous rock fracturing
Analysis of Acoustic Emission (AE) induced during brittle and porous rock fracturing at variety of loading conditions has been performed. On the base of advanced analysis of AE parameters, ultrasonic velocities and mechanical data we found that regardless of applied loading conditions the process of rock fracture can be separated into two main stages: (A) accumulation of non-correlated cracks localized almost randomly in the whole volume of uniformly stressed rock. (B) Final stage of sample fracturing could be characterized by appearance of AE nucleation site followed by initiation and propagation of the macroscopic fault. Contribution of tensile sources is reduced significantly, shear type and pore collapse type events dominate during propagation of a fracture process zone through the sample regardless of applied loading conditions. In the case of porous rock, nucleation of compaction bands could be clearly identified by the appearance of AE clusters inside the samples. Microstructural analysis of fractured samples shows excellent agreement between location of AE hypocenters and faults or the positions of compaction bands, confirming that advanced AE analysis is a powerful tool for the process of rock fracture investigation
Ultrasonic velocities, acoustic emission characteristics and crack damage of basalt and granite
Acoustic emissions (AE), compressional (P), shear (S) wave velocities, and volumetric strain
of Etna basalt and Aue granite were measured simultaneously during triaxial compression tests.
Deformation-induced AE activity and velocity changes were monitored using twelve P-wave sensors and
eight orthogonally polarized S-wave piezoelectric sensors; volumetric strain was measured using two pairs of
orthogonal strain gages glued directly to the rock surface. P-wave velocity in basalt is about 3 km/s at
atmospheric pressure, but increases by > 50% when the hydrostatic pressure is increased to 120 MPa. In
granite samples initialP-wave velocity is 5 km/s and increases with pressure by<20%. The pressure-induced
changes of elastic wave speed indicate dominantly compliant low-aspect ratio pores in both materials, in
addition Etna basalt also contains high-aspect ratio voids. In triaxial loading, stress-induced anisotropy of Pwave
velocities was significantly higher for basalt than for granite, with vertical velocity components being
faster than horizontal velocities. However, with increasing axial load, horizontal velocities show a small
increase for basalt but a significant decrease for granite. Using first motion polarity we determinedAE source
types generated during triaxial loading of the samples. With increasing differential stressAEactivity in granite
and basalt increased with a significant contribution of tensile events. Close to failure the relative contribution
of tensile events and horizontal wave velocities decreased significantly. A concomitant increase of doublecouple
events indicating shear, suggests shear cracks linking previously formed tensile cracks
Ultrasonic velocities, acoustic emission characteristics and crack damage of basalt and granite
Acoustic emissions (AE), compressional (P), shear (S) wave velocities, and volumetric strain
of Etna basalt and Aue granite were measured simultaneously during triaxial compression tests.
Deformation-induced AE activity and velocity changes were monitored using twelve P-wave sensors and
eight orthogonally polarized S-wave piezoelectric sensors; volumetric strain was measured using two pairs of
orthogonal strain gages glued directly to the rock surface. P-wave velocity in basalt is about 3 km/s at
atmospheric pressure, but increases by > 50% when the hydrostatic pressure is increased to 120 MPa. In
granite samples initialP-wave velocity is 5 km/s and increases with pressure by<20%. The pressure-induced
changes of elastic wave speed indicate dominantly compliant low-aspect ratio pores in both materials, in
addition Etna basalt also contains high-aspect ratio voids. In triaxial loading, stress-induced anisotropy of Pwave
velocities was significantly higher for basalt than for granite, with vertical velocity components being
faster than horizontal velocities. However, with increasing axial load, horizontal velocities show a small
increase for basalt but a significant decrease for granite. Using first motion polarity we determinedAE source
types generated during triaxial loading of the samples. With increasing differential stressAEactivity in granite
and basalt increased with a significant contribution of tensile events. Close to failure the relative contribution
of tensile events and horizontal wave velocities decreased significantly. A concomitant increase of doublecouple
events indicating shear, suggests shear cracks linking previously formed tensile cracks.Published974-993reserve
Influence of thermal and mechanical cracks on permeability and elastic wave velocities in a basalt from Mt. Etna volcano subjected to elevated pressure
We report simultaneous laboratory measurements of seismic velocities and fluid permeability on lava flow basalt from Etna (Italy). Results were obtained for dry and saturated samples deformed under triaxial compression. During each test, the effective pressure was first increased up to 190 MPa to investigate the effect of pre-existing crack closure on seismic properties. Then, the effective pressure was unloaded down to 20 MPa, a pressure which mirrors the stress field acting under a lava pile of approximately 1.5â2 km thick, and deviatoric stress was increased until failure of the specimens.
Using an effective medium model, the measured elastic wave velocities were inverted in terms of two crack densities: Ïi the crack density of the pre-existing thermal cracks and Ïv the crack density of the stress-induced cracks. In addition a link was established between elastic properties (elastic wave velocities Vp and Vs) and permeability using a statistical permeability model.
Our results show that the velocities increase with increasing hydrostatic pressure up to 190 MPa, due to the closure of the pre-existing thermal cracks. This is interpreted by a decrease of the crack density Ïi from ~ 1 to 0.2. The effect of pre-existing cracks closure is also highlighted by the permeability evolution which decreases of more than two orders of magnitude.
Under deviatoric loading, the velocities signature is interpreted, in the first stage of the loading, by the closure of the pre-existing thermal cracks. However, with increasing deviatoric loading newly-formed vertical cracks nucleate and propagate. This is clearly seen from the velocity signature and its interpretation in term of crack density, from the location of the acoustic emission sources, and from microstructural observations. This competition between pre-existing cracks closure and propagation of vertical cracks is also seen from the permeability evolution, and our study shows that mechanically-induced cracks has lesser influence on permeability change than pre-existing thermal cracks
Influence of thermal and mechanical cracks on permeability and elastic wave velocities in a basalt from Mt. Etna volcano subjected to elevated pressure
We report simultaneous laboratory measurements of seismic velocities and fluid permeability on lava flow
basalt from Etna (Italy). Results were obtained for dry and saturated samples deformed under triaxial
compression. During each test, the effective pressure was first increased up to 190 MPa to investigate the
effect of pre-existing crack closure on seismic properties. Then, the effective pressure was unloaded down to
20 MPa, a pressure which mirrors the stress field acting under a lava pile of approximately 1.5â2 km thick, and
deviatoric stress was increased until failure of the specimens.
Using an effective medium model, the measured elastic wave velocities were inverted in terms of two crack
densities: Ïi the crack density of the pre-existing thermal cracks and Ïv the crack density of the stress-induced
cracks. In addition a link was established between elastic properties (elastic wave velocities Vp and Vs) and
permeability using a statistical permeability model.
Our results show that the velocities increase with increasing hydrostatic pressure up to 190 MPa, due to the
closure of the pre-existing thermal cracks. This is interpreted by a decrease of the crack density Ïi from ~1 to
0.2. The effect of pre-existing cracks closure is also highlighted by the permeability evolution which decreases
of more than two orders of magnitude.
Under deviatoric loading, the velocities signature is interpreted, in the first stage of the loading, by the closure
of the pre-existing thermal cracks. However, with increasing deviatoric loading newly-formed vertical cracks
nucleate and propagate. This is clearly seen from the velocity signature and its interpretation in term of crack
density, the location of the acoustic emission sources, and from microstructural observations. This
competition between pre-existing cracks closure and propagation of vertical cracks is also seen from the
permeability evolution, and our study shows that mechanically-induced cracks has lesser influence on
permeability change than pre-existing thermal cracks.In press2.3. TTC - Laboratori di chimica e fisica delle rocceJCR Journalope
Stress induced elastic anisotropy of the Etnean basalt: Theoretical and laboratory examination
A theory, the stress-sensitivity approach, has been developed, which relates the elastic moduli of anisotropic rocks to the stress tensor and pore pressure for an arbitrary
symmetry of the applied load. The theory explains the stress-induced changes of seismic velocities in terms of
stress-induced changes of the pore space geometry. The stress dependent anisotropy is described in terms of Thomsenâs anisotropy parameters, g and d. To test the
theory we analyze the laboratory (high frequency) results of
deformation of an isotropically crack damaged dry lava
flow basalt from Mt. Etna volcano. The theory states that,
under an anisotropic (i.e. axisymmetric triaxial) load and in
the case of an initially isotropic rock, (1) the anisotropy
parameters are linear functions of the stress exponentials
(i. e. exponential functions of principal stress components)
and (2) the ratio of these anisotropy parameters as a function
of the stress is constant. In order to verify these
relationships, the stress exponentials and the anisotropy
parameters based on the measured velocities are computed
as well as the expected ratio of the Thomsenâs parameters.
Our experimental results are in very good agreement with
the theoretically predicted relations
Stress induced elastic anisotropy of the Etnean basalt: Theoretical and laboratory examination
A theory, the stress-sensitivity approach, has been developed, which relates the elastic moduli of anisotropic rocks to the stress tensor and pore pressure for an arbitrary
symmetry of the applied load. The theory explains the stress-induced changes of seismic velocities in terms of
stress-induced changes of the pore space geometry. The stress dependent anisotropy is described in terms of Thomsenâs anisotropy parameters, g and d. To test the
theory we analyze the laboratory (high frequency) results of
deformation of an isotropically crack damaged dry lava
flow basalt from Mt. Etna volcano. The theory states that,
under an anisotropic (i.e. axisymmetric triaxial) load and in
the case of an initially isotropic rock, (1) the anisotropy
parameters are linear functions of the stress exponentials
(i. e. exponential functions of principal stress components)
and (2) the ratio of these anisotropy parameters as a function
of the stress is constant. In order to verify these
relationships, the stress exponentials and the anisotropy
parameters based on the measured velocities are computed
as well as the expected ratio of the Thomsenâs parameters.
Our experimental results are in very good agreement with
the theoretically predicted relations.PublishedL113072.3. TTC - Laboratori di chimica e fisica delle rocceJCR Journalreserve
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