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| Titel |
Ultrasound P-wave velocities and amplitudes during triaxial deformation of three sandstone varieties: the chances to monitor different states of internal drainage |
| VerfasserIn |
M. Duda, J. Renner |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250070945
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| Zusammenfassung |
| Time-dependent variations in velocities and amplitudes of elastic waves are among the most
important sources of information to monitor natural or induced subsurface processes at
transient states of drainage. Laboratory experiments provide valuable information on the
characteristic properties of elastic wave propagation associated with different mechanical
states during deformation, in particular with respect to the role of variations in microstructure
and effective stress. Triaxial deformation characteristics of dry and saturated sandstone
samples (Ruhr sandstone, Wilkeson sandstone, Fontainebleau sandstone) at various confining
pressures up to 150 MPa and strain rates ranging from 10-7 and 10-3 s-1 were investigated
by simultaneously recorded ultrasound signals of P-waves propagating parallel to
the applied deviatoric stress with the aim to monitor different states of internal
drainage. During brittle deformation velocities and amplitudes initially increased,
exhibited a maximum prior to failure and decreased, whilst the magnitude of variations
decreased with increasing confining pressure and strongly differed between the three
sandstones. Differences between the deformation characteristics of dry and saturated
samples at equivalent effective pressures document the onset and magnitude of
dilatancy hardening of saturated samples associated with the transition from effectively
drained to undrained conditions at strain rates consistent with estimates from actual
measurements of hydraulic properties. Monitoring strain-rate dependent variations of
internal drainage with ultrasound P-waves was conspicuously limited for the three
sandstones varieties due to an inherent ambiguity of velocities under the applied pressure
and saturation conditions. However, the actual occurrence of insufficient drainage
was associated with characteristic variations in velocity and amplitude. Effectively
undrained conditions during deformation accompanied a reduced axial strain at
maximum amplitudes compared to dry experiments at otherwise constant experimental
conditions. The decrease in amplitudes at lower strains is likely attributed to pore
overpressure inducing stable crack propagation, when hydraulic diffusion is too low to
compensate for a reduction in pore space during the initial elastic compaction. Also, the
reduction in velocity prior to failure was less pronounced for saturated samples
under effectively undrained conditions than observed for dry and drained saturated
samples documenting local pore pressure diminution during inelastic dilation resulting
in dilatancy hardening. Critical strain rates for internal drainage independently
deduced from properties of ultrasound waves agreed within an order of magnitude
with critical strain rates estimated from both hydraulic and mechanical properties. |
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