Publication Date:
2017-04-04
Description:
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 1.5–2 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: ρi the crack density of the pre-existing thermal cracks and ρv 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 ρi 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.
Description:
In press
Description:
2.3. TTC - Laboratori di chimica e fisica delle rocce
Description:
JCR Journal
Description:
open
Keywords:
Elastic wave velocity, Permeability, Acoustic emission, Fracture, Basalt
;
04. Solid Earth::04.01. Earth Interior::04.01.04. Mineral physics and properties of rocks
;
04. Solid Earth::04.04. Geology::04.04.06. Rheology, friction, and structure of fault zones
;
04. Solid Earth::04.08. Volcanology::04.08.05. Volcanic rocks
;
04. Solid Earth::04.08. Volcanology::04.08.06. Volcano monitoring
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
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