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| Titel |
Experimental study of seismic attenuation in partially saturated porous media |
| VerfasserIn |
Julien Barrière, Clarisse Bordes, Pascale Sénéchal |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250042649
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| Zusammenfassung |
| Nowadays, it is well admitted that hydrogeological properties of the porous media (porosity,
fluid saturation and permeability) can influence seismic properties. In geophysics, the major
theory which links hydrogeological and seismic parameters is poroelasticity proposed by Biot
(1956). The Biot relaxation process is due to the relative displacement of fluid in comparison
to the solid which causes a significant attenuation of seismic waves, notably in
unconsolidated medium. In partially saturated medium, pore fluids are considered as
a perfect mixture and so called “effective fluid”. However, in more consolidated
rocks, the Biot theory is not sufficient to explain the attenuation level as measured
from field seismic and sonic log data. In the last decade, some authors provide new
theories to understand the attenuation caused by the interaction of the different
fluids.
Most experiments are done in the ultrasonic frequency range, where sources of attenuation
(like scattering or local fluid flow) are different as in the low frequency range where the
wavelength is greater than heterogeneities size. In this way, we propose a forward-looking
experiment with the use of a vertical impulsionnal seismic source which have a strong
amplitude spectrum ranging from 100Hz to 8kHz. We study three different unconsolidated
porous media at atmospheric pressure: fine-grained sand, coarsed-grained sand and
coarse gravel. Water content is measured with a calibrated capacitance probe and
temperature effects are corrected. Seismic wave propagation is recorded by piezoelectric
accelerometers designed for frequencies below 10kHz. The water injection is done by
imbibition.
We propose to analyse the attenuation in the [100Hz-1.5kHz] frequency range for the
studied media with various water saturation levels. The attenuation varies according
to the porous medium and the water content and appears more significant at dry
condition and at high saturation level. The weak cohesion at dry condition probably
explains the strong attenuation while the water injection consolidates the porous
media and consequently increases the quality factor. At high saturation level, other
phenomenona could be responsible for this increase of attenuation. We try to explain these
observed physical phenomenona with models using different theories. The input
parameters are principally poroelastic modulis, porosity and permeability. They will
be estimated from theoretical values and geotechnical laboratory measurements. |
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