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| Titel |
Experimental evidence of frequency and fluid effects in an icelandic basalt |
| VerfasserIn |
Mathilde Adelinet, Jerome Fortin, Yves Guéguen, Alexandre Schubnel, Laurent Geoffroy |
| 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 |
250037593
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| Zusammenfassung |
| An important area of geophysical research is based on the elastic properties variations in
rocks. In the laboratory, the responses of the rock elastic velocities to stress changes in
saturated or dry specimens have been studied extensively in the past using ultrasonic signals
in the megahertz frequency range. However when seismic wave velocities are involved, a
direct extrapolation from laboratory to field scale is not straightforward. Laboratory data are
indeed obtained in the ultrasonic range, whereas field data from seismological methods are
obtained at much lower frequencies (Hz). The interpretation of the seismic wave velocities,
measured at the field scale, in terms of fluids and/or in terms of physical properties of the
rock formation is complex.
From a theoretical point of view, when cracks or pores are connected, stress can induce
fluid flow from one inclusion to another; this is the squirt-flow effect. In saturated samples,
several authors have shown that the measured velocities at high frequencies are generally
faster than those predicted by the equations of Gassmann, which correspond to the low
frequency limit of Biot’s theory.
In this study, we investigated the frequency and fluid effects through an experimental
point of view. We conducted hydrostatic experiments on an icelandic basalt, characterized by
a bimodal porosity (~1% of cracks and ~7 % of equant pores; the total porosity
is ~8%). The bulk moduli were measured through two experimental methods: a
classical one using ultrasonic P- and S-waves velocities (frequency 106 Hz), and a
new one based on oscillation tests (frequency 10-2 Hz). Measurements have been
performed in the pressure range of 0- 200 MPa, both for low frequency and high
frequency.
In dry conditions, no significant differences are observed between high and low
frequency bulk moduli. However, in saturated conditions, two effects are highlighted:
a physico-chemical effect emphasized by a difference between drained and dry
moduli, and a squirt-flow effect evidenced by a difference between high and low
frequency undrained moduli. This second effect disappears at high pressure, as
cracks are closed. This observation is consistent with a squirt flow from cracks to
pores.
Reference :
M. Adelinet, J. Fortin, Y. Guéguen, A. Schubnel and L. Geoffroy. Frequency and fluid
effects on elastic properties of basalt: Experimental investigations. In press in Geophysical
Research Letters |
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