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Attenuation at low seismic frequencies in partially saturated rocks Measurements and description of a new apparatus



Attenuation at low seismic frequencies in partially saturated rocks Measurements and description of a new apparatus



Journal of Applied Geophysics 86(none)



Wave attenuation at low seismic frequencies (.1-1Hz) in the earth crust has been explained by stress-induced fluid flow in partially saturated porous media. We present the pressure vessel called Broad Band Attenuation Vessel (BBAV) and two series of attenuation (Q E -1 ) measurements conducted on Berea sandstone. The BBAV employs the sub-resonance method to measure seismic wave attenuation in the frequency range from .1 to 1Hz, under confining pressure up to 25MPa and generating a bulk strain around 1-6 in a cylindrical sample with maximum size of 76mm in diameter and 25mm in length. The BBAV has been successfully designed, built and tested. The calibrations obtained with aluminum (EN AW-682) and Polymethyl-methacrylate (PMMA or Plexiglas) agree with literature values. Two 2% porosity and 1.97x1 -13 - 9.87x1 -13 m2 permeability Berea sandstone samples were tested. The stress conditions were: i) unconfined, ii) confined at 2MPa and iii) confined at 15MPa. Dry samples exhibited always attenuation around .1, while saturated samples exhibited attenuation between .1 and .4. Attenuation values in ?6% water saturated samples were frequency-dependent only for confining pressures ?2MPa. One explanation to this observation, which requires more experiments to be established, is that for confining pressures >1MPa the microcracks in the sample would be closed, impeding attenuation related to squirt flow. A new machine to measure attenuation at low seismic frequencies is presented. Calibrations performed with aluminum and PMMA samples are reported. We present two series of attenuation data, at low frequencies, for Berea sandstone. are reported as function of confining pressure and saturation. Frequency dependent attenuation varies with saturation and confining pressure.

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Accession: 036507020

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DOI: 10.1016/j.jappgeo.2012.07.008


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