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

Wave attenuation at low seismic frequencies (0.1-100 Hz) 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 0.01 to 100 Hz, under confining pressure up to 25 MPa and generating a bulk strain around 10(-6) in a cylindrical sample with maximum size of 76 mm in diameter and 250 mm in length. The BBAV has been successfully designed, built and tested. The calibrations obtained with aluminum (EN AW-6082) and Polymethyl-methacrylate (PMMA or Plexiglas) agree with literature values. Two 20% porosity and 1.97 x 10(-13) -9.87 x 10(-13) m(2) permeability Berea sandstone samples were tested. The stress conditions were: i) unconfined, ii) confined at 2 MPa and iii) confined at 15 MPa. Dry samples exhibited always attenuation around 0.01, while saturated samples exhibited attenuation between 0.01 and 0.04. Attenuation values in >= 60% water saturated samples were frequency-dependent only for confining pressure <= 2 MPa. One explanation to this observation, which requires more experiments to be established, is that for confining pressures >10 MPa the microcracks in the sample would be closed, impeding attenuation related to squirt flow. (C) 2012 Elsevier B.V. All rights reserved.