Electrical Impedance Spectroscopy Used as a Tool for the Detection of Fractures in Rock Samples Exposed to either Hydrostatic or Triaxial Pressure Conditions

Pressure induced variations of the pore geometry werestudied by means of complex electrical dispersionmeasurements on natural marble (97% calcite, 3%quartz) and synthetic calcite samples (95% calcite,5% quartz; Siddiqi et al., 1997). Hydrostatic (up to 160 MPa) pressures were used toexamine the closing of fractures, whereas, triaxial pressures (P c =3 MPa; P u =1 to100 MPa) revealed information on forming and growth ofcracks. Porosity data ranged from 1 to 4 vol%.Permeabilities of the synthetic samples covered therange from 0.02 to 4 μD, whereas the naturalmarble exhibited a permeability of less than 5 nD. BETinner surface areas ranged from 0.29 to 0.7m 2 /g. The frequency dispersion was measured inthe frequency range 1 kHz to 1 MHz. This frequencyrange is extensive enough to fit model data to themeasured data. The interpretation of the model datarevealed information on bulk conduction processes andinterfacial phenomena as well. The latter one isdirectly related to variations in the pore geometryand resulted in a correlation between the BET surfaceand the model parameter sensitive to the closing,opening, forming and growth of fractures. Asignificant `precursor' signal before failure wasfound in the triaxial experiments. Copyright Kluwer Academic Publishers 2000