HR: 1330h
AN: H42B-1078    [PDF]
TI: Frequency-Dependent Fracture Specific Stiffness
AU: * Pyrak-Nolte, L J
EM: ljpn@physics.purdue.edu
AF: Purdue University, Department of Physics 525 Northwestern Avenue, West Lafayette, IN 47907-2036 United States
AU: * Pyrak-Nolte, L J
EM: ljpn@physics.purdue.edu
AF: Purdue University, Department of Earth and Atmospheric Sciences, West Lafayette, IN 47907-2051 United States
AU: Folz, M A
EM: folzma@purdue.edu
AF: Purdue University, Department of Physics 525 Northwestern Avenue, West Lafayette, IN 47907-2036 United States
AU: Acosta-Colon, A
EM: aacosta@physics.purdue.edu
AF: Purdue University, Department of Earth and Atmospheric Sciences, West Lafayette, IN 47907-2051 United States
AB: Monitoring the hydraulic properties of fractures remotely through their seismic signatures is an important goal for field hydrology. Empirical studies have shown that the hydraulic properties of a fracture are implicitly related to the fracture specific stiffness through the amount and distribution of contact area and apertures that arise from two rough surfaces in contact. Complicating this simple picture are seismic measurements that indicate frequency-dependent stiffness, i.e., a scale-dependent fracture stiffness where the scale is set by the wavelength. Thus relating the hydraulic properties of fractures to seismic measurements becomes a scale dependent problem. We have performed laboratory experiments to examine the phenomenon of frequency dependent fracture specific stiffness to aid in the assessment of the hydraulic properties of a fracture using seismic techniques. To this end, we have developed a photolithographic technique with which we can construct synthetic fractures of known fracture geometry with feature sizes controlled over several orders of magnitude. The synthetic fracture (and the control non-fractured samples) are made from acrylic cylinders that measure 15.0 cm in diameter by 7.7 cm in height. The diameter of the samples enables us to sample the acoustic properties of the fracture using acoustic lens over regions that range in scale from 10 mm to 60 mm. A confinement cell controls the normal stress on the fracture. Seismic measurements were made with broadband compressional-mode piezoelectric transducers enabling one-order of magnitude in frequency. We found that when the wavelength is smaller than the asperity size, a linear dependence of fracture specific stiffness on frequency occurs. In this geometric ray regime the asymptotic value of the transmission function provides a direct measure of the contact area of the fracture. On the other hand, when the asperity spacing is less than an eighth of a wavelength, the fracture behaves as a displacement discontinuity and exhibits a frequency-independent fracture specific stiffness. For intermediate asperity spacings, mixed behavior (that may include resonant scattering) was observed. By understanding how to interpret fracture specific stiffness as a function of frequency, we seek to develop a better interpretation of the hydraulic properties of fractures based on seismic measurements. Acknowledgments: The Authors acknowledge support of this research by the Geosciences Research Program, Office of Basic Energy Sciences, US Department of Energy. LJPN wishes to acknowledge Purdue University Faculty Scholar
DE: 1829 Groundwater hydrology
DE: 1899 General or miscellaneous
DE: 5102 Acoustic properties
DE: 5104 Fracture and flow
DE: 5114 Permeability and porosity
SC: Hydrology [H]
MN: 2003 Fall Meeting