The Effect Of Scale On Seismic Measurements Of Fracture Properties

Remotely determining the hydraulic properties of a fractured rock is very important for characterization and monitoring of shallow fractured aquifers. Laboratory studies have shown that the hydraulic properties of a fractured medium are related to the specific stiffness of a fracture. In addition, fracture specific stiffness can be quantified from seismic measurements of attenuation and velocity. However, quantifying fracture specific stiffness from seismic methods is a scale-dependent process because there are length scales associated with the fracture geometry that controls stiffness, with the wavelength of the seismic probe and with the size of the source/receiver illumination of the fracture plane. We have developed an acoustic lens system to investigate the aforementioned length scales that affect geophysical interpretation of fracture properties. Using compressional mode piezoelectric transducers (1Mhz) in conjunction with a set of acoustic lenses, the acoustic system produces a collimated wavefront with diameters of 15 mm, 30 mm, 45 mm and 60 mm. The seismic wavelength varies over one order of magnitude. To study the scaling behavior of the fractured medium, the same region of a sample is scanned with the collimated wavefronts with different diameters. The number of measurements depends on the size of the collimated wavefront. The recorded signals are analyzed using wavelet transformation to examine wave attenuation, dispersion and velocity as a function of scale. In this study, cylindrical carbonate rocks and acrylic samples measuring 150 mm in diameter by 76 mm in height were used. After characterizing the intact rock samples, a fracture was induced using Brazil testing. For the acrylic samples, diffraction gratings were milled into the acrylic. For homogeneous samples, the average seismic properties were scale independent. However, heterogeneous fracture geometry that produced strong scattering resulted in scale dependent seismic properties. Scale dependent seismic attributes will affect the interpretation of the hydraulic properties of the fracture from seismic measurements. Acknowledgments: Geosciences Research Program, Office of Basic Energy Sciences US Department of Energy. LJPN wishes to acknowledge University Faculty Scholar program at Purdue University