Detecting the Presence of a Sub-Porosity in an Open Fluid-Filled Fractures

Fractures and joints in the field often contain debris within the void spaces. These debris originate from many different mechanisms: organic and/or inorganic chemical reactions/mineralization, sediment transport, formation of the fracture, mechanical weathering or combinations of these processes. In many cases, the presence of debris forms a "sub-porosity" within the fracture void space. The "sub-porosity" may partially fill voids thereby reducing the local porosity to lengths scales on the order of sub-microns to tens of microns. It is quite clear that a sub-porosity affects fracture porosity, permeability and storativity. In this study, we investigate how the existence of a sub-porosity affects seismic wave propagation and consequently our ability to probe changes in the subsurface caused by the formation or alteration of a sub-porosity. Laboratory experiments were performed to examine acoustic wave scattering from packings of acrylic beads used to create a sub-porosity within a synthetic fracture. The experimental setup consisted of a synthetic fracture created by the separation of two Lucite cylinders. The aperture of the fracture was controlled using a computer- controlled linear actuator to increment the aperture from contact to 20 mm in increments of 50 microns. The sub- porosity in the fracture was created by using acrylic beads with diameters of 3.0 mm, 5.4 mm, and 9.4 mm. Compressional waves were propagated across the fracture using piezoelectric transducer to send and receive the signal. The ratio of the seismic wavelength to bead diameter ranged from approximately 9 to 0.1 for a broadband frequency range of 0.1 MHz to 1.0 MHz. The seismic response of the fracture was first measured without a sub-porosity for three different conditions: dry, partially saturated and fully saturated. To study the effects of the sub-porosity on wave propagation across a fracture, a single layer of beads was added to the fracture and measured for the same three conditions described above. A time-frequency analysis of the received signals was performed to examine the velocity dispersion of the received signals. The transmission data were compared for a fixed aperture of 10 mm. For a 10 mm aperture filled with water, the velocity dispersion was 550 m/s/MHz because of resonant scattering within the water-layer between the two fracture surfaces. The addition of a single layer of beads reduced the thickness of the water-layer as the bead size increased from 3.0 mm to 5.4 mm to 9.4mm. A low-frequency low-amplitude signal arrives first in samples with a sub-porosity because the low frequency components of the signal scatter the least. From a time-frequency analysis, as the bead size increased the velocity dispersion of the signal decreased from 227 m/s/MHz to 62 m/s/MHz. Two-scattering modes compete when a sub-porosity partially fills an open water-filled fracture: resonance scattering within water-layer and scattering from the sub-porosity (beads). The ability to distinguish open fluid-filled fractures from those partially filled by a sub-porosity depends on the aperture of the fracture, the size of the grains composing the sub-porosity, the relative thickness of the sub-porosity layer to the thickness of the water-layer and the wavelength of the seismic signal. All of these length scales affect both the hydraulic and seismic response of a fracture. Acknowledgments: The authors wish to acknowledge support of this work by the Geosciences Research Program, Office of Basic Energy Sciences US Department of Energy (DEFG02-97ER14785 08).