ABSTRACT: Natural fractures are often filled with debris. This debris originates from many different mechanisms: organic and/or inorganic chemical reactions (such as 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. 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 fractures properties caused by the formation or alteration of a sub-porosity. Laboratory experiments were performed to examine acoustic wave scattering from packings of spherical beads used to create a sub-porosity within a synthetic fracture. The sub-porosity in the fracture was created by using spherical beads with a range of diameters from 500 µm to 7.79 mm. Compressional waves were transmitted across the fracture using contact piezoelectric transducer. The analysis was performed for a fixed aperture with different sub-porosity created by single and multiple layers. Interpretation of the acoustic response depends critically on understanding the length scales associated with the layers (bead diameter, layer thickness), the aperture of the fracture and seismic length scales (wavelengths, field of view). The detection of multiple layers of debris within a fracture is possible by understanding wave interference, dispersion and the reflections in the waveform.
1. INTRODUCTION
Fractures and joints in the field often contain debris within the void spaces (aperture). These debris originate from many different mechanisms: organic and/or inorganic chemical reactions (such as 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 material that composes this sub-porosity can differ or be the same in chemical and physical properties as that from the fracture walls. 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 know that the sub-porosity affects fracture porosity, permeability and storativity. Several laboratory studies have measured the acoustic properties of spherical beads in fluids. Most of these measurements are obtained using seismic reflection methods [1] to simulate marine field measurements. Other experiments consist of simulating sediment transport where the spherical beads are submerged in a fluid and free to move [2,8]. These and other studies have shown that beads produce elastic and acoustic scattering signatures that depend on the relative size of the bead to the wavelength, ¿, of the acousticsignal. There are different scattering regimes based on the size of the scatterer, a, and the wave number, k (k=2p/¿) of the signal that exhibit different frequencydependent attenuation coefficients. For Rayleigh scattering (ka<1), the attenuation coefficient is proportional to the fourth power of frequency. For resonance (Mie) scattering, the attenuation coefficient depends on the first power of frequency when the wavelength approaches the size of the scatterer. Mie scattering is commonly observed in monodispersions of glass beads in fluidized beds [2]. The characterization and understanding of the mechanical, seismic and hydraulic properties of fractures is affected by the size of the sediments filling a fracture or a mechanical discontinuity.
