ABSTRACT:

Fractures are often the principal source of hydraulic and mechanical anisotropy in a rock mass. To measure the seismic properties of the fracture samples, we use a wavefront imaging method to acquire the full arriving waveform in two spatial dimensions and one temporal dimension. From this 3D dataset, we reconstruct the complete longitudinal components of acoustic wavefronts that have propagated though a sample, and thereby obtain a direct visualization of discontinuous features in the arriving wavefronts produced by the presence of discrete fractures. This technique is applied to one intact aluminum sample and one aluminum sample containing synthetic fractures spaced 12mm apart. The specific stiffnesses of the fractures are controlled by applying stress (0MPa7MPa) around the circumference of the sample.

INTRODUCTION

One of the principal sources of frequency-dependent seismic anisotropy in a fractured rock arises from sets of multiple fractures. The spacing between multiple parallel fractures introduces a characteristic length scale for the rock mass. ^ seismic wave will be increasingly scattered as the wavelength of the seismic signal approaches the characteristic length scale of the medium, in this case the fracture spacing. This characteristic length scale results in a frequency-dependent seismic response that manifests itself as scattering, resonance (Nakagawa, 1998), and wave guiding (Nihei et al., 1999). For plane waves propagating perpendicular to the fractures, this characteristic length scale results in a frequencydependent stop-band behavior where propagating modes cease to exist between certain frequencies that are related to the stiffness of the fracture and fracture spacing (Nakagawa, 1998). Our primary experimental goal in this paper is to image seismic waves propagation parallel to multiple-parallel fractures using a spatially localized source, i.e., a source that is localized within a single layer defined by two parallel fractures.

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