Abstract

The engineering properties of a geologic material are greatly affected by the presence of macropores. Previous research has demonstrated that the size, location, and proximity of macropores influences both the strength and stiffness of specimens. Knowledge of the distribution of macropores in a specimen prior to testing would be useful for a number of reasons. We are currently developing a non-destructive method called cross-specimen acoustic tomography (CSAT) to determine the number, location, and size of the macropores in a laboratory specimen. The CSAT method uses a set of piezoelectric sensors that generate and receive high frequency acoustic waves. We measure the travel times of the acoustic waves through a specimen and then use a commercially available tomography software package to invert the data. The inverted velocity model is in turn used to locate the voids within the specimen. The verification of two dimensional (cross-sectional) results from plaster specimens containing large macropores of Styrofoam show the technique is promising and worthy of further development.

INTRODUCTION

Previous research has shown that the presence of macropores (large void spaces) has a significant effect on the structural behavior of geologic materials. Prior studies have used laboratory produced brittle specimens [1], numerical modeling [2], and real rock specimens in attempts to quantify the effects of large voids on engineering properties [3]. These studies showed that at the same level of macroporosity there can be large differences in strength and stiffness. This implies that the size, shape, and location of macropores within a specimen control the engineering properties of the specimen. As such, determining the size, shape, and location of macropores in a rock specimen is important for evaluating the laboratory test results.

There are a number of common imaging techniques available to characterize the internal structure(s) within geological specimens. These techniques include X-ray computed tomography [4], magnetic resonance imaging [5], ultrasound [6], and computerized axial tomography [7]. A comprehensive discussion of such techniques is beyond the scope of this paper. However it is important to note that the above techniques are medical imaging techniques which require expensive equipment utilizing highly skilled personnel for operation.

The motivation for this work is to develop a simple yet robust non-destructive method to characterize macropores within laboratory specimens prior to destructive testing. Characterizing the macropores would include quantitatively determining the size, shape, and location of macropores. The technique that is being developed has been named cross-specimen acoustic tomography (CSAT). The goal is to be able to use the CSAT method to characterize the macropores and use the information to gain an understanding of how the macropores influence the variability of the engineering properties of macroporous rock. For this initial study, we used three plaster of Paris cylindrical specimens (15.2 cm diameter by 30.5 cm length) containing a known number and size of Styrofoam inclusions.

THEORY

Elastic wave tomography is a technique that has been used in structural and geotechnical engineering to determine the locations of inclusions and defects in materials such as concrete [6].

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