Applications of X-ray computed tomography in the geosciences

Abstract

X-ray computed tomography (CT) is a non-destructive technique with wide applications in various geological disciplines. It reveals the internal structure of objects, determined by variations in density and atomic composition. Large numbers of parallel 2D sections can be obtained, which allows 3D imaging of selected features. Important applications are the study of porosity and fluid flow, applied to investigations in the fields of petroleum geology, rock mechanics and soil science. Expected future developments include the combined use of CT systems with different resolutions, the wider use of related X-ray techniques and the integration of CT data with results of compatible non-destructive techniques.X-ray computed tomography (CT) is a nondestructive technique that allows visualization of the internal structure of objects, determined mainly by variations in density and atomic composition. It requires the acquisition of one-or two-dimensional radiographs for different positions during step-wise rotation around a central axis, whereby either the source and detector or the sample are moved. This is followed by the reconstruction of two-dimensional cross-sections perpendicular to the axis of rotation.CT images record differences in the degree of attenuation of the X-rays, which is materialand energy-dependent. The interactions that are responsible for this attenuation are mainly Compton scattering and photoelectric absorption. The contribution of the photoelectric effect depends on the effective atomic number and is especially important at low energies. At high energies, the Compton effect predominates and attenuation is mainly determined by density.X-ray CT was developed as a medical imaging technique in the early 1970s (Hounsfield 1972(Hounsfield , 1973. The possibility of its use in geology and engineering was soon recognised, resulting in large numbers of publications from the early 1980s onwards. Early applications include studies in the fields of soil science (Petrovic et al. In this introductory paper, we present some general information about the technique and a brief overview of its applications in geology, with references to some recent studies. More detailed information can be found in recent review articles by Duliu (1999) and Ketcham & Carlson (2001). Acquisition of transmission dataThe basic components of X-ray CT scanners are an X-ray source, a detector and a rotation system. Various possible configurations exist, whereby the selected configuration is determined by sample size and the desired resolution, besides availability and access restrictions. Ideally, the X-ray beam should be parallel rather than fanor cone-shaped (with a finite size of the origin), in which case the resolution is only determined by detector quality. High resolution (10 ~tm) can also be attained with microfocus X-ray tubes. Another aspect of acquisition is the energy spectrum of the X-ray source. Unless the X-rays are produced by radioactive decay, they are always polychromatic with a wide range in energy. This compli...

Investigation of porosity and permeability effects from microstructure changes during limestone dissolution

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Paradoxical co-existing base metal sulphides in the mantle: The multi-event record preserved in Loch Roag peridotite xenoliths, North Atlantic Craton

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Rôle des liquides sulfurés dans la formation des minéralisations riches en éléments du groupe du platine :

“…However, this technique can also be used for qualitative and quantitative analysis of geological materials (Ketcham and Carlson, 2001) such as metamorphic rocks (Carlson and Denison, 1992;Denison and Carlson, 1997;Denison et al, 1997), sedimentary rocks (Risen and Becker, 2003;Michaud et al, 2003), soils (Delerue et al, 2003), rare meteorites (Kondo et al, 1997), fossils (Brochu, 2000) or ores (Kyle and Ketcham, 2003;Kyle et al, 2004). The details of the principle and the functioning of the X-ray CT have been developed in many papers (Denison and Carlson, 1997;Denison et al, 1997;Ketcham and Carlson, 2001;Mees et al, 2003;Ketcham and Iturrino, 2005) and only a brief description is given in the following. In the present study, the samples were scanned at the MRS (Institut Nationale de la Recherche Scientifique, Quebec, Canada) using an X-ray CT scanner "Siemens Somatom Volume Access".…”