The Importance of the Heel Effect in X-Ray Ct Imaging of Soils

Non-destructive and non-invasive X-ray computed tomography (CT) is increasingly used in environmental geotechnics research. As a result of recent advances in technology and image processing techniques, CT with rapid scanning now has the potential to track changes in soil structure or soil water conditions as they happen, rather than as previously on a specimen in (temporary) stasis. Gathering meaningful data in a short scan time requires compromises to be made on parameters such as exposure time, and / or the use of higher X-ray intensities and energies. Data processing and imaging processing - including the removal of any artefacts, which can cause errors in interpretation of soil structure or phase proportions - then become especially important. One such artefact is the heel effect. It has been recognised in medical imaging, owing to its association with high scan energies. However, it has not previously been identified in soil imaging, despite the trend towards using higher energies. This paper presents an investigation into the potential for the heel effect to affect the soil property determination. It is shown for the first time that a noticeable heel effect will be present in CT images of soils and derived phase proportion data, when certain types of X-ray reflection targets are used. A correction method for the heel effect is presented, use of which will prevent significant errors in derived soil parameters such as water content.