Computed Tomographic Evaluation of Earth Materials with Varying Resolutions.

Evaluation of earth materials with computed tomography (CT) provides information on geometrical pore parameters and spatial variability within the structure in two- (2-D) and three-dimensions (3-D) that conventional procedures do not provide.  This information can be used to improve flow and energy-transfer models, assist in development of techniques for storage of materials such as water, and develop web-based training tools to explain structural differences in materials.  This paper describes differences in geometrical pore parameters of two- and three-dimensional reconstructions for soil materials and explains the benefits and weaknesses of these techniques relative to sample size, image resolution, and image analysis.  Tomography for soil samples scanned at 200-, 75-, and 9-µm resolutions were used as model data to accomplish those objectives.  Samples were scanned using a medical scanner, high-resolution scanner, and synchrotron micro-tomography.  Currently available scanning techniques and image analysis software can be used to discriminate differences among soil materials to understand the effects of management, natural causes, and/or a combination of influencing factors.  Selected land-use results indicated that images at higher resolution provide additional information such as path tortuosity, pore connectivity, and pore geometry in 3-D scale while low resolution scanners provided details on number of pores, 2-D features, and area of pores.  Low and high resolution techniques both exhibited the same trend for number of pores although the high resolution technique detected more pores.  Soil samples scanned at a 75- and 9-µm resolution discriminate samples by pore connectivity parameters.  Between the two high resolution images, the 9-µm resolution showed pores with much higher coordination numbers and a stronger relationship between the probability and coordination numbers as compared to the 75-µm resolution.  However, 9-µm resolution may not have estimated the total pore length or volume (longer than the sample size) that were detected by the 75-µm esolution.  The comparison indicated that 200-µm resolution can be used to differentiate treatment effects for a larger number of samples and high resolution could be used to obtain additional information on geometrical parameters for selected samples.  This implies that imaging resolution could be based on parameters of interest.  Analysis of a combination of smaller-sized samples along with larger samples may provide information on more holistic treatments, reaction, and process effects.