Pore geometry imaging and its quantitative description is a key factor for advances in the knowledge of physical, chemical and biological soil processes. More specifically it is a powerful tool for to evaluate , for example, changes in soil structure due to different irrigation and soil tillage practices.Earlier 3D representations of the internal structure of the soil have been obtained using medical tomographic systems (NMR and X-ray CT). However, images provided using such equipments, show strong limitations in terms of spatial resolution. In the last decade very good results have then been obtained using imaging from very expensive systems based on synchrotron radiation.More recently, X-ray Micro-Tomography has resulted the most widely applied in scientific community being the technique showing the best compromise between costs, resolution and size of the images.Conversely, the conceptually simpler but destructive method of “serial sectioning” has been progressively neglected for technical problems in sample preparation and time consumption to obtain an adequate number of serial sections for correct 3D reconstruction of soil pore geometry.

INTRODUCTION

In this work main results from a comparison between the following two techniques of 3D image reconstruction of soil are reported:

1.X-ray microtmography2.“Serial Sectioning” or Mechanical Tomographyin order to point out advantages, shortcomings and

to define the different potential.

AIMS

A cylindrical undisturbed soil sample 6.5cm in diameter and 6.5cm height of an Ap horizon of an alluvial soil collected in vineyards at EER-INTA in Lujan de Cuyo, Mendoza, has been reconstructed using both a desktop X-ray micro-tomograph Skyscan 1172 and the new automatic serial sectioning system SSAT (Sequential Section Automatic Tomography) set up at CNR ISAFOM in Ercolano (Italy) with the aim to overcome most of the typical limitations of such a technique.

MATERIAL AND METHODS

3D imaging of soil pore network: two different approachesM. Matrecano1, B. Di Matteo1, G.Mele1, and F. Terribile2,

(1) CNR ISAFOM, Soil Physics, Ercolano NA, Italy (giacomo.mele@cnr.it), (2) DISSPAPA,Università Federico II. Napoli, Italy

European Geosciences Union - General Assembly 2009 - Vienna, Austria, 19 – 24 April 2009SSS37 - X-ray Computed Tomography in Geo-sciences: 3D visualization and quantification

sorgente

campione

detector

TECHNIQUE

It is an ultimate desktop x-ray microtomograph.The cone micro-focus beam source is based on a tungsten X-ray tube having focal spot of 7 and 5 microns (at 10W and 4W isowatts respectively). Voltage can be set from 20 to 100 kV with current which meets 250mA. Il detector is an high resolution CCD camera (4000x2624 pixels) coupled with a FOS (Fiber Optic plate with X-ray Scintillator) which allows high X-ray sensitivity and wide acquisition area.System has copper and aluminum filter plates which can be used if needed in order to increase the tungsten energy spectrum.

Image reconstruction software is based on last generation algorithms which apply both “convolution” and “back-projection” procedures. Filtering and correction procedures for “ring artifact” and “beam hardening” (due to the polychromatic source) are included in order to enhance accuracy of the reconstructed images. Rotation of the sample is variously programmable and can be set specifically to avoid artifact due to metal materials in the sample.

From“projection” images , sections normal to the rotating axis are reconstructed by reiterate calculation of inverse Radon transforms according to the different incidence angles of the X-ray beams.This procedure, high demanding in terms of computing time, is called also “back projection” alghorithm.Images of trasversal sections are then “superimposed” for three-dimensional geometry reconstruction.

A sequence of “radiographies” of a rotating undisturbed soil sample are done obtaining projection images of its x-ray- attenuation capacity at different angles of rotation (such a property is strongly related to the density).

The X-ray Microtomograph SKYSCAN 1172

“Serial Sectioning” or Mechanical Tomography

The SSAT has been set up at Istituto per i Sistemi Agricoli e Forestali del Mediterraneo – CNR, Ercolano NA, Italy.An high precision surface grinder Jones&Shipman 540X (already used to produce soil thin sections) is coupled with a lightning and cleaning system, and a digital image acquisition unit.This latter is a digital reflex camera Nikon D200 mounting AF Micro-Nikkor 60mm f/2.8D optic. UV (365nm) lightning is provided by two Spectroline lamps each one having two 8W tubes. It is possible to use also different illumination sources (eg. Visible, IR etc…)Cleaning system of the soil surface from cooling oil consists of a wiper blade driven by an air actuated piston. The whole working flow has been completely automated by interfacing each unit with an electronic board specifically designed and assembled. Functioning and control of the system has required a specific software..Up to 2000 grinding and acquisition cycles can be autonomously performed. At each grinding cycle it is possible to control up to three different lightning devices also in combination. Hence up to 8 image acquisitions per cycle can be done allowing each one a different spectral response in order to identify also soil matrix features different from the simple solid pore phases.

Flat layers of soil are successively removed from an undisturbed sample impregnated with fluorescent resin. Upper surfaces sequentially exposed are lightened using UV lamps, digital images are acquired, binarized and, finally, superimposed in order to obtain the 3D image reconstruction.This technique destroys the sample and, besides the time needed for impregnation and polymerization, it is usually very tedious and time consuming for the repetitive manual operations of grinding, cleaning of polished surfaces, illumination and image acquisition.

Specific SSAT artifact is a certain unalignment between consecutive sections. This problem can be easily overcome via software.Defects in fluorescent resin impregnation of the sample can occur.

SSAT (Sequential Section Automatic Tomography)

SECTIONS, TYPICAL ARTIFACTS, RECONSTRUCTION

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Pore size(mm)

Pore size distribution Comparison between pore size distribution measurments from image analysis using “successive opening” algorithm has been performed on both reconstructed volumes without application of any filter or image enhancement. It has showed an overall larger porosity in the case of the SKYSCAN 1172 for the pore size classes up to 1.4 mm.Higher porosity values have resulted in the case of SSAT for pore size classes larger than 1.4 mm.Such a behaviour can be partially explained by the the more noisy images generated from the SKYSCAN 1172 which produce a number of false particles. These latter partition larger pores assigning them to lwer size classes. Difference in total porosity is mainly due to the different voxel resolution achieved with the two equipments.

FURTHER COMPARISON ELEMENTS

CostMainteinance cost

per year

Time (man) for sample

preparation

Time (machine) for 3D image acquisition

SKYSCAN 1172 € 250.000 € 5.000 0.5 hours 1-36 hours

SSAT € 80.000 € 2.000 3.5 hours 24-60 hours

SKYSCAN 1172 X-ray micro-tomograph can provide 3D images of better resolution than the SSAT on samples having the same size. The SSAT produces less noisy images specially if large samples have to be reconstructed.

SSAT has showed better flexibility in sample size although both techniques allow to investigate REVs (Representative Elementary Volumes) for most of the macroscopic properties which describe soil physical-chemical processes.

Undoubted advantages of the X-ray Skyscan 1172 micro-tomograph are the not-destructivity and the ease of preparation of the soil samples while the SSAT can exhibit lower overall costs and its potential to provide three-dimensional maps of other soil features different from the simple solid/ pore phases .

Both approaches does not show differences in terms of image analysys procedures which can be applied . Only different minor specific image pre- processing is needed.

CONCLUSION

X-ray Microtomography

Polichromatic X-ray cone beam source

Inverse transform of RADON (Kak, A. C. and Slaney, M. Principles of Computerized Tomographic Imaging. IEEE Press, 1988. )

SECTIONS, TYPICAL ARTIFACTS, RECONSTRUCTION

Reconstruction procedure can produce typical “ring artifacts” (see picture on the left). Shining spots are also possible rising clear strikes if very dense particles are present in the samples. Such phenomena are due to the “beam hardening” of the polychromatic sources. These artifact can be successfully corrected via software.

Max (cylindrical) volume:Diameter 6.8 cm x Height 6.8 cm with a resolution of 8.6mm per voxel.Best nominal resolution is about 1mm (using lower sample size).

Total Porosity: 36%

TECHNIQUE

General scheme of the tomographyc system

Control hardware

Software interfaceFlux of automatic procedures

Resolutioon of the reconstructed volume: 25mm per voxel.Best resolution (for smaller volumes) is about 7mm.Max reconstructable volume : 14x14x25 cm

Total Porosity: 31%