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Scintillator + CCD


Approximately 1 .5 meters Rotation stage


X -ray source


> A high-resolution X -ray tom ography device at The Australian National University . The rotating sam ple stage and charge-coupled device ( CCD) cam era slide on a track , enab ling adj ustm ent of the distance b etw een the cam era, sam ple and X -ray source. With this device, a sam ple can b e m agni ed from 1. 1 to m ore than 100 tim es its original size. The stage rotates w ith m illidegree accuracy and can


b e  tted w ith  uid pum ps for im aging  ow through porous m edia. ( Figure courtesy of The Australian National University . )


image a 60-mm specimen with a 30-micron pixel size. They can also zoom in for high-resolution scanning to image a 4-mm specimen with 2-micron pixels. Approximately 3,000 projections are needed to generate a 2,0483 voxel tomogram. Between each projection, the sample stage is rotated 0.12° . The entire process takes 12 to 24 hours, depending on the type of sample and the filtering steps required to reduce sampling artifacts. The resulting 24 gigabytes of projection data are


9 . Ab b reviations for m icrocom puterized tom ography range from µ CT ( w here the Greek letter m uis a standard sy m b ol for the pre x “ m icro” ) to uCT ( w here u is a sub stitute for m u) to m CT ( w here the m stands for m icro) to X MT for X -ray Microtom ography .


10. K ay ser A, Gras R, Curtis A and Wood R: “ Visualizing Internal Rock Structures: New Approach Spans Five Scale-Orders, ” Offshore64 , no. 8 ( August 2004 ) : 129 – 13 1.


11. K etcham RA and Carlson WD: “ Acq uisition, Optim ization and Interpretation of X -Ray Com puted Tom ographic Im agery : Applications to the Geosciences, ” Com puters & Geosciences27 , no. 4 ( May 2001) : 3 81– 4 00.


12. Sak ellariou A, Saw k ins TJ , Senden TJ and Lim ay e A:


“ X -Ray Tom ography for Mesoscale Phy sics Applications, ” Phy sica A3 3 9 , no. 1-2 ( August 2004 ) : 15 2– 15 8.


Sak ellariou A, Saw k ins TJ , Senden TJ , K nack stedt MA, Turner ML, J ones AC, Saadatfar M, Rob erts RJ , Lim ay e A, Arns CA, Sheppard AP and Sok RM: “ An


X -Ray Tom ography Facility for Q uantitative Prediction of Mechanical and Transport Properties in Geological, Biological and Sy nthetic Sy stem s, ” in Bonse U ( ed) : Developm ents in X -Ray Tom ography IV, Proceedings of SPIE— The International Society for Optical Engineering, Vol. 5 5 3 5 . Bellingham , Washington, USA: SPIE Press ( 2004 ) : 4 7 3 – 4 7 4 .


13 . This test eq uipm ent includes pum ps or other devices used to study  uid  ow or m echanical com paction.


14 . Rather than ex posing  lm to light, CCD technology captures im ages in a techniq ue sim ilar to com m on digital photography . A CCD uses a thin silicon w afer to record light pulses given off b y a scintillator. The CCD silicon w afer is divided into several thousand individual light-sensitive cells. When a light pulse from the scintillator im pinges on one of these cells, the photoelectric effect converts the light to a tiny electrical charge. The charge w ithin a cell increases w ith every light pulse that hits the cell. Each cell on the CCD silicon


w afer corresponds in size and location to an im age pix el. The pix el’ s intensity is determ ined b y the m agnitude of the charge w ithin a corresponding cell.


processed by supercomputer, and it takes 128 central processing units about 2 hours to generate the tomogram.


Visualization Technology


Once individual radiographic projections have been compiled into a 3D data volume file, the data can be loaded into an immersive visuali- zation environment for detailed examination. With Inside Reality virtual reality technology, the data can be imaged and manipulated like any other volume of 3D data. Originally developed to help visualize seismic volumes based on miles or kilometers of data, Inside Reality technology can also handle data volumes based on much finer, submillimeter scales.


Geoscientists utilize this advanced visuali- zation technology to view a data volume from any direction. This capability enables bedding planes


and fracture planes of rock samples to be viewed orthogonally, even when the physical sample has been cut obliquely to these planes. Sedimentary and structural features of the rock sample are typically analyzed in the form of slices or transparency views through a volume. While the scanning process relies on density differences to distinguish features within a sample, the visualization process depends largely on opacity


differences. One way to expose features deep within a volume comprising millions of voxels is to render surrounding voxels invisible.


Opacity rendering is the key to


visualization. Each voxel is assigned a value along a transparency-opacity spectrum, thus making some voxels stand out while others fade away. Without this capability, the opacity of outer voxels would hide all features lying within the volume. Voxel-based technology can be used to determine the volume and geometry of rock grains, cement, matrix and pore space within a sample. Using Inside Reality opacity-rendering tools, geoscientists can assign different values of the opacity-transparency spectrum to various components within a volume. This technique allows


geoscientists materials of different density


to distinguish between values. For


example, the distribution of cement between mineral grains shows up as a distinctive color, while setting pore space to zero-opacity makes it transparent, thus showing the spaces between grains. This allows the viewer to separate rock grains from cement, matrix and pore space to reveal internal sedimentary and structural features (below).


> Sandstone pores. An opacity  lter is used to render different features in volum e w indow s using Inside Reality softw are. The left w indow ab ove and b ehind the y ellow arrow show s only q uartz grains


( light green) in this eolian sandstone from the Rotliegendes form ation in Germ any . A volum e show ing only pore space ( b lue) is in the b ack ground on the right. The sm aller volum e in the foreground on the right show s late diagenetic b arite cem ent ( red) . The slice m ak ing up the b ase im age indicates q uartz


( gray ) , pore space ( b lue) , b arite ( red) and carb onate cem ent ( orange) . The y ellow arrow for scale is 1 m m long.


Spring 2006


9


1.0 m m


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