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Evolving to Industrial Strength


devices is often expressed not in terms of pho- tosites, but rather in megapixels. A 1.2-megapixel device, for instance, might have an area of 1,280 x 960 (1,228,800 pixels), while higher resolution would be attained by a 3.1- megapixel device measuring 2,048 x 1,5 36 (3,145 ,728 pixels).


Image resolution can then be affected by the medium on which it is displayed. A rela- tively low-resolution computer monitor might be described as a 640 x 480 display. This means that the monitor has a width of 640 pix- els, spread across a height of 480 lines, totaling 307,200 pixels. If those pixels were spread across a 15 -inch monitor, then any image displayed on that monitor would be allotted 5 0 dots per inch. To increase resolu- tion, either the screen size must be reduced or more pixels must be packed into the screen. Modern applications generally take both approaches, squeezing a huge number of pixels into a smaller area. To image a 3D object, the pixel is expanded into another dimension. A third coordinate (z) is added to the x-y location to precisely define the pixel’s position within the volume of a 3D object, thereby creating a voxel— short for volume pixel. In CT images, the z-coordinate often denotes depth, and is dic- tated merely by the position that a tomographic slice holds within a volume formed by stacking together numerous closely spaced slices (previous page, bottom). In addition to x, y and z coordinates, a voxel can define a point by a given attribute value. In the case of CT scans, that value is density, which is a function of the sample’s trans- parency to X-rays. Density values can be tied to a color spectrum, while a range of intensi- ties can control the opacity of a voxel on a computer screen. With this information and 3D rendering software, a two-dimensional image of a 3D object can be generated for viewing at various angles on a computer screen.


1. Although ex perts m ay correctly assert that photosites are not actually pix els, the term s are b ecom ing increasingly interchangeab le in the popular vernacu- lar, thank s largely to the b road appeal of digital photography , in w hich m anufacturers of digital cam - eras describ e resolution in term s of m egapix els.


Density contrasts within a rock body can be imaged just as they can within a human body (below). By the mid 1980s, CT technology was making significant applications.


inroads into


determination of bulk density of rock samples, CT scanning was adapted to visualize microbial desulfurization of coal, displacement of heavy oil, and oil flow through carbonate cores.7


geoscience In addition to quantitative


It didn’t take long for those outside the medical field to recognize the potential of CT technology for nondestructive evaluation of materials. Geoscientists soon joined the ranks of other researchers, particularly those in the field of materials testing, who sought increasingly finer detail for imaging internal structures. This capability has largely been realized through development of industrial-strength CT systems, which can employ more powerful X-rays, a tighter focal point and longer exposure times than those used in the medical field.8


M ineral


Q uartz Calcite Anhydrite B arite


Celestite


Density, g/ cm3 2 .6 4 2 .7 1 2 .9 8 4 .0 9 3 .7 9


M ineral


G ypsum Dolomite I llite


Chlorite H ematite


Density, g/ cm3 2 .3 5 2 .8 5 2 .5 2 2 .7 6 5 .1 8


> Density values of various m inerals com m only found in sedim entary rock .


X -ray s used to visualize rock structures are affected, in part, b y differences in density and m ineralogy w ithin a sam ple.


In the early days of CT rock scans, it was not unusual for geoscientists to work out agreements with the only institution in town that could provide access to such sophisticated technology. Often in the dark of night, with as little attention as possible, core samples from the oilpatch would be wheeled into the pristine and sterile setting of a hospital CAT-scanning facility for imaging and analysis (below).


7 . K ay ser A, K ellner A, Holzapfel H-W, van der Bilt G, Warner S and Gras R: “ 3 D Visualization of a Rock Sam ple, ” in Doré AG and Vining BA ( eds) : Petroleum Geology : North-West Europe and Glob al Perspectives – Proceedings of the 6th Petroleum Geology Conference. London: The Geological Society ( 2005 ) : 1613 – 1620.


Vinegar HJ : “ X -ray CT and NMR Im aging of Rock s, ”


J ournal of Petroleum Technology 3 8, no. 3 ( March 19 86) : 25 7 – 25 9 .


8. For m ore on high-resolution X -ray CT: University of Tex as High-Resolution X -ray Com puted Tom ography Facility . http: / / w w w . ctlab . geo. utex as. edu/ overview / index . php# anchor1-1 ( accessed J anuary 3 0, 2006) .


> A different k ind of patient. A section of w hole core is placed on a sliding gurney prior to im aging at a hospital CAT-scan facility .


Spring 2006 7


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