Abstract. The adequacy for laboratory testing of four dolomite cores from the Culebra Dolomite of the Rustler Formation at the Waste Isolation Pilot Plant near Carlsbad, NewMexico, were evaluated using representative elementary volume (REV) theory. Gamma ray computerized tomography created three-dimensional grids of bulk density and macropore index over volumes from 1.4 x 10 -7 to 1.6 L. Three different methods for both volume averaging and REV analysis were applied and compared. Both density and macropore index converged to single values with increasing volume, which meets the most common qualitative definition of a REV. Statistical test results for the relatively homogeneous samples indicate that volumes larger than 1 to 7 mL have constant properties. Contrarily, a highly varied sample required 250 and 373 mL to achieve invariant density and macropore characteristics, respectively. Prismatic volume averaging was found to be better than slice averaging, while a qualitative test for the REV provided similar results as a rigorous statistical method. All cores were larger than the REV but were significantly different from one another. This implies that multiple cores are necessary to determine the entire range of transport properties within the rock. In practice, the concept has also been applied to characterize both nonhomogeneous porous media and large-scale properties in fractured media [Bear, 1993]. In those cases a REV was defined for both matrix and fractures. Expansion of the definition has added complications to sampling, testing, and verification procedures and has produced some debate. As Baveye and Sposito [1984] observed, while intuitively appealing, no known data had been presented to quantify the dimension and operational significance of the REV. Only recently, have Buchter et al. [1994] extrapolated limited two-dimensional porosity data to directly describe a three-dimensional REV relationship, and Clausnitzer and Hopmans [1999] presented a simple REV plot for glass beads. Thus, while almost every advanced groundwater textbook has a hypothetical plot of the REV, there is little insight on the actual shape of the REV curve, much less wisdom into how the concept may be applied.To some extent the concept of the REV has become obsolete because of the recent advances in characterization of spatial variability. We no longer expect to represent an aquifer or soil with a single value of porosity and conductivity but instead model those systems with complex stochastic representations. One may rightly ask if the REV concept holds any value beyond the narrow distinction between pore-scale processes and the bulk porous medium. To that, we answer the concept is still important in the design and interpretation of laboratory col-1199

Abstract. Tomographic images of porous media are complex distributions of linear attenuation coefficients that reflect the combined effects of scanning spatial resolution, photon statistical measurement errors, and true material densities. We address how the true voxel-scale attenuation distribution and measurement errors are convoluted to yield measured density frequency distributions. A deconvolution algorithm is demonstrated that uses the measured density frequency distributions and known photon statistical errors to quantify average cross-section volume contents of pure components and a mixedcomponent phase. The mixed-component phase represents regions where components are intertwined or varied in spaces smaller than the scanning resolution. This approach is applied to a complex core of the Culebra Dolomite Member of the Rustler Formation collected at the Waste Isolation Pilot Plant, near Carlsbad, New Mexico. The methodology provides a quantitative measure of the volume content of gypsum, dolomite, and mixedcomponents, and heterogeneity in the sample. This research demonstrates how porous media compositions can be quantified by applying a statistical approach to tomographic measurements obtained from gamma ray computerized tomography (gamma CT). More importantly, the relationship between the density frequency distribution and the component volume content and heterogeneity in a sample can be determined. These procedures are general in nature and may also be applied to X ray and synchrotron images, subject to the limitations of the specific machine and porous media system. Tomography Measurements