Representative elementary volume estimation for porosity, moisture saturation, and air-water interfacial areas in unsaturated porous media: Data quality implications

Achieving a representative elementary volume (REV) has become a de facto criterion for demonstrating the quality of mu CT measurements in porous media systems. However, the data quality implications of an REV requirement have not been previously examines. In this work, deterministic REVs for porosity, moisture saturation (S(W)), and air-water interfacial area (A(I)) were estimated using a set of 49 mu CT images of eight unsaturated homogeneous porous media with heterogeneity in moisture distributions present in varying degrees. Estimated porosity REVs were <8 mm(3) for all cases, smaller than typical mu CT image sizes (similar to 100 mm(3)). Estimated S(W) and A(I) REVs were <55 mm(3) for cases with homogeneous moisture distributions but could not be estimated for cases with heterogeneous moisture distributions, due to the absence of a distinct "REV plateau" within the maximum imaged volume. Conventionally, S(W) and A(I) data from such non-REV cases would be excluded. The implications of excluding data on the basis of REV were examined by comparing A(I)-S(W) data measured on image windows of increasing size against the expected linear A(I)-S(W) relationship. At measurement scales exceeding porosity REV, random fluctuations in A(I)-S(W) data were excluded, even for cases containing heterogeneous moisture distributions. In contrast, requiring measurement scales to exceed S(W) and A(I) REV appeared overly restrictive and resulted in visible loss of reliable A(I)-S(W) data. We attribute these findings to overestimation of REVs due to inherently problematic estimation of deterministic REVs in real systems. Implications of these findings for ensuring mu CT data quality and the efficient use of mu CT data are discussed.