Use of gamma-ray geotomography to measure dry bulk density and unsaturated flow through tuff

Abstract

Gamma-ray geotomography was used to measure the dry bulk density and porosity distributions in two cores of slightly-welded tuff. The cores were then subjected to a point or non-point source of water, and water movement through the cores was observed using tomography. Three numerical methods, the Algebraic Reconstruction Technique (ART), modified ART, and Singular Value Decomposition (SVD) were used to reconstruct the tomographs, and SVD was found to be superior for the simple tomographs of this study. Several tomographic models were used to determine the maximum image resolution from a given set of data. The effects of increasing the number of gamma-ray measurements on resolution were also studied. Finally, the tomographs were compared with laboratory measurements for validation. The tomographically predicted dry bulk density distributions were similar to the measured dry bulk density distributions within each core. However, the percent saturation tomographs were found to be inaccurate when water was rapidly moving through the cores because of the considerable time required to obtain the numerous gamma-ray attenuation measurements.