Determining rock discontinuity fracture aperture and infilling characteristics using 3D X ray tomography with automated approach and multi method validation

This paper presents an automated method for calculating the aperture of rock discontinuities using X-ray computed tomography data. The methodology was applied to two distinct calcareous samples of the Scaglia Rossa and Maiolica geological formations (Umbria-Marche stratigraphic succession, Italy), exhibiting different discontinuity characteristics. In one sample, an open, unfilled fracture was analyzed to determine its aperture. For the second sample, characterized by fractures filled with clay minerals, the thickness of the infilling material was evaluated. After extracting the 3D fracture network with the U-Net neural network, we applied an automated method to calculate aperture or infilling thickness. The method is based on a Matlab algorithm for 2D image analysis combined with a curve-fitting technique. Aperture/thickness was computed across three distinct volume portions of the samples – top, center, and bottom – and across the entire volume, with statistical analyses performed on all sections. This approach allows for the determination of potential variations in aperture/thickness at the sample scale. Additionally, we characterized the infilling material using X-ray diffractometry, Energy Dispersive Spectroscopy, and optical petrographic analysis. Furthermore, the contrast in grey levels between the rock matrix and the discontinuity was examined using 2D tomography images. Tomographic results were validated against measurements from a high-resolution digital microscope and a manual caliper. This comparison highlights the value of integrating multiple techniques to improve accuracy and reliability in aperture and thickness determination.