Mapping fracture aperture as a function of normal stress using a combination of casting, image analysis and modeling techniques

A combination of casting, image analysis and modeling techniques have been used to produce aperture maps of natural fractures under a normal load. Images of the fracture casts are corrected for trends introduced by the inhomogeneity of the light source, and a precision-engineered mold is used to make a calibration wedge for each fracture that is used to determine the relationship between grey-level and aperture. The resulting aperture maps are input to an analytical model that analyzes how the void spaces deform under stress. For natural fractures in granite core samples, results from the model are compared to LVDT measurements of displacement across the fracture, and maps of contact area generated via image analysis of the fracture casts. The laboratory measurements and modeling results both indicate the importance of accounting for the deformation of fracture surfaces in calculating changes in fracture aperture with stress; the aperture maps obtained via image analysis alone, assuming that aperture changes uniformly across the fracture, over-estimate contact area and do not accurately portray changes in void geometry that occur under load. The research demonstrates the important role that image processing can play in understanding and modeling the hydromechanical behavior of natural fractures, and the importance of building a methodology for transforming images into physically meaningful data. © 1997