ABSTRACT: Experimental studies on fracture flow processes are vital to understand the dependence of fracture permeability on various complex processes. A key parameter in such experiments is the fracture aperture, which can be determined using light transmission techniques when using transparent fracture models. For this, scientific-grade monochrome cameras are used to record light intensity under different conditions. The output data of scientific cameras is linearly related to the radiance scene, which allows one to do quantitative analysis using such data. On the contrary, consumer-grade digital (CGD) cameras usually output nonlinear color images (e.g. JPG or TIFF files), which are not suitable for most scientific uses. In this paper, we outline an image analysis procedure to use CGD cameras for light intensity measurements. We demonstrate that the direct use of raw data from such cameras is adequate for scientific quantification and prevents the introduction of artefacts from further image processing that usually occurs within the built-in software of most cameras.
1. INTRODUCTION
Experimental studies have greatly contributed to our understanding of the dependence of fracture permeability on various complex processes (e.g. fracture deformation, dissolution). Recently, universal scaling relationships between fracture stiffness and fluid flow have been proposed (Pyrak-Nolte and Nolte, 2016). Theoretical and numerical investigations have led to a number of models, for instance, for fracture deformation and closure (e.g. Kling et al., 2018), and corrections to the widely used Local Cubic Law (Brown, 1987). Yet, experimentation remains an essential part of fracture flow studies since it can validate the models, point out inconsistencies of the models, and reveal new physical aspects. Important challenges in experimental investigations of fracture flow are related to multiple processes occurring simultaneously, as well as specimen heterogeneity and test repeatability.
Systematic investigations require controlled variation of the geometric- and mechanical properties of the fractured specimens. Certainly, numerical simulations can address the above-mentioned issues. However, the validity of such simulations depends on a series of assumptions that not always hold. The use of fracture replica and fracture analogs presents a great advantage here, it provides the possibility of efficient specimen replication, so one can conduct systematic experimental investigations while controlling the geometric and material properties.
