Quantitative Evaluation of the Fracturing State of Crystalline Rocks Using Infrared Thermography

• Published in: Rock mechanics and rock engineering 2023-09, Vol.56 (9), p.6337-6355
• Main Authors: Franzosi, Federico, Casiraghi, Stefano, Colombo, Roberto, Crippa, Chiara, Agliardi, Federico
• Format: Article
• Language: English
• Subjects: Abundance; Boundary conditions; Capacitance; Civil Engineering; Computed tomography; Cooling; Cooling curves; Crystalline rocks; Earth and Environmental Science; Earth Sciences; Environmental conditions; Fracturing; Geophysical methods; Geophysics/Geodesy; Gneiss; Infrared imaging; Laboratories; Laboratory experimentation; Lithology; Mechanical properties; Methods; Mica; Nondestructive testing; Original Paper; Parameters; Physics; Porosity; Quantitative analysis; Random sampling; Rock; Rock masses; Samples; Schist; Schists; Sediment samples; Shape; Statistical analysis; Statistical sampling; Thermal response; Thermography; Three dimensional models
• ISSN: 0723-2632; 1434-453X
• DOI: 10.1007/s00603-023-03389-x

The fracturing state of rocks is a fundamental control on their hydro-mechanical properties. It can be quantified in the laboratory by non-destructive geophysical techniques that are hardly applicable in situ, where biased mapping and statistical sampling strategies are usually exploited.  We explore the suitabilty of infrared thermography (IRT) to develop a quantitative, physics-based approach to predict rock fracturing starting from laboratory scales and conditions. To this aim, we performed an experimental study on the cooling behaviour of pre-fractured gneiss and mica schist samples, whose 3D fracture networks were reconstructed using Micro-CT and quantified by unbiased fracture abundance measures. We carried out cooling experiments in both controlled (laboratory) and natural (outdoor) environmental conditions and monitored temperature with a thermal camera. We extracted multi-temporal thermograms to reconstruct the spatial patterns and time histories of temperature during cooling. Their synthetic description show statistically significant correlations with fracture abundance measures. More intensely fractured rocks cool at faster rates and outdoor experiments show that differences in thermal response can be detected even in natural environmental conditions. 3D FEM models reproducing laboratory experiments outline the fundamental control of fracture pattern and convective boundary conditions on cooling dynamics. Based on a lumped capacitance approach, we provided a synthetic description of cooling curves in terms of a Curve Shape Parameter, independent on absolute thermal boundary conditions and lithology. This provides a starting point toward the development of a quantitative methodology for the contactless in situ assessment of rock mass fracturing. Highlights We use Infrared Thermography (IRT) to quantify the cooling behaviour of rocks with different fracturing states in the laboratory. Rock samples characterized by higher fracture intensity / porosity cool faster than less fractured rocks. We model experimental cooling curves with a lumped capacitance equation and derive a Curve Shape Parameter independent on boundary conditions and lithology. The Curve Shape Parameters shows clear correlations with fracture abundance measures. Results lay the foundations to develop an upscaled methodology to predict the fracturing state of rock masses.