Changing of Rock Fragments Equivalent Emissivity and Its Impact on Localized Positive Infrared Brightness Temperature as Rock Fractured

Although thermal infrared imaging has been developed as an important tool of remote sensing rock mechanics (RSRM) since the 1990s, the relationship between infrared emissivity and rock fragments, which is essential for interpreting positive infrared abnormity, has not been investigated. In this letter, the infrared brightness temperature (IBT) ( $T_{b}$ ) of sandstone, marble, and granite, including an intact rock specimen and its fragments of six-level sizes, was experimentally detected with an infrared imaging system indoor. The relationship between the detected $T_{b}$ and the measured size of rock fragments is investigated, and the equivalent emissivity $\varepsilon _{\!\!f}$ of rock fragments of different sizes is obtained. It was discovered that $\varepsilon _{\!\!f}$ of rock fragments rose up to 8.43% compared to that of intact rock and behaved the maximum as the fragment sizes get close to the mineral particle scale (MPS) of the rock. $T_{b}$ is hence locally lifted by 6.16 K. This letter revealed that the rise of $\varepsilon _{\!\!f}$ is a third remote sensing mechanism, besides the known thermoelastic and friction thermal effects, of local $T_{b}$ enhancement in process of rock loaded to fracturing, which provides new experimental supports to infrared imaging detection on rock fracturing and IBT-based stability analysis in rock engineering and geostructures.