3D Thermal Monitoring of Jointed Rock Masses through Infrared Thermography and Photogrammetry

Guglielmo Grechi,Salvatore Martino,Gian Marco Marmoni,Matteo Fiorucci

doi:10.3390/rs13050957

Abstract

The study of strain effects in thermally-forced rock masses has gathered growing interest from engineering geology researchers in the last decade. In this framework, digital photogrammetry and infrared thermography have become two of the most exploited remote surveying techniques in engineering geology applications because they can provide useful information concerning geomechanical and thermal conditions of these complex natural systems where the mechanical role of joints cannot be neglected. In this paper, a methodology is proposed for generating point clouds of rock masses prone to failure, combining the high geometric accuracy of RGB optical images and the thermal information derived by infrared thermography surveys. Multiple 3D thermal point clouds and a high-resolution RGB point cloud were separately generated and co-registered by acquiring thermograms at different times of the day and in different seasons using commercial software for Structure from Motion and point cloud analysis. Temperature attributes of thermal point clouds were merged with the reference high-resolution optical point cloud to obtain a composite 3D model storing accurate geometric information and multitemporal surface temperature distributions. The quality of merged point clouds was evaluated by comparing temperature distributions derived by 2D thermograms and 3D thermal models, with a view to estimating their accuracy in describing surface thermal fields. Moreover, a preliminary attempt was made to test the feasibility of this approach in investigating the thermal behavior of complex natural systems such as jointed rock masses by analyzing the spatial distribution and temporal evolution of surface temperature ranges under different climatic conditions. The obtained results show that despite the low resolution of the IR sensor, the geometric accuracy and the correspondence between 2D and 3D temperature measurements are high enough to consider 3D thermal point clouds suitable to describe surface temperature distributions and adequate for monitoring purposes of jointed rock mass.

Highlights

The analysis of the mechanical effects in terms of induced strains by cyclical thermal stresses on rock masses is considered as a nontrivial issue in the field of geological risk mitigation relative to slope instabilities that can lead to high hazard scenarios due to their impulsiveness and high frequency of occurrence
A preliminary attempt was made to test the feasibility of this approach in investigating the thermal behavior of complex natural systems such as jointed rock masses by analyzing the spatial distribution and temporal evolution of surface temperature ranges under different climatic conditions
Through the Through the observation the rock block from it different angles, it is possible to monitor observation of the rock blockoffrom different angles, is possible to monitor the evolution of the evolution temperatures for the whole the prestemperatures forofthe whole monitoring period,monitoring evidencingperiod, how theevidencing presence ofhow differently ence of surfaces differently oriented surfaces leads to a marked heterogeneity the surface oriented leads to a marked heterogeneity of the surface thermaloffields (Figurether5), especially during the5), maximum insolation stages (i.e., 11:30)

Summary

Introduction

The analysis of the mechanical effects in terms of induced strains by cyclical thermal stresses on rock masses is considered as a nontrivial issue in the field of geological risk mitigation relative to slope instabilities that can lead to high hazard scenarios due to their impulsiveness and high frequency of occurrence. Thermal expansion–contraction cycles cause the stress fields to undergo perturbations that can induce both the growth of pre-existing cracks and the genesis of new ones in a subcritical regime (i.e., subcritical crack-growth), which is commonly associated with constant or cyclic loading [1] These preparatory processes can induce cumulative inelastic deformations causing rock mass damage, driving it toward shallow slope failure (e.g., rockfalls and rocktopples) when transient stressors (triggers) occur [4,5]. Due to the severity and continuity of such external stressors, the analysis of thermomechanical effects on jointed rock masses requires the complete and exhaustive characterization of near-surface thermal fields With this approach, the intensity and the evolution in time and space of temperatures can be constrained with respect to local climatic conditions, morphological features (i.e., surface irregularities and differently exposed surfaces) and jointing conditions of the target under investigation. Within this framework, infrared thermography (IRT) is considered one of the most useful tools for the characterization and monitoring of near-surface temperatures

Methods

Results

Discussion

Conclusion