Thermal Remote Sensing from UAVs: A Review on Methods in Coastal Cliffs Prone to Landslides

Abstract

Coastal retreat is a non-recoverable phenomenon that—together with a relevant proneness to landslides—has economic, social and environmental impacts. Quantitative data on geological and geomorphologic features of such areas can help to predict and quantify the phenomena and to propose mitigation measures to reduce their impact. Coastal areas are often inaccessible for sampling and in situ surveys, in particular where steeply sloping cliffs are present. Uses and capability of infrared thermography (IRT) were reviewed, highlighting its suitability in geological and landslides hazard applications. Thanks to the high resolution of the cameras on the market, unmanned aerial vehicle-based IRT allows to acquire large amounts of data from inaccessible steep cliffs. Coupled structure-from-motion photogrammetry and coregistration of data can improve accuracy of IRT data. According to the strengths recognized in the reviewed literature, a three-step methodological approach to produce IRTs was proposed.

An Infrared Thermography Approach to Evaluate the Strength of a Rock Cliff

“…The rock strength, defined as the resistance to permanent deformation by flow or fracture, is often estimated by the Schmidt (rebound) hammer test [8][9][10][11][12][13][14]. Direct surveying and groundbased monitoring, however, can be unfeasible if the rock outcrops are located in inaccessible areas, such as steep mountain ridges or coastal cliffs [15][16][17][18]. Efforts are therefore being made on formulating alternative methods relying on remote sensing techniques for the definition of input data for empirical and physically-based models [19][20][21][22][23][24].…”

“…The potential of IRT in geosciences has been first demonstrated in the seminal work of Hudson [42]. Since then, several applications have been proposed thanks to the technological development of thermal sensors and acquisition systems [16]. Volcano monitoring [43][44][45][46], underground mining [36], cave exploration [47], and geothermal analyses [48] can benefit from IRT technologies.…”

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

“…Additionally, when there is a lack of encoded metadata containing exterior orientation parameters of cameras (as in this case study) target positioning represents the only solution for a reliable 3D reconstruction of the scanned scene. Consequently, when inaccessible or high-risk sites represent monitoring areas, especially in engineering geological issues, the task of target positioning may not be feasible [45,46]. A possible approach to partially solve this problem is the employment of UAV platforms equipped with Real Time Kinematic Global Positioning Systems (RTKGPS) [47] that can provide camera positions with high accuracy (i.e., errors in the order of centimeters).…”

Integrating Drone Technology into an Innovative Agrometeorological Methodology for the Precise and Real-Time Estimation of Crop Water Requirements

“…The thermal camera used in the present study was the DJI Zenmuse XT2 (12 MP), which incorporates a high-resolution forward-looking infrared FLIR Tau 2 thermal sensor (FLIR Systems, Wilsonville, OR, USA; resolution 640 × 512 pixels, lens 9 mm; thermal sensitivity of <50 mK) and a 4 K visual camera (1/1.7" active-pixel CMOS) with a leading stabilization and machine intelligence technology (Figure 9c) [60]. In general, the Zenmuse XT2 camera used in the present study is a very high technological and sophisticated camera that performs radiometric calibration of the acquired thermal images through its advanced digital system, which records the absolute temperature.…”