APPLICATIONS OF UAV THERMAL IMAGERY FOR ROCKFALL HAZARD MAPPING: A CASE STUDY FROM ZION NATIONAL PARK, UTAH

Drones have a well-established utility in hazards mapping using overlapping imagery to generate 3D orthomosaics. However, thermal unmanned aerial vehicle (UAV) surveys are an underutilized tool. In this case study, we used an Autel Evo II 640T equipped with an Uncooled VOx Microbolometer Infrared (IR) camera to examine a rockfall-prone area susceptible to thermal cycling.

The Utah Geological Survey (UGS) performed UAV surveys of Cable Mountain in Zion National Park following two rockfall events occurring in August 2019 and November 2023. Cable Mountain is a tall (~730 m from UAV launch site), northwest-facing cliff composed of massive sandstone— a landscape that is extremely challenging for automated flight plans even when using advanced software. Because of this, all flights required manual piloting for safety and efficiency. In April 2024, UGS pilots implemented the first UAV lidar flight of Cable Mountain accompanied by three IR UAV surveys to examine effects of thermal cycling. We targeted times of the day that would show the greatest differences in thermal contrast: early morning no sun, afternoon maximum sun, and full shadow after sunset.

In addition to addressing the challenges of this field area from a piloting perspective, we developed a workflow to create thermal orthomosaics, which are used to identify thermal anomalies rather than a measure of absolute temperature. In the thermal orthomosaics, the morning flight showed fairly even temperature with little contrast (range of ~5°C), whereas the afternoon flight showed the highest thermal contrast (range of ~25°C) when exposed to full sun. Several possible factors could influence the thermal signal, including aspect, rock color, and air circulation in open fractures. Comparing the thermal orthomosaics with other UAV imagery helps to explain certain anomalies while repeat flights could further help to isolate and track open fractures.

As a result of our outcrop mapping, we determined that the advantage of the Autel 640T over other thermal UAV systems is the fully maneuverable gimbal mount, allowing complete control over the camera angles and flight path. Thermal UAV surveys are an emerging tool for rockfall hazard mapping with potential for further development.