Introduction
Monitoring and detecting rockfalls on cliffs is essential for reducing risks to infrastructure, human safety, and ecosystems in steep terrains. Traditional methods often rely on expensive equipment and labor-intensive surveys, restricting their use to high-risk areas. Single-camera monitoring offers a cost-effective and scalable alternative, using advancements in image processing and change detection algorithms to identify rockfall events. Challenges such as lighting variations and environmental noise, require adapting existing algorithms for effective deployment. This study introduces a low-cost, autonomous system using high-resolution images from a single camera combined with an unsupervised deep learning approach to efficiently detect rockfalls.
Methodology
Prototype Design and Installation
The proposed monitoring system integrates commercially available components for ease of deploymen. A Sony-Alpha-7RM4A camera paired with a 400mm lens captures high-resolution images of fine-scale changes on the rockface. A Gigapan pan/tilt mechanism provides precise control for acquiring mosaics covering large-areas. A Raspberry-Pi controller automates image capture, data transfer, power monitoring, and remote transmission. Solar panels mounted on an adjustable frame provide continuous power, while weatherproof housing protects the components.
Operating autonomously, the system captures hourly images between 6:00-AM and 6:00-PM daily. Images are stored locally and transmitted remotely when connectivity permits. The system also logs operational data to support maintenance. Installed at the St. Eynard cliff in Biviers, France, the prototype captures mosaics of 102 high-resolution images per acquisition, enabling daily monitoring.

Image Preprocessing and Change Detection
The system captures around 40 GB of images daily, with a resolution of about 1 cm, under varying conditions such as lighting changes, vegetation growth, weather effects, and camera vibrations. These factors pose challenges for detecting rockfall-related changes, necessitating a robust image processing chain.
First, images are organized into tiles corresponding to specific regions of the mosaic and renamed using timestamps. Blurry or poorly lit images are filtered out using methods like Laplacian variance and gradient analysis. Images are then coregistered within each tile using the Scale-Invariant-Feature-Transform method, ensuring consistent pixel-level correspondence across the time series. Preprocessed tiles are assembled into a coherent mosaic of the study area.
Traditional threshold-based change detection methods are ineffective in large study areas due to diverse changes and lack of ground truth. To overcome this, a Siamese-Variational-Autoencoder (SVAE) was developed. The SVAE uses a U-Net-like architecture to extract latent features, an attention mechanism to focus on critical features, and a change-detection branch to generate precise change maps. Loss functions, including Kullback-Leibler divergence, perceptual, and texture ensure robust latent representations and preserve image fidelity, enabling effective detection of subtle changes while minimizing false positives.
Finally, processed images are georeferenced, translating detected changes into geographic coordinates to extract attributes such as rockfall size and location.


Applications
This framework has been successfully implemented at the St. Eynard cliffs. Detection results were validated against complementary datasets, including lidar and seismic data, demonstrating the system's reliability and effectiveness in real-world applications. This research was funded, in whole or in part, by the French National Research Agency (ANR) under the project C2R-IA (https://anrc2ria.fr/, grant ANR-22-CE56-0005-06).