Rockfall monitoring: comparing several strategies for surveying detached blocks and their volume, from TLS point clouds and GigaPan pictures

Abstract

[EN] Rockfalls are fast slope instabilities frequent in mountainous areas, which cause damage in infrastructures (roads and railways), buildings, vehicles and people. Nowadays, several continuous and discontinuous techniques are available to monitoring the prone areas in order to manage the associated risk. One side task is to detect changes in the source zones (rock cliffs with recurrent events) in order to assess the rockfall activity and calibrate the Magnitude-Frequency curves. Long range and high precision Terrestrial Laser Scanning (TLS) is currently used for this purpose, sometimes in combination with high-resolution pictures taken from UAV or from the ground (with a GigaPan setup, for instance). Some detected changes along time may correspond to precursory displacements while others are due to blocks detached from the cliffs. In our contribution, we present the use of the aforementioned geomatic techniques (TLS and GigaPan) within several algorithms /strategies (Cloud to Cloud, Cloud to Mesh and M3C2) inside some two commercial computer programs and open source program in order to detect and measure the differences along the successive field campaigns. This work has being carried out within the frame of the GeoRisk research project, with field data from the ICGC. In particular, we test the strategies in three sites in the Montserrat Massif (Spain) –Mirador de l'Oliver, Canal dels Aritjols and Mirador dels Apòstols– during four measuring epochs along 2019-2021. The results show that rock volumes as small as 0.001 m3 can be detected in a regular basis.