Dense ultrasonic imaging of thermo-mechanical stress changes in a limestone cliff

Rock fracturing driven by temperature fluctuations, rainfall, and freeze–thaw cycles governs both long-term landscape evolution and the onset of rock-slope instabilities. However, the thermo-mechanical stress field that develops within the first decimeters of exposed rock—and its role in sub-critical crack growth—remains poorly constrained, largely because it cannot be directly observed at relevant spatial and temporal scales in-situ.

We present a new dense ultrasonic monitoring experiment designed to image near-surface stress, rigidity, and damage in an unstable limestone cliff. The system consists of more than 50 permanently installed ultrasonic transducers deployed over a 4 m² area on a 50-m-high limestone pillar located in the foothills of Larzac, southern France. Half of the sensors operate as emitters and half as receivers, allowing repeated, highly redundant measurements of travel times and waveforms across hundreds of ray paths. Using acousto-elasticity, temporal changes in ultrasonic velocity provide a quantitative proxy for stress and crack evolution, while waveform decorrelation enable tracking of micro-damage and scattering.

The high spatial density of the array enables 2-D and potentially 3-D tomographic imaging of stiffness and damage within the rock surface layer, resolving gradients that are invisible to sparse instrumentation or bulk resonance methods. First results reveal pronounced diurnal velocity variations that correlate with surface temperature and solar radiation, indicating strong thermo-elastic control on near-surface stress and fracture opening.

This new monitoring approach opens the door to direct, time-lapse imaging of climate-driven damage in rock slopes, providing a critical link between environmental forcing, sub-critical cracking, and the progressive weakening that precedes rockfall and cliff collapse.

This work was funded by the European Research Council (ERC) under grant No. 101142154 - Crack The Rock project.