Acoustic Fingerprints - Tracing Irreversible Damage in Natural Cliffs

Progressive damage and failure in rock masses is governed by multi-scale processes ranging from micro-crack growth to meter-scale fracture opening. We present an active acoustic monitoring approach that captures these evolving processes through time-lapse waveform fingerprinting, providing a quantitative measure of the temporal evolution of rock mass stiffness and scattering properties.

We deployed acoustic sensors on three highly fractured rock cliffs (two limestone sites in southern France and one gneiss site in western Switzerland) and conducted repeated active acoustic measurements every few minutes over periods of several weeks to more than one year. Each source-receiver path yields a unique acoustic response whose complexity increases with fracture density and scattering. By tracking phase shifts and waveform distortions, we 'draw' time-lapse waveform fingerprints that are highly sensitive to small changes in crack density, fracture aperture, and contact stiffness.

The waveform fingerprints reveal strong repeatability under similar meteorological conditions, with coincident patterns observed on days sharing comparable temperature and moisture regimes. Distinct fingerprints emerge under different rock cracking and damage states reflecting reversible thermo-hydro-mechanical effects. Some rocks are indeed more reactive to external forcings than others. At longer timescales, partial but incomplete recovery of the fingerprints is observed. In the one-year data set, major fingerprint features reappear under similar climatic conditions, but with persistent residual changes, indicating the accumulation of irreversible damage within the rock mass.

Future work could apply diffuse acoustic wave spectroscopy and acoustic correlation-based imaging to spatially locate damage and quantify fracture growth, enabling the transition from qualitative fingerprints to quantitative maps of rock degradation.