Array-based ambient vibration modal analysis describes fracture-controlled mode shapes at a natural rock arch (Utah, USA)

Abstract. Fracture generation and propagation are primary mechanisms of structural degradation in natural rock arches and other freestanding rock landforms. However, methods to detect structural changes arising from fracturing are limited, particularly at sites with difficult access and high cultural value. Here we show how ambient vibration modal analysis can be used to identify fracture-controlled resonance modes at a sandstone arch in Utah (USA) aiding the selection of relevant modes for structural health monitoring. We characterized modal properties of Hunter Canyon Arch (i.e., resonance frequencies, damping ratios, and mode shapes) using spectral and cross-correlation analyses of data generated from an array of nodal geophones. Results revealed properties of nine resonance modes with frequencies between 1 and 12 Hz, damping ratios between 0.6 and 4.3 %, and an assortment of 3D mode shapes. Experimental data were then compared to numerical models implementing both homogeneous media and heterogeneous configurations generated through discretization of compliant zones in areas of mapped fractures. Results showed that all numerical solutions replicated the first two resonance modes of the arch, indicating these are insensitive to structural complexity derived from fractures and thus may be poor targets for monitoring. Meanwhile, heterogenous models with implemented fracture zones succeeded in matching the frequency and shape of one additional higher mode, indicating this mode is sensitive to fracture properties and thus most likely to respond to structural change from fracture propagation. Evolutionary crack damage modelling confirmed the sensitivity of this mode, and conversely the relative insensitivity of other modes, to simulated fracture propagation. While examination of fundamental modes is common in structural health monitoring studies, our results suggest that identifying changes in higher-order modes, i.e., those determined to be affected by fractured areas, may be more informative for characterizing structural damage in monitoring applications.