Abstract
Abstract. Natural rock arches are rare and beautiful geologic landforms with important cultural value. As such, their management requires periodic assessment of structural integrity to understand environmental and anthropogenic influences on arch stability. Measurements of passive seismic vibrations represent a rapid and non-invasive technique to describe the dynamic properties of natural arches, including resonant frequencies, modal damping ratios, and mode shapes, which can be monitored over time for structural health assessment. However, commonly applied spectral analysis tools are often limited in their ability to resolve characteristics of closely spaced or complex higher-order modes. Therefore, we investigate two techniques well-established in the field of civil engineering through application to a set of natural arches previously characterized using polarization analysis and spectral peak-picking techniques. Results from enhanced frequency domain decomposition and parametric covariance-driven stochastic subspace identification modal analyses showed generally good agreement with spectral peak-picking and frequency-dependent polarization analyses. However, we show that these advanced techniques offer the capability to resolve closely spaced modes including their corresponding modal damping ratios. In addition, due to preservation of phase information, enhanced frequency domain decomposition allows for direct and convenient three-dimensional visualization of mode shapes. These techniques provide detailed characterization of dynamic parameters, which can be monitored to detect structural changes indicating damage and failure, and in addition have the potential to improve numerical models used for arch stability assessment. Results of our study encourage broad adoption and application of these advanced modal analysis techniques for dynamic analysis of a wide range of geological features.
Highlights
Natural rock arches form by erosion (Bruthans et al, 2014; Ostanin et al, 2017) and are major tourist attractions worldwide
Note that Geimer et al (2020a) allowed a polarity flip for mode shapes with sub-horizontal incidence angles equal to or larger than 85◦ in order to compare to numerical models
We demonstrate the ability of enhanced frequency domain decomposition (EFDD) to retrieve the fulllength normal-mode shapes at Squint Arch, where data acquired by a nodal geophone array during a separate experiment are available (Fig. 4b; raw power spectra are shown in Fig. A1 a–c)
Summary
Natural rock arches form by erosion (Bruthans et al, 2014; Ostanin et al, 2017) and are major tourist attractions worldwide. The stability of engineered structures, such as buildings and bridges, has been increasingly analyzed using measurements of their vibrational properties associated with resonance. Understanding this dynamic response to ambient loading forms the basis for the field of structural health monitoring (SHM; Doebling et al, 1996). SHM concepts have been applied at natural rock arches and other geological formations to improve site characterization and hazard assessment associated with failure of these features (e.g., Bottelin et al, 2013; Burjánek et al, 2018; Iannucci et al, 2020; Kleinbrod et al, 2019; Mercerat et al, 2021; Moore et al, 2018).
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