Fracture Network Imaging on Rock Slope Instabilities Using Resonance Mode Analysis

Abstract

We performed modal analysis using frequency domain decomposition of ambient seismic vibration data collected on large rock slope instabilities. This technique enables a robust detection of resonance frequencies and provides the corresponding mode shape vectors. We applied the technique to synthetic and field data sets acquired by seismometer arrays on two rock instabilities in Switzerland. We found that, at the fundamental mode, the entire instability vibrates in‐phase with the dominant mode shape vector oriented perpendicular to dominant fracture systems. At higher frequencies, different compartments of the instability resonate antiphase. Therefore, delineating the zero crossings between the phases allows dominant fractures to be efficiently mapped. Approximately 1 hr of ambient vibration data suffices to apply the method successfully. The method also potentially detects hidden fractures that cannot be observed by geological field mapping. In addition, this approach combines classic amplification and polarization analysis into one technique, simplifying data processing efforts.

Monitoring the Preonzo Rock Slope Instability Using Resonance Mode Analysis

“…Compared to the findings by Häusler et al (2019), who identified f2 at 4.7 Hz in a dataset acquired using a similar temporary seismic array in 2012, the mode shapes are not perfectly repeatable. Note that the area of the southeastern high amplification area was not covered in the array by Häusler et al (2019). Even if only those locations are considered which were previously surveyed in 2012, the resonance frequency f2 is much more pronounced in the data acquired in 2019 than in the data of 2012 (comparison in Figure S4).…”

“…Burjánek et al (2018) also observed strong polarization phenomena perpendicular to the failure scarp and the open fractures. This normal mode behavior was later reassessed by Häusler et al (2019) using frequency domain decomposition (FDD) modal analysis. This study also analyzed higher modes and concluded that the observed mode shapes and amplification factors can only be explained by an additional rear fracture potentially limiting the extend of the entire rock instability (dashed line in Figure 2).…”

“…The frequency domain decomposition (FDD) introduced by Brincker et al (2001b) is an operational modal analysis technique popular in civil engineering (e.g., Michel et al, 2010) that determines the natural modes of vibration of a structure, providing resonance frequencies and mode shapes. Recently, FDD was applied on natural rock structures such as unstable rock columns (Bottelin et al, 2013), landslide release areas (Häusler et al, 2019), and large…”

“…To determine the dynamic response (resonance frequencies and mode shapes) of the instability across a larger area around the monitoring stations, we repeated the modal analysis experiment described by Häusler et al. (2019) by installing additional temporary seismic arrays. The new data set acquired in June 2019 comprises 23 seismometer locations, whereas the first data set collected in May 2012 consists of 12 points.…”

A Bayesian Approach to Predict Blast-Induced Damage of High Rock Slope Using Vibration and Sonic Data

“…Unlike blasting vibration velocities that are easily influenced by external conditions, natural frequencies of rock masses are intrinsic characteristics and relatively simple to obtain without knowing near-field vibration data. Based on commonly recorded blasting vibration data, natural frequencies of rock masses can be extracted by diverse methods, such as Fourier spectra [ 37 , 38 ], the power spectral density (PSD) [ 39 , 40 ], the transfer function [ 41 , 42 ], the frequency domain decomposition method [ 43 , 44 ], spectral ratios [ 45 , 46 ], and polarization analysis [ 40 , 47 ]. Researchers have developed a number of techniques to identify the location and degree of damage in structures using the change in the natural frequency in structural health monitoring [ 48 , 49 , 50 ].…”