Aug 2026· e-Journal of Nondestructive Testing· Vol 31· 0 citations
Abstract
Conventionally, contact sensors such as accelerometers are used to conduct vibration-based structural health monitoring (VBSHM), providing accurate measurements but with deployment significantly limited by installation and maintenance challenges. Remote sensing alternatives for VBSHM have consequently garnered interest for alleviating monitoring costs. Several techniques have been developed previously: uncrewed aerial vehicles (UAVs) equipped with cameras for digital image correlation, and both ground-based lidar and millimetre-wave Doppler radar in either real or synthetic aperture modes – however these all require onsite proximity, limiting their coverage. Spaceborne sensing offers a less occluded vantage point from an orbiting platform, both features vastly improving sensor coverage. Synthetic aperture radar (SAR) is a widely used technology, where in an SHM context interferometric SAR (InSAR) has been noted for utility in measuring the long-term displacement of structural elements. As InSAR functions by comparing multiple SAR acquisitions of an area, its sampling rate – dictated by the revisit time of EO missions – is too low for VBSHM purposes. The alternative presented here is micro-Doppler SAR (MDSAR), which measures motion using a single SAR image and can achieve the sampling rates required, providing a complement to conventional SHM approaches. MDSAR functions by estimating the Doppler shift caused by a vibrating target, which is observed in a SAR signal. These shifts can be related to a target velocity along the line of sight, and sampled at rates matching the oscillation frequencies of large structures.
This paper presents an MDSAR technique, showing time history and spectral results both of calibration tests and a validation experiment conducted on a bridge. The input data are high-resolution, single-pass SAR images of real-world targets obtained through commercial SAR companies, with synchronous ground truth measurements gathered by conventional sensing. Calibration measurements of radar targets showed good agreement between time histories for velocities down to RMS 0.66 mm/s for 2 Hz oscillation with a 0.4 Hz amplitude modulation similar to that observed for bridges, with RMSE values of 64%, and a spectral residual value less than the frequency resolution of 0.07 Hz.
Validation experiments were carried out on the South Portland Street Suspension Bridge in Glasgow, UK, with ground-truth from an installed accelerometer monitoring system. They are consistent with calibration tests and demonstrate the feasibility of measuring vibrational velocities as low as 1 mm/s with MDSAR. In the time domain, the average measurement error is approximately 1 mm/s, comparable to the true velocities of the bridge. In the frequency domain, the technique performs well in identifying the dominant vibrational frequency, with a residual less than the frequency resolution of 0.06 Hz determined by the SAR acquisition duration of 16 s. Full modal identification (of mode shape components) is currently limited by the characteristics of current SAR missions, including radar wavelength, signal-to-noise ratio, and acquisition time. In the absence of modal coupling, partial information e.g. modeshape phase components, is retrievable. Although MDSAR cannot yet supplant onsite SHM methods, it can still provide valuable insights which could integrate into hybrid systems and scope remains for refinement of measurements.
Vibrational monitoring of structures traditionally relies on contact sensors such as accelerometers, displacement transducers, and strain gauges, which provide reliable physical information for structural health assessment. However, these sensors require manual installation and direct access to the structure, resulting in practical limitations in terms of installation time, safety, and accessibility, while long-term maintenance may compromise the overall Structural Health Monitoring (SHM) system reliability. These constraints often hinder the systematic implementation of SHM, particularly for large-scale infrastructures such as bridges. Alternative approaches present complementary limitations: vision-based techniques, such as digital image correlation, are sensitive to lighting conditions, camera calibration, and line-of-sight occlusions, whereas fibre-optic sensing systems, despite their high accuracy and distributed capabilities, require permanent installation and physical integration within the structure, limiting their suitability for rapid or temporary monitoring campaigns.
To address these challenges, Ground-Based Interferometric RADAR (GB-InRA) technology has emerged as a promising non-contact alternative for vibration monitoring, enabling safe measurements where visual inspection or contact sensor deployment are impractical. Interferometric radar enables the detection of sub-millimetric displacements by measuring the phase difference between transmitted and received signals. In this context, Real Aperture Radar (RAR), which provides one-dimensional line-of-sight measurements, is typically preferred for vibration monitoring over Synthetic Aperture Radar (SAR), which enables two-dimensional imaging. Nevertheless, radar-based measurements remain sensitive to instrument-to-target distance and antenna tilt, which must be carefully calibrated to ensure reliable results.
In the perspective of bridge monitoring, this work presents an experimental study conducted on a simply supported flexible steel beam (Figure 1). Vibrations were measured using an IBIS-FS microwave interferometric radar and compared against conventional displacement transducers adopted as ground truth. Three corner reflectors were installed at distinct beam locations to enable the estimation of the first three vibration mode frequencies and shapes. Impulsive hammer excitations induced sub-millimetric displacements, allowing a quantitative assessment of radar sensitivity and measurement accuracy. Preliminary results indicate displacement amplitude errors ranging between 0.02 mm and 0.2 mm for reference amplitudes between 0.5 mm and 12 mm, when compared to displacement transducers. Different configurations of vertical tilt and radar-to-target distance were also investigated to evaluate their influence on displacement estimation and modal identification. Overall, the findings demonstrate the effectiveness and practical applicability of GB-InRA as a robust, non-invasive tool for vibration monitoring and modal identification of civil structures.
Chiara Suppi, R. Andreotti, Vikram Kumar et al.· e-Journal of Nondestructive...· 0 citations
Many existing methods for vibration-based monitoring of bridges rely on expensive sensor networks or require direct physical contact with the structure, which limits their widespread adoption, especially across large networks of ageing infrastructures. Recent advancements in non-contact radar technology enable precise and multichannel vibration measurements from a distance, allowing operation safely without interfering with traffic or requiring complex installation. This feature offers a practical and scalable approach to monitoring hard-to-access bridges or structures. However, commercial solutions remain expensive and are often constrained by limited beam coverage, focusing on single-point measurements.
Addressing the existing drawbacks and augmented past research, this study presents an application of low-cost frequency modulated continuous wave (FMCW) radar-based vibration monitoring bridges through experimental validation. This research also investigates multi-point vibration measurements, thanks to the use of multiple transmitting and receiving antennas in radar enabling full-field vibration measurements of the structure. The fundamental working principle of the radar systems is by emitting electromagnetic waves continuously with a linearly changing frequency and comparing its frequency to the reflected waves from the target. To monitor sub-millimetre-level displacements, which are more often observed in the real-world infrastructure systems, the radar systems use the interferometry technique to analyse the phase shift in the reflected signal to accurately estimate the small displacements.
The present study adopts the two-step strategy to validate the predictive capabilities of the FMCW radar system, as shown in Figure. The initial step involves the validation of low-cost sensors against commercial solutions on a corner reflector attached to a shake table, enhancing the signal-to-noise ratio and thereby avoiding interference from static clutter. Subsequently, validation of multiple-point vibration measurements for different targets located within the same range is carried out using multiple transmitting and receiving antennas. This approach increases confidence in the radar system's capability to evaluate the full-field vibration of a structure, rather than relying solely on single-point measurements. Second, the radars are tested for the real-world application on a bridge under the traffic or railway loadings. The field campaign is carried out to evaluate the dynamic behaviour of the bridge, such as its modal properties under ambient traffic conditions. The results reveal that the low-cost radar system effectively captures the dynamic behaviour of both the corner reflectors in the laboratory setup and the bridge during the field campaign. This enables informed decision-making for the stakeholder and asset managers for timely, actionable insight for maintenance and safety interventions by relying on the low-cost FMCW radar systems.
Vikram Kumar, Enrico Tubaldi, Carmine Clemente et al.· e-Journal of Nondestructive...· 0 citations
Spaceborne Synthetic Aperture Radar (SAR) is a non-contact remote sensing technology that detects surface deformation by analyzing the phase differences between radar images acquired over the same area at different times. Due to its extensive coverage, high spatial resolution, and all-weather operational capability, spaceborne SAR has become an established technique for large-scale, continuous monitoring of civil infrastructure. Transportation networks constitute a fundamental component of urban infrastructure, playing a pivotal role in enabling efficient mobility and fostering regional economic development. Extreme weather events severely threaten the durability and operational safety of transportation networks. However, limited funding restricts the deployment of traditional sensors for detailed and comprehensive monitoring of the entire transportation network system. In this research, a stack of Sentinel SAR images acquired over a two-years period is collected from the Copernicus Data Space Ecosystem, and subsequently processed with Persistent Scatterer Interferometric Synthetic Aperture Radar (PS-InSAR) technology. A dedicated post-processing procedure, consisting of PS points refinement and clustering analysis, is applied to the displacement time series derived from the PS-InSAR processing. Then, statistical control limits method is employed to evaluate the risk levels across transportation network. Finally, the reliability and effectiveness of the proposed risk assessment framework are validated through a specific bridge case study. These findings demonstrate the potential of the proposed framework for large-scale, risk-informed assessment of transportation networks, thereby contributing to more proactive and data-driven transportation network management strategies.
Yi Xu, You Dong, Yi-qing Ni· e-Journal of Nondestructive...· 0 citations
Displacement monitoring represents an important component of structural control and preservation of cultural heritage assets, especially when foundation settlements are involved. The continuous acquisition of displacement data, when integrated with environmental information, allows for the distinction of deformation components associated with environmental variations and those related to structural modifications.
Traditional structural monitoring systems generally rely on the installation of contact sensors directly on the building surface. While this approach is well established, and can provide high-accuracy measurements, its efficacy largely depends on the number, spatial distribution, and proper calibration of the sensors. However, deploying and maintaining a dense network of contact instruments can be costly and logistically demanding. In addition, many historical and artistic monuments are subject to strict conservation regulations that limit or prohibit the installation of invasive instrumentation, thus promoting the development of non-contact monitoring solutions capable of ensuring reliable measurements without altering the physical or aesthetic integrity of the structure. Although non-contact techniques are already mature for large-deformation applications (e.g., slopes and infrastructure), their use in heritage contexts, typically characterized by small displacements and complex geometries, still requires further consolidation and methodological refinement to achieve comparable accuracy and long-term reliability.
Within this framework, the present study investigates the application of Ground-Based Interferometric Real Aperture Radar (GBInRAR) technology as a non-invasive method for the structural monitoring of masonry towers. This technique, developed and refined over the past few decades, enables displacement measurements with sub-millimetric precision by comparing the phase differences of radar signals acquired at successive time intervals. The radar system, positioned at a distance from the target, allows for continuous observation of multiple points along the line of sight, offering a global view of the structural response and temporal evolution of the monitored asset.
The bell tower of the Cathedral of Santa Maria Assunta in Pisa, universally known as the Leaning Tower, was selected as the case study. Over the centuries, the monument has experienced significant settlements and deformations, prompting extensive stabilization works and the establishment of a long-term contact-based monitoring system to ensure its safety. Despite these efforts, several aspects of its structural behavior, and their coupling with environmental variables, remain insufficiently characterized.
Motivated by these gaps, the study presents the results of acquisition campaigns conducted with GBInRAR technology during different periods of the year, along with a comparative analysis of these data and those obtained from the existing monitoring system, while also exploring correlations between observed displacements and environmental parameters. The results aim to highlight the main advantages, limitations, and potential applications of radar-based monitoring in cultural heritage conservation, supporting the development of reliable and non-invasive strategies for the long-term protection and structural behavior assessment of historical monuments.
Lorenzo Gianfranceschi, L. Vignali, A. De Falco· e-Journal of Nondestructive...· 0 citations
Rockfall hazards have increasingly drawn global attention due to their devastating impacts on human lives, infrastructure, and the environment. Traditional monitoring approaches for these events typically rely on ground-based radar, video surveillance, and point sensors such as accelerometers and strain gauges. Although these methods provide valuable insights into ground motion and event characteristics, they are often constrained by sparse spatial coverage, limited resolution, susceptibility to environmental interference, and high installation and maintenance expenses. Distributed Acoustic Sensing (DAS) technology mitigates these issues by converting optical fiber cables, extending tens of kilometers, into a continuous array of virtual sensors with meter spatial resolution. Previous studies have demonstrated that DAS can capture high-frequency seismic signals generated by artificially triggered rockfalls, exhibiting strong correlation with conventional seismic records. In this study, we conducted an extensive DAS data collection campaign that captured routine operational signals from various highway segments, diverse vehicle types, and different time periods. The testing section, approximately 1 km long, is situated along an highway in Quzhou, Zhejiang Province, China. Subsequently, we manually simulated rockfall events by allowing wooden stakes of varying weights to fall naturally at different locations, thereby emulating the characteristics of real rockfall incidents. Given the substantial daily sample size and the rigorous requirements for a low false alarm rate, a single model proved insufficient. Therefore, by integrating features from the time-frequency domain and spatial resolution, we fused an audio classification model- PANNs with Self-Attention and Convolution to develop a DAS-based long-range rockfall monitoring and early warning system. This model delivers accurate, real-time monitoring over extensive distances, achieving a recall rate exceeding 92% while maintaining a false alarm rate of only 0.058%. Moreover, the system can provide timely alerts regarding incidents, offer recommendations to road maintenance departments, and significantly mitigate secondary disasters such as traffic accidents and congestion triggered by rockfalls.
Liang Lyu, Jian-Fu Lin, Guoxu Chen et al.· e-Journal of Nondestructive...· 0 citations
Accurate bathymetric data are essential for the design and monitoring of coastal structures, but conventional multibeam surveys are costly and often impractical in shallow or confined areas. We evaluate a single-beam echosounder (SBES, ECT400) suspended beneath an unmanned aerial vehicle (UAV) as a rapid method with low logistical requirements for bathymetric monitoring of coastal infrastructure. Fieldwork was performed in an operational dry dock that was alternately drained and filled, enabling direct geometric validation against an ultra-high-resolution photogrammetric DEM (0.55 cm GSD). The co-registered dataset comprises N = 16,137 sonar returns to depths of ≈ 8 m. The UAV-mounted SBES produced a mean depth difference of 0.15 m (SD = 0.58 m) relative to the photogrammetric reference. From these residuals we estimate a 95% Minimum Detectable Change (MDC95) of ≈ 0.5 m when changes are assessed by aggregating repeated co-located passes. These results indicate that the UAV-SBES workflow is suitable as a Tier-1 screening tool for structural-health monitoring, effective for detecting metre- to decimetre-scale changes and triaging sites for targeted high-precision follow-up, but not for micrometre/mm-scale deformation monitoring. The method’s portability and vessel-free operation make it especially useful for frequent inspections in shallow, confined coastal settings.
Bethsaide Souza-Santos, M. Arza-García, J. Ortiz-Sanz et al.· Journal of Civil Structural...· 0 citations
