**1. Introduction**

In the last decade, a growing sensitivity has developed towards problems related to the integrity of structures, both in the construction sector and in that of road and railway infrastructures, as well as in that of industrial plants and products.

To check the functionality and health of these structures, natural vibration frequencies and oscillation amplitudes are among the most important parameters. This is even truer for slender or tall structures, such as very tall buildings, piles and spars of bridges and towers. For this kind of structure, it is mandatory to know the behavior when dynamic loads are applied (e.g., strong winds, vibrations due to traffic, earthquake-induced shaking). The ever more widespread presence of tall structures in urbanized and even densely inhabited areas makes their control more useful, which also helps avoid risks to the safety of people. For this reason, the development of simple methodologies for the quick inspection of slender or tall structures is of interest for SHM.

Wind towers are high cantilever structures, which are subject at one end to strong horizontal loads and cyclical stresses due to the rotors; these stresses are reflected in particular on the foundations and on the foundation soil. Thus, for these structures, the control of the frequencies is of fundamental importance.

Several techniques have been adopted for SHM [1]. Among these, techniques based on the measurement of the structural vibration are increasingly used; a review of vibration-based methods can be found in [2,3]. Among the various techniques proposed for measuring the frequencies of structures, those based on the acquisition of accelerometers and velocimeters have been widely used for decades [4,5]. The use of micro-electro-mechanical systems (MEMS)-based sensors and wireless connected sensors is growing, in order to set up even very dense control networks [6,7]. Global navigation satellite system (GNSS) receivers o ffer the possibility of extracting vibration frequencies [8]; as for the aforementioned techniques, the limit of GNSS is the point-like property of its measurements; moreover, it is necessary to position the GNSS receiver on the point to be monitored. Total stations (TS) are also used, thanks to their increased sampling rate [9] and the possibility of performing long-range monitoring with high precision using appropriate atmospheric correction techniques [10]. Another non-contact technique, continuously growing thanks to the increasing resolution of charge-coupled devices (CCD) and complementary metal–oxide–semiconductor (CMOS) sensors, and to the high frame rate of the most recent cameras, is based on photogrammetry and image processing [11,12].

Ground-based real aperture radar (GB-RAR) o ffers the possibility to monitor the dynamic characteristics of the structures while ensuring, at the same time, precise measurements of vibration frequencies and displacements. This long-range, non-contact methodology is even more widely adopted for monitoring large structures, for post-disaster interventions and for SHM [13–18].

TLS is a recent technique, still in the evolution phase, used in a wide range of applications and in particular for structural monitoring. Its strength lies in its ability to acquire a very large number of points in a short time, thus making it possible to survey even very complex objects, and to obtain detailed 3D models, thanks to point cloud processing software that is increasingly evolved. TLS, usually mounted on a tripod in fixed positions, is mainly used to monitor the deformations of the structures by measuring the di fferences between the surfaces scanned at di fferent times [19]. In the last decade, several TLS applications can be counted for the monitoring of dynamic phenomena and the measurement of structure vibrations. Kim and Kim [20] used a Riegl VZ-400 ® to perform dynamic displacement measurement. The smoothing of the acquired data was performed by using a kriging approach. Two experimental tests were conducted in laboratory on a small object and under ideal conditions. Neitzel et al. [21] showed a comparison between a TLS and a sensor system based on low cost MEMS accelerometers for measuring the vibrations of a bridge. They used a Zoller + Froehlich Imager 5003 ® in 1D mode; thus, measurements were performed on a single point. A GB-RAR was used for reference purposes. In this case, the accelerometers showed a better accuracy than TLS.

Schill and Eichhorn investigated the movements and frequencies of two wind towers. For each test, they used two Zoller+Fröhlich ® 9012 profilers, with sight lines on the rotor plane and perpendicular to it [22].

In this work, a technique is proposed for the measurement of natural vibration frequencies and oscillation amplitudes of structures, based on the TLS acquisition of scan lines and GB-RAR interferometry. The technique used to process the TLS acquisitions, in order to obtain a better precision than the instrumental one, is explained. A test is carried out on a wind tower and data is acquired both during normal activity and during the deactivation phase of the wind turbine. The frequencies and the oscillation amplitudes obtained from the processing of the collected data are compared with the theoretical ones, resulting from an analytical solution. A comparison is made with the results

obtained by GB-RAR, which acquires data simultaneously with the TLS. The results obtained, and the comparison with the theoretical solution and with GB-RAR, are shown also through figures and tables and the synergistic use of the two instruments is discussed.

The structure of the article is as follows: Section 2 introduces the TLS technique in line scanner mode and the technique used for data processing, in order to optimize the results, along with a brief introduction to the GB-RAR technique. Section 3 shows the test results. Section 4 provides discussion of the results. Finally, in Section 5, some conclusions are drawn.
