## 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 offer 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) offers 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; 14; 15; 16; 17; 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 differences between the surfaces scanned at different times [19]. In the last decade, several TLS applications can be counted for the monitoring of dynamic phenomena and the measurement of structure vibrations. [PERSON] and [PERSON] [20] used a Riegl VZ-400(r) to perform dynamic displacement measurement. The smoothing of the acquired data was performed by using a kriging approach.