Luca Piroddi

Comparison of natural and artificial forcing to study the dynamic behaviour of bell towers in low wind context by means of ground-based radar interferometry: the case of the Leaning Tower in Pisa

The study of Cultural Heritage assets needs the application of non-destructive and non-invasive monitoring techniques. In particular, monuments and historical buildings which are open to the visitors and/or subject to important stress must be studied for their dynamic response.In the last 10 years the new ground-based radar interferometry technology has been developed allowing to monitor displacements from a point of sight far from the studied targets. It virtually provides a continuous mapping of displacements of the observed structures up to 10 µm with a range resolution of 0.75 m.In this paper, the application of ground-based interferometry on one very important historical building, the Leaning Tower of Pisa in Italy, is reported. The analysis of these kind of structures is important to catch their dynamic response to natural actions in general, and also to assess the effects due to pedestrian and users, and consequently to define functional capabilities and levels of acceptable dynamic stress. The studied structure was subject to artificial loading by synchronous movement of about 20 people. Artificial forcing led the structure to a resonance condition with the same frequency of the one due to the natural noise excitation, which was separately measured, and with an oscillation amplitude more than thirty times greater than the natural one (in conditions of weak wind). During the passive stages of the survey the recorded structural vibrations were very closed to the instrumental sensitivity, making difficult to distinguish vibration amplitudes amplifications of various segments at various heights.Through the spectral analysis of the acquired data it was possible to estimate the vibration frequencies of the first modal shapes of the structure along two orthogonal directions. The power spectra of the passive survey data have the same maximum frequency of the active but contain more noise at low frequency.

Ambient Vibration Tests of an Arch Dam with Different Reservoir Water Levels: Experimental Results and Comparison with Finite Element Modelling

This paper deals with the ambient vibration tests performed in an arch dam in two different working conditions in order to assess the effect produced by two different reservoir water levels on the structural vibration properties. The study consists of an experimental part and a numerical part. The experimental tests were carried out in two different periods of the year, at the beginning of autumn (October 2012) and at the end of winter (March 2013), respectively. The measurements were performed using a fast technique based on asynchronous records of microtremor time-series. In-contact single-station measurements were done by means of one single high resolution triaxial tromometer and two low-frequency seismometers, placed in different points of the structure. The Standard Spectral Ratio method has been used to evaluate the natural frequencies of vibration of the structure. A 3D finite element model of the arch dam-reservoir-foundation system has been developed to verify analytically determined vibration properties, such as natural frequencies and mode shapes, and their changes linked to water level with the experimental results.

Fast Dynamic Control of Damaged Historical Buildings: A New Useful Approach for Structural Health Monitoring after an Earthquake

The structures damage conditions assessment requires numerous precautions to ensure the safety of people during site visits and inspections. Among several methods providing useful information about the conservation status of the structures, dynamic monitoring techniques are suitable to retrieve the global behavior of the buildings. The anomalous features diagnosis of the structural dynamic response is an index of alterations of the material state and, in the worst cases, is related to the presence of damaged structural elements. This paper proposes the use of remote control systems for the structural evaluation of the damage state of buildings and describes the results achieved in an interesting application: the experimental dynamic analysis carried out on the inaccessible damaged bell tower of the Church of Santi Giacomo and Filippo in Mirandola (Italy). The study is based on observations performed using the IBIS-S ground-based radar interferometer to remotely measure the displacements of several elements of the building above 0.01 mm amplitude. This totally noninvasive and nondestructive approach has proved to be reliably implemented as a useful method to structural health monitoring procedures and especially for extensive and fast inspection analyses aiming at the first evaluation of the damage level and the soundness of slender buildings after earthquakes.

Integrated geophysical and aerial sensing methods for archaeology: A case history in the Punic site of Villamar (Sardinia, Italy)

In this paper, the authors present a recent integrated survey carried out on an archaeological urban site, generally free of buildings, except some temporary structures related to excavated areas where multi-chamber tombs were found. The two methods used to investigate this site were thermal infrared and ground penetrating radar (GPR). The thermography was carried out with the sensor mounted under a helium balloon simultaneously with a photographic camera. In order to have a synthetic view of the surface thermal behavior, a simplified version of the existing night thermal gradient algorithm was applied. By this approach, we have a wide extension of thermal maps due to the balloon oscillation, because we are able to compute the maps despite collecting few acquisition samples. By the integration of GPR and the thermal imaging, we can evaluate the depth of the thermal influence of possible archaeological targets, such as buried Punic tombs or walls belonging to the succeeding medieval buildings, which have been subsequently destroyed. The thermal anomalies present correspondences to the radar time slices obtained from 30 to 50 cm. Furthermore, by superimposing historical aerial pictures on the GPR and thermal imaging data, we can identify these anomalies as the foundations of the destroyed buildings.

Synergical Use of Passive and Active Methods to Reconstruct the Subsoil in Urban Areas

In urban areas it is often difficult to carry out not only direct but also indirect investigations, such as geophysical surveys. In fact, because of traffic noise, power lines and pipelines it is not possible to obtain reliable and repetitive measures using seismic, magnetic, electrical and electromagnetic methods. In addition, the asphalt and the paving of the sidewalks don’t facilitate the placing of geophones and electrodes. So only a few methods can usefully be applied. The gravimetric method in particular makes it possible to reconstruct maps of vulnerability that can be a useful document for the planning of appraisal or the prevention of risk of collapse for buildings. Finally the gravimetric maps represent a basic document for municipal urban development plans. Recently a seismic passive method was also developed that uses a 3-component seismic system called Tromino, which enables us to measure over time for periods of several hours, the spatial components of acceleration and velocity produced by natural tremors and vehicular traffic. Recently seismic land streamer devices have also been produced, that allow us to make profiles of seismic refraction on unpierceable areas and also to drag the system along a profile increasing its length.

Vibration analysis of historic bell towers by means of contact and remote sensing measurements

Abstract The dynamic behaviour of two slender structures with very similar geometry has been investigated in order to evaluate the role played by the construction materials; the comparison has thus been conducted on their vibration properties as resonance frequencies, damping coefficients and mode shapes. The studied structures are two bell towers of a church which were built in two different historical times, with an interval of about one century, using different construction techniques and materials. The experimental tests were carried out by means of output-only measurements of ambient vibration using both contact and non-contact techniques. The signals have been acquired using a tri-directional tromometer or two short period seismometers, both placed in prearranged station points on the structures. Furthermore, the vibrations of the structures have also been measured with the IBIS-S microwave interferometer which is able to provide submillimetric displacements along the radar Line Of Sight without need of any contact with the surface. Therefore, the experimental dynamic response of the church-towers system has been estimated integrating both velocity and displacement data. Though the vibration of the structures had low magnitude, both surveys allowed us to identify the main linear dynamic properties of the structures. Based on these passive surveys, a linear finite element model was calibrated in order to confirm the relationship between the materials and vibration properties. The final model has been locally validated by means of in situ acoustic measurements.

Applications of new technologies of ground-based interferometric radar to the study of Cultural Heritage buildings

Differential Synthetic Aperture Radar Interferometry is a technique that, by making the phase difference of SAR image pairs, allows to extract the information relevant to the earth surface displacements. The result of the phase difference, referred to as Interferometric Phase or Interferogram, contains different terms: the deformation phase term, a topographic phase contribution, an atmospheric phase term and the noise contibution. Applying an algorithm referred to as Small Baseline Subset (SBAS) proposed by I.R.E.A. - C.N.R. it is possible to know the displacement of buildings and constructed parts (roads, bridges ecc.) which have a dimension of about 10 meters The results demonstrate that the DInSAR technique is a fast and precise method to detect and follow the temporal evolution of deformations, so it is a powerful technique for monitoring the territory and to map its vulnerability.

Time and Spectral Multiresolution Remote Sensing for the Study of Ancient Wall Drawings at San Salvatore Hypogeum, Italy

Historic and prehistoric sites are often a serious challenge for geophysical techniques, which could give enhanced and essential information to define identity and health of cultural heritage assets. Multispectral remote sensing techniques have been applied to ancient wall pictures of a hypogeum actually under a rural church in San Salvatore (Italy). Multispectral survey was jointly done with state of art surveying for historic architecture and was compared with a 40s’ photographic survey. This integration has been the way to fully explore and exploit the signs present in wall drawings evidences of hypogeum frequentations. Adopting multiresolution approach for the construction of scientific datasets and maintaining full dimensionality for their evaluation and interpretation, jointly with some synoptic images, allowed us to build a user friendly Decision Support System potentially used by authorities assigned to the conservation, restoration and management of ancient heritage goods. In this case study, the informative layering showed to be a very effective method to find cross-information present in different surveys data, which are essential for planning restoration activities but also for dissemination and fruition of cultural heritage sites. The application of applied geophysics techniques to cultural heritage and artistic targets proved to be a disciplinary strategic application field.

Terrestrial remote sensing and microtremor measurements for the study of the vibrations of a rock mass with large underground cavities

This note describes an experimental study aimed at evaluate the vibration properties of a limestone rock vault, subjected to the forcing action produced by an intense vehicular traffic. The site of study occupies an area of about 3,000 m2. The subsoil of the site is characterised by shallow underground cavities due to the past quarry activities. Ambient vibration measurements were done by means of seismometer stations placed on the ground surface. Microtremor stations were arranged above the cavity and in the surrounding area. An innovative coherent radar sensor was installed inside the underground cavity in order to measure time series of displacement related to different reflector points located on the internal surface of the vault. The experimental vibration properties were derived by means of frequency analyses of both seismic and interferometric data in order to compare the results achieved by using the seismic transducers and the unconventional microwave sensor. Furthermore the preliminary results of numerical simulations were discussed.