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Le tecniche geofisiche sono i metodi più efficienti per ottenere informazioni sulle strutture presenti nel sottosuolo. Ad oggi, la tomografia di resistività elettrica (ERT) è il metodo più utilizzato per il rilevamento di vuoti sotterranei, tuttavia, la combinazione con dati derivanti da diversi metodi geofisici è l’approccio più adatto per la determinazione delle cavità. Negli ultimi anni, la ERT è stata sempre più spesso congiunta alla tomografia sismica a rifrazione (SRT) al fine di ottenere interpretazioni più robuste anche utilizzando un approccio di tipo statistico. La cluster analysis eseguita su unità statistiche definite da valori di resistività elettrica, velocità delle onde P e densità sismica, ha permesso di ottenere interpretazioni ottimali delle strutture del sottosuolo. In base al numero di cluster, le mappe di distribuzione sono state costruite in uno spazio multiparametrico, consentendo di definire determinati limiti di variabilità per i parametri selezionati. Geophysical techniques are the most efficient methods for obtaining information on the structures present in the subsoil. Today electrical resistivity tomography (ERT) is the most used method for the detection of underground voids, however, the combination of experimental data from different geophysical methods is the most suitable approach for the determination of cavities. In recent years, the ERT has been joined more and more often to the technique of seismic refraction tomography (SRT) in order to obtain more robust interpretations also using a clustering approach. The cluster analysis performed on static units defined by electrical resistivity values, P wave velocities, and seismic density on coincident sections, allowed to interpret the subsoil structures. Based on the number of clusters, cluster distribution maps have been constructed in the multi-parameter space, allowing to define certain variability limits for the selected parameters.

We present the first results of a preliminary geological and geophysical survey carried out in the archaeological site of Santa Venera al Pozzo (Catania, Italy). The site dates back to the Roman Age, (1st century BC). Geological surveys highlighted a set of remarkable fractures affecting some archeological remains, suggesting the occurrence of a capable fault zone through the area. Multidisciplinary geophysical surveys (seismic refraction and electrical resistivity tomographies, together with aerial photographic and thermographic survey) identified a main tectonic discontinuity ascribable to the fault zone, allowing us to infer that the fractures observed at surface could be the evidence of coseismic ruptures.

A geophysical study by means of electrical resistivity and induced polarization tomographies was carried out in the landfill site of Bellolampo (Palermo, Italy) to check the integrity of the landfill liner and to detect possible leachate plumes. Preliminarily, synthetic tests were performed, simulating acquisition data along profiles placed above resistivity distribution, suitable to represent landfills protected with an high-density polyethylene (HDPE) liner at the bottom or with possible leachate plumes. Optimized array data sets for parallel measurements have been tested in order to assess the resolution and the effectiveness of tomographic images, comparing inversions with and without the constraint of the HDPE liner position. Based on the obtained results, three electrical tomographies have been planned and carried out directly over the landfill and nearby. The results provided useful information for identifying the electrical properties of the rock, waste, and leachate; for identifying snags and disruption of the tank bottom liner; and for pointing out possible polluting leachate nearby and below the landfill.

The Horizontal to Vertical Spectral Ratio (HVSR) noise method (Nakamura, 1989) is nowadays widely used to estimate the resonance frequencies of geological structures (Bonnefoy-Claudet, 2006). However, often HVSR is also used to obtain information on the depth of the seismic bedrock and on thickness and seismic velocity of the process overburden deposits, using inversion techniques of the H/V curve (Fäh et al., 2003). This nevertheless produce results with large uncertainty intervals of parameters, and then must be necessarily constrained by detailed stratigraphic information. An application of HVSR inversion is presented in order to verify the effectiveness of this technique for purposes of reconstruction of the buried geological setting where stratigraphic constraints are available. The study area lies in the south-eastern zone of the Palermo plain, known as "Bandita". It falls within the geological context of the Monti di Palermo (Abate et al., 1978), resulting from the superposition of structural-stratigraphic units arising from the Miocene compressive deformation of limestone and dolomite shallow-marine deposits (Panormide Platform), and Mesozoic limestone and siliciclastic deep sea deposits (Imerese Basin), with overlying Oligo-Miocene terrigenous covers (Numidian Flysch). Specifically, the reconstruction of the stratigraphy in the studied area has been obtained by a seismic-stratigraphic three-dimensional modelling. This allowed for geophysical and geognostic data derived from surveys carried out during this study, as well as previously by the municipality of Palermo.

The control of the safety assessment of a dam is largely dependent on knowledge of the mechanical parameters of the actual construction, as well as its geological substratum. The application of geophysical methods can be used in existing dams to evaluate the materials (strength properties) and to verify if they match design expectations, as well as to contribute to the safety control. In particular seismics methods can considerably contribute to the examination indicating the less consolidated material and the degradation of mechanical parameters. For the characterization of the concrete conservation three seismic tomographies and a vertical seismic profile on the dam body were performed.The vertical seismic profile showed P-wave velocity ranging between 3800 m/s 4750 m/s and S-wave velocity ranging between 1100 m/s and 2520 m/s. Also in this case lower values are present in the upper part of the dam body, as well as show the three seismic tomographies. Even Poisson ratios show typical values of the concrete in good condition, varying between 0.29 and 0.41 (at the upper part of the dam body), which are representative values for concrete.

The use of HVSR technique allows in many cases (Bonnefoy-Claudet et al. 2006) to obtain detailed reconstruction of the roof of the seismic bedrock (Di Stefano et al. 2014) and to identify areas with similar seismic behaviour. Theoretical considerations (Nakamura 1989) and experimental tests showed that amplification of horizontal motions between bottom and top of a sedimentary cover is well related to the ratio between the spectra of the horizontal and vertical components of the ground velocity (Nakamura 2000). This ratio is a measure of ellipticity of Rayleigh wave polarization, overlooking Love and body waves contribution. Assuming that subsoil can be represented as a stack of homogeneous horizontal layers and imposing some geometric and/or physical constraints it is possible to estimate the parameters of the shear wave velocity model (Fäh et al. 2003; Parolai et al. 2000). The integration of data related to HVSR and active techniques based on the analysis of surface waves can greatly reduce the uncertainties on the interpretation models. Because the inversion of HVSR curves implies monodimensional distribution of Vs, before inversing the data we used a cluster analysis technique to subdivide them into subsets attributable to areas with low horizontal velocity gradients and therefore similar seismic responses. The data of each cluster were then interpreted by imposing conditions of maximum similarity between the 1D models relating to each measurement point. Clustering methods are widely used in different research fields (Hartigan, 1975, Adelfio et al., 2012; D’Alessandro et al., 2013). In general, the cluster analysis is a good tool whenever you have to classify a large amount of information into meaningful and manageable groups. A modified centroid-based algorithm has been applied to HVSR datasets acquired for studies of seismic microzoning in various urban centers of Sicilian towns (Capizzi et al., 2014). The results obtained for Modica and Enna towns are shown. HVSR data were previously properly processed to extract frequency and amplitude of peaks by a code based on clustering of HVSR curves determined in sliding time windows (D’Alessandro et al. 2014).

The goal of this work is to study how the reliability of inverse model depends on a few basic parameters, as the combination of potential spacing and dipolar distance and, consequently, the number of measurements and of current dipoles, considering also how error affects inversion. The number of current dipole used is crucial, when using multichannel resistivity-meters, because it determines the overall acquisition time. A systematic comparison is presented between four 2D resistivity models and their images, obtained by the inversion of synthetic datasets relating to four different arrays: dipole-dipole (DD), pole-dipole (PD), Wenner-Schlumberger (WS) and multiple gradient (MG). For DD, PD and WS arrays a progression of eight different datasets are considered, by increasing the number of current dipoles but obtaining approximately the same amount of measures, and so increasing the investigation time. For MG array a progression of six datasets is obtained by increasing the current dipoles and so the lateral coverage. The goal is to study how this affect the resolution and the reliability of the tomographic inversion, particularly in presence of buried structures. Both noise-free and noisy data have been calculated and inverted. The results are compared using quality parameters of the reliability of the inversion. These are calculated for each cell of the first inverse model, and subsequently a mean value is obtained for the entire section or for areas coinciding with the abnormal structures. The sequences are also tested with field data to assess the validity of the theoretical results.

A modified centroid-based algorithm has been applied to HVSR (Horizontal to Vertical Spectral Ratio) datasets (Nakamura, 2000) acquired for studies of seismic microzoning in various urban centers of Sicilian towns also aimed to obtain detailed reconstruction of the roof of the seismic bedrock (Di Stefano et al. 2014). HVSR data were previously properly processed to extract frequency and amplitude of peaks by a code based on clustering of HVSR curves determined in sliding time windows. In centroid-based clustering, clusters are represented by a central vector, which may not necessarily be a member of the data set. After fixing the number of clusters, the algorithm find the cluster centers and assign each HVSR peaks to the cluster, such that the squared distances from the cluster centroid are minimized. Then it calculates the new means to be the centroids in the next step. The algorithm converges to a (local) optimum when the assignments no longer change. There is no guarantee that the global optimum is found using this algorithm. The proposed algorithm doesn’t fix the number of k clusters and choose automatically for each k value the initial centroids from data set. In particular, the UTM coordinates, amplitude and lithology values are the same for all k initial centroids and corresponding to their average value of all the units to be partitioned. The differentiation on the initial coordinates of the centroids was only based on frequency of the H/V peaks. The distance of each unit from the initial centroids and those obtained after each iteration was calculated as the weighted sum of the Euclidean normalized distances of all the variables considered, UTM coordinates, frequency, amplitude and lithology. The choice of weights has been optimized taking into account, for each k number of groups, the intra-cluster and inter-cluster variances. Also the choice of the optimal number of k classes was supported by the analysis of intra-cluster and inter-cluster variances, but basically remains a subjective choice, which is based on priori information and contextual data. The distribution in the frequency domain of H/V peaks obtained using the described centroid-based algorithm for different k values have been used to reconstruct geological discontinuities and to separate the stratigraphic peaks from those morphological. In many cases the clustering analysis of HVSR data showed excellent results, allowing to group peaks that can be attributed to the same seismic structures. However, the choice of the partition is strongly linked to the choice of weights for the calculation of the distance and to the geological and stratigraphic knowledge of the area. In other cases, the results showed similar results regardless of a priori choices. The obtained results underline how the most appropriate clustering algorithm for a particular problem often needs to be chosen experimentally.

At today the use of inversion of HVSR curves is mainly limited to derive average parameters of the shear wave velocity, although recently they have been used also for a detailed reconstruction of the roof of the seismic bedrock (Di Stefano et al. 2014). Since ambient vibrations may contain waves travelling in all directions, as body waves and Rayleigh and Love waves, a limit of this method lies in the uncertain composition of seismic noise, in the lack of knowledge about the microseismic field and in the subjective choices regarding the data processing. This work aims to verify the potential and limits of the HVSR inversion for the purposes of geological reconstruction of the subsoil in heavy urbanized areas, where few stratigraphic and geophysical constraints are available, and the only information is practically derived from the geological maps. The test site is the area of Altavilla Milicia (Sicily), characterized by outcrops of marine terraces and continental Pleistocene deposits that do not indicate directly the oldest geological formations, which, however, outcrop in the surrounding areas. Microtremor measurements were carried out on 20 recording points equally spaced of about 300 meters along two alignments: the line P in NW-SE direction near parallel to the coast and the line Q in NNE-SSW direction crossing the town. The HVSR inversion has been carried out using the Neighborhood Algorithm (Wathelet et al. 2004) implemented by the Geopsy software. Because of the lack of boreholes, starting models for inversions were chosen by assigning to each seismic layer ranges of thickness and velocity typical of the lithology expected, as deduced from the geological formations outcropping in the area and from previous geophysical surveys. The 1D seismic models obtained by results of HVSR inversion were correlated and laterally interpolated in order to construct two-dimensional geophysical sections, considering typical seismic velocities for each lithology concerned. In the study area, the outcropping Plio-Pleistocene clastic succession covers the Numidian Flysch silico-clastic deposits and the substrate related to slope to deep water Imerese Succession. The study area is characterized by a Miocene compression and later Plio-Pleistocene transcurrent and extensional tectonics (Catalano et al. 2013). The profiles display seismic layers mostly dipping towards N. Profile P (NW-SE-trending) shows displacements whereby seismic layers are abruptly downthrown towards N. This can be interpreted as high angle transcurrent and extensional faults, that downthrow a backlimb of a major fold towards the coast. Results of this test demonstrate that HVSR inversion can be a valuable method for the reconstruction of the geometry of the main subsurface structures, if supported by the right geological-structural information.

L’analisi delle proprietà strutturali di una scultura in marmo, attraverso l’impiego di tecniche di indagine non invasive da realizzare direttamente in situ, è di fondamentale importanza per la valutazione dello stato di conservazione e, conseguentemente, per il monitoraggio di eventuali fenomeni di degrado che possono compromettere l’integrità dell’opera d’arte. Lo studio condotto sul bassorilievo raffigurante la Madonna con Bambino conservato presso la Cappella del Monastero delle Clarisse Totus Tuus di Gorizia ha permesso di verificare la complementarietà delle informazioni restituite da tecniche di analisi della superficie (acquisizioni in fluorescenza ultravioletta) e tecniche di analisi del volume (tomografia ultrasonica). Le indagini in fluorescenza ultravioletta consentono di individuare e differenziare i materiali (originali, di restauro o di degrado) presenti sulla superficie lapidea. La tomografia ultrasonica, attraverso la stima delle velocità ultrasoniche caratteristiche del materiale, permette di ottenere una caratterizzazione strutturale dell’intero volume dell’opera. Generalmente, tale indagine è associata a un rilievo pacometrico necessario per rilevare la presenza di perni metallici all'interno dell'opera o nel supporto di ancoraggio, anche per interpretare eventuali anomalia riscontrate. Nel caso studio presentato, il modello tomografico ad ultrasuoni ha evidenziato la presenza di piccole zone ad alta velocità correlabili alla presenza di perni metallici e da una zona a maggiore degrado che si concentra nella parte inferiore dell’opera. Le indagini in fluorescenza ultravioletta hanno mostrato la reale estensione della frattura che coinvolge la porzione inferiore del bassorilievo, verificando il perimetro dell’area caratterizzata da un maggior degrado ed evidenziata dall’indagine ad ultrasuoni.

The Horizontal to Vertical Spectral Ratio (HVSR) technique, applied to ambient noise, is widely used to quickly estimate the fundamental frequency of a site. The HVSR technique is based on numerous assumptions about the propagation medium and the nature of the seismic noise, often hard to verified. In addition, in order to obtain reliable results, several acquisition and processing criteria must be respected. One of the most controversial aspects in the technique implementation are the reliable criteria for the identification in the microtremors signals of time windows appropriate for the calculation of the HVSR curves. Several authors suggest to remove spikes and transients because they bring information highly dependent from the source and cannot be used to estimate the resonance frequency of the site. Other authors instead, suggest that transients in seismic noise carry essential information and therefore should not be removed. The time windows suitable for the HVSR techniques are generally arbitrarily identified by the operator by a simple visual inspection of the signals in time or spectral domain. This can lead to an incorrect determination and interpretation of the HVSR curves. This technique is often used in the first level seismic microzonation. An incorrect processing of the seismic noise may provide unreliable results and HVSR curves difficult to interpret, an so making hard the identification of areas with similar seismic response. In this work we used the cluster analysis to analyze the HVSR curves as function of time and space. Cluster analysis is generally used to group objects characterized by a high level of similarity. We test different clustering algorithms to seek inside the seismic noise, time windows characterized by very similar HVSR curves. The implemented algorithms were applied to about 500 records of seismic noise each 46 minutes long. This analysis allowed us to identify inside each record, groups of very similar HVSR curves, allowing us to easily split those linked to the site effects from those related to the source effects. The separation of the two effects allowed us to determine robust HVSR curves better related to the local site effects. The clustering techniques were used also to group HVSR determined for different sites and identify areas with very similar behaviour in seismic perspective.

The seismic microzoning is a technique of analysis of a territory which aims to recognize, at a small scale, the local geological and geomorphologic conditions that may significantly affect the characteristics of the seismic motion, generating stress on structures that could produce permanent and critical effects. In other words, this technique has the objective to predict and evaluate possible site effects as a result of an earthquake. The first phase of the seismic microzoning is the detailed partition of the territory in homogeneous areas with respect to the expected behavior of soils during an earthquake. The seismic micro-zoning is a tool for prevention and reduction of seismic risk particularly effective when used already in the process of planning. The present paper focus on a recent study applied to the Oliveri, a coastal village settled on a coastal plain in north-eastern Sicily.