Strain rate analysis over the Central Apennines from GPS velocities: the development of a new free software

Displacements or velocities obtained by GPS data processing over repeated surveys can provide useful information on tensional states of terrestrial crust, in those areas in which many stations well spatially distributed are present. In particular, the strain (or strain rate) can be computed over the nodes of a regular grid with suitable size to define a high density deformation field. A new method was deployed to generate easily and quickly the deformation pattern from GPS velocities and to evaluate the significance: values, related to an assigned grid point, can be truly considered only if the GPS stations are well distributed around it. The approach validation was performed by means of synthetic data derived from the theoretical displacement field generated by a Mogi model source. A complete analysis on the velocity pattern of the CaGeoNet network (Central Apennine chain, Italy) was performed providing strain rates and showing both extensional and compressive behaviour at the same values, along the Apennine chain axis

Terrestrial laser scanner and retro-reflective targets: an experiment for

Artificial targets are generally used in the terrestrial laser scanner (TLS) practice for data georeferencing, since they are well recognized and modelled from the point cloud, and their positions can be contemporarily measured by topographical techniques. The accuracy of target identification directly influences the georeferencing quality. In particular, retroreflective materials can cause anomalies in range measurement due to the too high amplitude of returned pulse. If the received pulse intensity exceeds the limits of the sensor dynamic range, the receiver saturates producing a truncated pulse preventing the correct time-of-flight computation. A series of experiments were performed in order to test the performances of a specific instrument (Optech ILRIS 3D) for the acquisition of artificial targets made of retro-reflective material resulting in very high reflectance. Dealing with ranges lower than about 300 m, two cases were clearly observed, that is wrong distance measurement of points over the high reflecting surfaces, and the presence of haloes around these surfaces. Neglecting these phenomena, has serious implications and can lead to a wrong georeferencing. The experiments were executed and data analyzed providing a qualitative and semi-quantitative phenomenon description. Finally, the design of a target that can be easily recognized and correctly modelled was ideated and proposed

Effects of surface irregularities on intensity data from laser scanning: an experimental approach.

The results of an experiment carried out with the aim to investigate the role of surface irregularities on the intensity data provided by a terrestrial laser scanner (TLS) survey are reported here. Depending on surface roughness, the interaction between an electromagnetic wave and microscopic irregularities leads to a Lambertian-like diffusive light reflection, allowing the TLS to receive the backscattered component of the signal. The described experiment consists in a series of TLS-based acquisitions of a rotating artificial target specifically conceived in order to highlight the effects on the intensity data due to surface irregularity. This target is articulated in a flat plate and in an irregular surface, whose macro-roughness has a characteristic length with the same order of the spot size. Results point out the different behavior of the plates. The intensity of the signal backscattered by the planar element decreases if the incidence angle increases, whereas the intensity of the signal backscattered by the irregular surface is almost constant if the incidence angle varies. Since the typical surfaces acquired in a geological/geophysical survey are generally irregular, these results imply that the intensity data can be easily used in order to evaluate the reflectance of the material at the considered wavelength, e.g. for pattern recognition purposes

Remote sensing of volcanic terrains by terrestrial laser scanner: preliminary reflectance and RGB implications for studying Vesuvius crater (Italy)

This work focuses on the use of terrestrial laser scanner (TLS) in the characterization of volcanic environments. A TLS survey of the Vesuvius crater (Somma-Vesuvius volcano, Italy) allows the construction of an accurate, georeferenced digital model of different sectors of the crater. In each sector, the intensity is computed for each point as the ratio between the emitted amplitude and the received one, normalized to the maximum signal, providing the radiometric information. Moreover, the RGB colours of the observed surfaces can be captured by means of a calibrated camera mounted on the TLS instrument. In this way, multi-band information is given, since a long range TLS operates in the near infrared band. The reflectance and RGB data are compared in order to verify if they are independent enough to be complementary for model analysis and inspection. Results show that the integration of RGB and intensity data can fully characterize this volcanic environment. The collected data are able to discriminate different volcanic deposits and to detect their stratigraphic features. In addition, our results shed light on the spatial extension of landslides and on the dimensions of rock fall/flow deposits affecting the inner walls of the crater. The remotely acquired TLS information from the Vesuvius crater is compared with that from a sedimentary terrain (coal-shale quarry) to detect possible similarities/differences between these two geological environments

OPTECHTM ILRIS-3D TERRESTRIAL LASER SCANNER: SHORT USER GUIDE

The OptecTM ILRIS-3D Terrestrial Laser Scanner (TLS) has been recently acquired by the Istituto Nazionale di Geofisica e Vulcanologia (INGV) Sezione di Bolgna in the context of a TTC program for volcanoes monitoring (TTC 1.3 Controllo Geodetico delle Aree Vulcaniche Attive) supported by the Dipartimento di Protezione Civile. Several experiments were performed by INGV since 2004 to study the level of precision for surface modelling by means of laser scanner long range instruments, in order to detect the best suitable standard for rapid and simple acquisition in volcanic area (Pesci et al., 2007). In particular, during the MESIMEX experiment (October 2006), a national exercitation organized by the Dipartimento di Protezione Civile (DPC) exploited to simulate a volcanic eruption in Naples, the second TLS survey of the whole Vesuvius crater was executed and a large mass variations were estimated revealing the collapse of a portion of the crater. The alignment and comparison of point clouds (2006–2005) show high variations over a large portion of the NE slope and a volume variation of about 6850 m3 was computed. The analysis was performed in almost real time by means of direct comparisons between scans, indicating the laser scanning as one of the most reliable technique for fast monitoring in crisis time (Pesci et al. 2008a). The main characteristics recommended for surveying in volcanic areas were the laser device eyes safety, the achievable very long range (> 1 km), the precision of measurements and final accuracy in data modelling, the acquisition velocity, the instrument portability in terms of weight and size and the ability to manage scanner by means of PC pocket. The ILRIS-3D scanner was chosen based on the previously described recommended points. The simple operation needed for scan execution and the possibility to plan and realize a complete survey by means of only two operators confirmed ILRIS-3D as the best choice for volcanic applications. This technical report is a simple and effective user guide for laser scanner management providing all the necessary instruction from instrument settings, remote connection, data storage, downloading and preprocessing. Authors proposal is to make operators independent enough to scan and carry out survey in interested areas also without a specific experiences in LIDAR (Light Detection And Ranging) monitoring

Remote Sensing and Geodetic Measurements for Volcanic Slope Monitoring: Surface Variations Measured at Northern Flank of La Fossa Cone (Vulcano Island, Italy)

Abstract: Results of recent monitoring activities on potentially unstable areas of the NW volcano flank of La Fossa cone (Vulcano Island, Italy) are shown here. They are obtained by integration of data by aerial photogrammetry, terrestrial laser scanning (TLS) and GPS taken in the 1996–2011 time span. A comparison between multi-temporal models built from remote sensing data (photogrammetry and TLS) highlights areas characterized by ~7–10 cm/y positive differences (i.e., elevation increase) in the upper crown of the slope. The GPS measurements confirm these results. Areas characterized by negative differences, related to both mass collapses or small surface lowering, also exist. The higher differences, positive and negative, are always observed in zones affected by higher fumarolic activity. In the 2010–2012 time span, ground motions in the northern part of the crater rim, immediately above the upper part of observed area, are also observed. The results show different trends for both vertical and horizontal displacements of points distributed along the rim, with a magnitude of some centimeters, thus revealing a complex kinematics. A slope stability analysis shows that the safety factors estimated from these data do not OPEN ACCESS Remote Sens. 2013, 5 2239 indicate evidence of possible imminent failures. Nevertheless, new time series are needed to detect possible changes with the time of the stability conditions, and the monitoring has to go on

EXPERIENCE IN MOBILE LASER SCANNING BY MEANS OF LYNX SYSTEM IN L’AQUILA CITY

The terrestrial laser scanner is an efficient topographical instrumentation used to acquire a redundant number of points distributed over a physical surface. The goal of laser scanning is the definition of very accurate models of the studied areas. In this way, deformations or changes can be monitored by means of repeated surveys in different epochs [Pesci et al., 2005; 2007]. The laser signal is characterized by highly collimated, monochromatic, and coherent radiation that is well suitable for very short impulse generation in the nanosecond scale. The operating methodology of a time-of-flight laser scanner is similar to a laser range-finder, measuring the time it takes a laser pulse to travel from a transmitter to the surface surveyed, and back to a detector device. The range d is computed using the relation d = ct / 2, where t is the time of flight and c is the speed of light. The advantage of this instruments is the laser beam deflection over a very accurate angular grid, that can be obtained by oscillating and rotating mirrors, thus providing a wide coverage area between adjacent points. Each point is collected into a local reference system consisting of the origin at the instrument sensor, well-known angular parameters, and very accurate measurements of range. Together with point coordinates (x, y, z) , radiometric values related to the surveyed object’s reflectivity can be calculated from returned signal energy. The maximum measurable range depends on the illuminated material roughness and color, and the laser wavelength [Fidera et al. 2004, Pesci and Teza, 2008]. Divergence values for new generation long-range scanners are extremely reduced, illuminating very small surface elements for each shot. The spot dimension increases linearly with the distance, and is always greater than the lower limit of the instantaneous field of view (IFOV) due to physical diffraction. Effective laser scanner characteristics are defined by a set of parameters, including: range resolution (depending on telemeter efficiency), single point measurement accuracy (depending on the internal electronic device, signal-to-noise ratio and critical time needed for pulse recognition), beam divergence (which defines the IFOV, depending on laser wavelength), and minimum angular step (depending on the internal mirrors calibrated system) [Wehr and Lohr 1999]. Overlap is the laser scanning strategy that can reduce errors, because redundant points are acquired belonging to the same illuminated area. A common overlap is obtained by fixing the ratio between spot dimension (the area illuminated by a single pulse with a given divergence) and angular step so that a given point is measured 10 times. For instance, if the divergence is 3 mrad and angular variation about 0.3 mrad, at 100 m distance, an element included in a 3 cm area is observed 10 times. The final result of a laser scanner application is a very dense point cloud, with radiometric reflectivity data for each point

Analyzing Virtual Reference Station for GPS surveying: experiments and applications in a test site of the northern Apennine (Italy).

The availability of a GPS network of 10-20 km mean size, provides good topographical support for the measurement of ground displacements, even at a local scale such as a landslide. In particular, a series of multitemporal kinematic or rapid-static GPS acquisitions of a landslide allows a good characterization of its displacements if the measurements are referred to a GPS reference network. Nevertheless, a wider network formed by stations located at long distances, for example at several tens of kilometers, characterized by large spacing, can lead to results affected by high noise, degrading the accuracy of final point positions. In order to obtain an adequate GPS reference network, some virtual reference stations (VRSs) can be introduced, even if a network refinement based on VRS cannot reach the same accuracy of a real local network. Some experiments, including measurements on a real landslide, have been performed in order to evaluate the performance of this technique. The results point out that the standard deviation of the obtained solutions is about two or three times larger than those which can be reached using a real local network

Esperienza di misura mediante lo strumento tromino per lo studio delle vibrazioni e delle sollecitazioni naturali e antropiche

Il rumore sismico ambientale è l’insieme delle piccole vibrazioni sismiche presenti ovunque sulla superficie terrestre e generate da sorgenti naturali o antropiche. Tra gli esempi più significativi, si può pensare agli effetti delle perturbazioni atmosferiche sulle onde oceaniche ed alla loro propagazione sul continente come onde di superficie, al traffico veicolare e alle attività industriali, che producono onde superficiali di Rayleigh, e, in generale, all’attività dinamica terrestre. Le onde sismiche che ne derivano sono tipicamente a bassa energia, con ampiezze dell’ordine di 10-4/10-2 mm [Okada, 2003]. Inoltre, in base al contenuto in frequenza inferiore o superiore a 0.5 Hz, si parla rispettivamente di microsismica (primariamente di origine naturale) o microtremore (di origine generalmente antropica). Il rumore sismico ambientale è una sorgente di eccitazione per la risonanza del sottosuolo e degli edifici, da cui la possibilità di estrarre da esso, mediante opportune tecniche di analisi, informazioni interessanti sui sistemi risonanti studiati. Il rumore sismico può usarsi per lo studio della stratigrafia del terreno, sulla base dell’analisi degli spettri di potenza dei segnali e dei rapporti spettrali [Kanai e Tanaka, 1954; Lermo, 1993; Yamanaka et al., 1993]. In particolare, il metodo dei rapporti spettrali H/V è particolarmente interessante perché consente di ottenere informazioni affidabili utilizzando strumentazione di basso costo e facile impiego. Esso è basato sul calcolo del rapporto degli spettri di Fourier del rumore nel piano orizzontale H (generalmente lo spettro H viene calcolato come media degli spettri di Fourier delle componenti orizzontali NS ed EW) e della componente verticale V [Nakamura, 1989]. Il significato teorico del rapporto spettrale H/V è abbastanza immediato nel caso in cui si consideri un mezzo semplice formato da due soli strati: il bedrock, cioè lo strato duro e profondo, ed uno strato superficiale più soffice. Si immagini che l’onda di superficie che viaggia nello strato superficiale sia riflessa all’interfaccia tra gli strati e interferisca costruttivamente con le onde incidenti, sommandosi e raggiungendo ampiezze massime per l’effetto di risonanza. Ciò accade quando la lunghezza dell’onda incidente !m è tale che = 4H /(2m !1) m " , con m =1, 2,L, dove il fattore 4 /(2m!1) ) deriva dal fatto che, all’interfaccia tra un mezzo soffice ed uno duro, avviene inversione di fase. Le corrispondenti frequenze di risonanza sono pertanto date da (2 1) 4 = m ! H V f s m , (1) dove s V è la velocità di propagazione delle onde di superficie nel mezzo considerato. Poiché il modo fondamentale m = 1 è nettamente dominante rispetto a quelli superiori, si ha semplicemente H V f s r 4 = , (2) frequenza di risonanza che può individuarsi quale picco del rapporto H/V. Le frequenze proprie del sottosuolo possono essere quindi eccitate dal rumore di fondo e diventare visibili nello spettro del rumore sismico misurato in superficie. Il rumore sismico ambientale può anche essere utilizzato per identificare le frequenze proprie di vibrazione di un edificio. Un tale tipo di applicazione è profondamente diverso da quella che impiega i rapporti H/V, e i metodi di processamento dell’informazione sono piuttosto diversi, ma la strumentazione da utilizzare è esattamente la stessa perché in entrambi i casi l’obiettivo è estrarre informazione significativa da debole rumore sismico. In particolare, è attualmente disponibile uno strumento particolarmente compatto, economico e di semplice utilizzo come il tomografo digitale Tromino [Castellaro et al. 2005], grazie al quale questi metodi di sismica passiva hanno iniziato ad avere larga diffusione nella valutazione degli effetti di sito [Mulargia et al. 2007]. Nel caso in cui la frequenza di risonanza del sottosuolo coincida con quella di un edificio presente, in caso di terremoto può aversi un fenomeno di accoppiamento fra le due modalità di vibrazione. Questo effetto di amplificazione sismica produrrà un grande aumento della sollecitazione sugli edifici. Per questo motivo, l’amplificazione sismica è oggi considerata la prima causa dei danni indotti dal 6 terremoto e per questo motivo una attenta analisi delle frequenze caratteristiche dei siti viene effettuata nella fase di progettazione degli edifici. Va inoltre considerato che la stima delle frequenze di risonanza degli edifici può essere monitorata nel tempo, nella ben fondata ipotesi che un cambiamento sensibile dei modi principali della struttura sia legata al danneggiamento o all’alterazione della struttura stessa. In generale, un danneggiamento si traduce infatti in una diminuzione sia della frequenza di risonanza per ciascun modo, sia nella diminuzione del rapporto di smorzamento, tanto che l’analisi modale sperimentale (experimental modal analysis, o EMA) è correntemente utilizzata nella valutazione con tecniche non distruttive dello stato di salute di una struttura. In questo lavoro viene descritta una esperienza effettuata dall’INGV (Sezione di Bologna) di osservazione e di analisi dei dati rilevati mediante Tromino in un edificio situato in una zona altamente trafficata da mezzi e veicoli pesanti. Gli obiettivi di questa prima esperienza sono: (i) misurare le vibrazioni a cui gli edifici sono soggetti per effetto del traffico stradale, in modo da verificare se le normative vigenti in merito sono rispettate oppure no; (ii) fornire una prima stima delle frequenze di risonanza dell’edificio, da verificare in futuro in ulteriori sessioni di misura, operando in modo da rendere possibile un’analisi modale sperimentale completa a partire dai dati acquisiti ora