Ramón González-Drigo

Characterization of a Romanesque Bridge in Galicia (Spain)

This article presents the characterization of a mediaeval bridge located in Fillaboa, Galicia (northern Spain). The study of this bridge involves data acquisition about the structure (geometry, visual inspection of damages, and nondestructive testing), the evaluation of the possible damage mechanisms compatible with the observed cracks and fissures, and the dynamic evaluation of the structure. This bridge is a masonry four-lancet arches bridge, with damage on the piers and abutments. Two non-invasive methodologies are applied to obtain information about the bridge: ground-penetrating radar (GPR) survey and ambient vibration noise measurements. The drawings of the structure were created using close-range photogrammetry (CRP). A finite elements model of the structure was obtained prior to the vibration field measurements, as a preliminary evaluation. Data obtained from GPR and the geometry determined with CRP were the information used in this preliminary model of the bridge. This model was improved using the dynamic field test to compare model behavior and to validate the numerical results. A second and more accurate model was then obtained by using finite elements according to the experimentally measured modal frequencies (the possible first three transversal vibration modes of the bridge).

Nanozonation in Dense Cities: Testing a Combined Methodology in Barcelona City (Spain)

Microzonation is widely used in seismic risk evaluations to define the predominant period values, which are usually associated with extended areas of a few hundred meters. However, the representative values corresponding to these areas are obtained from few measurements in each area. Thereby, results are accurate only in the case of depth-dependent soils. However, not detected narrow and sharp lateral changes in soil are potentially the cause of imprecision and could be a source of specific errors. This article aims to present several tests conducted in order to emphasise the importance of accurate selection of points, to underscore the necessity of more precise and detailed evaluations, and to suggest a possible methodology to select the most appropriate data acquisition points. Results highlight the need to divide microzonation areas into smaller zones for a precise evaluation in locations where sudden changes in soil characteristics exist. Therefore, in such sites the requirement of nanozonation appears; defining zones with the same soil response. Distance between vibration measurements could be the main problem for nanozonation; data acquisition in areas with irregular geology can be time consuming when a precise analysis is required. In the most complicated environments or in dense cities, it could even be unfeasible. Consequently, it is necessary to establish a functional methodology to adequately distribute the measurement points throughout the area. On this occasion, three sites in Barcelona city were studied. This city is surrounded by mountains at NW, W, and S, and by the Mediterranean Sea at N and E. As a consequence, the shallow geology is characterized by many paleochannels and streams that are currently buried. These geological structures most likely affect the soil response. Several tests were carried out to determine this dependence. The tests were based on Ground Penetrating Radar (GPR) surveys to define the paleochannels position and on vibration measurements in order to define properly the soil response. The results from both methods were compared to the known geology to accurately define the effect of the shallow geological structures in the predominant period and in the GPR images. Areas with the same geological unit but different materials were identified in the GPR images, allowing the selection of the most appropriate distance between vibration measurements in each place. As a final result, predominant periods that were measured over the same geological unit but over different material showed changes higher than the 40% in short distances. This procedure could improve the soil response maps, including nanozonation.

GPR resolution in cultural heritage applications

The non-destructive study of historical buildings, archaeological sites and other Cultural Heritage structures requires high resolution methodologies and a good knowledge of the potential of the different methods. Laboratory measurements provide valuable information about the ability to detect different targets and to determine structural problems, but these data must be compared to the results obtained in real and complex structures. In this work, we present experimental GPR measurements made in order to determine the spatial resolution under laboratory conditions. These results were compared to the data obtained in different GPR surveys applied to Cultural Heritage. The information obtained in drillings, in visual inspections and in old documentation about the historical buildings and archaeological sites is used to determine the resolution in each case.

Analysis of the attenuation in soils and water content in remote sensing surveying

Ground-penetrating radar (GPR) is a high resolution surveying method applied to civil engineering, surface geology, archaeology and other disciplines. Mainly it is used solving the direct problem and obtaining a model of the studied medium. Otherwise, the study of the inverse problem could provide other valuable information: the electromagnetic properties of the medium. These parameters are obtained from the changes of the velocity, attenuation and frequency of the recorded wave. The physical properties of the medium related to those wave parameters are, mainly, the water content and the porosity. Several lab experiences are performed in order to obtain these parameters from different soil samples. Porosity and water content are measured and controlled. Velocity is obtained by measuring the two-way travel time of the reflected wave and comparing wave reflected amplitudes on the surface of the samples. Attenuation coefficients are determined from the analysis of the amplitude of the wave traveling in different thickness samples. Frequencies velocities and wave attenuation are analyzed in the different cases in order to characterize those different media and to relate its water content and its porosity with these measured parameters. The experimental results were also compared with the complex refraction index model (CRIM).

Experimental analysis of the resolution in shallow GPR survey

Ground-penetrating radar (GPR) is a high resolution surveying method applied to civil engineering, surface geology, archaeology and other disciplines. Today, GPR is an effective technique for investigating the integrity of concrete structures. As a non destructive technique, it is particularly suited for the assessment of large structures such as prestressed concrete bridges, highways, railway tracks and tunnels. A significant parameter in GPR high frequency surveys is the horizontal resolution. This parameter indicates the capability of the method to detect anomalies and to discriminate between adjacent elements. In concrete structures analysis the horizontal resolution lead to determine the exact position of reinforcing elements. This paper presents the basics of GPR, its limits, and the experimental measurements and the signals post-processing performed in order to determine the horizontal resolution of a 1.6 GHz antenna in concrete structures assessments.

Characteristics of the GPR field pattern antennas

Ground-Penetrating Radar has become a popular non-destructive and non-invasive tool in different kind of applications: civil engineering, archaeology, concrete and masonry analysis, etc. The selection of the antenna frequencies depends on the application, but each antenna has a radiation pattern and some characteristics that have influence in the final interpretation and in the model obtained for the studied medium. The knowledge of these features and its coupling effects with the medium could improve the results of the GPR prospecting studies. In this work, some experimental procedures were carried out in order to obtain the 1.6 GHz centre frequency antenna characteristics in the air and in one material medium and to compare them. First, the study of the attenuation due to geometrical spreading was performed. This result was compared with the amplitude attenuation in a material medium, deduced from the GPR experimental data. Second, the shape of the radiation pattern was estimated in laboratory for different distances between the target and the antenna. Near field and far field were considered during the experimental data acquisition. Third, the relative amplitude of the reflected wave (in dB) was obtained depending on the relative position of the antenna over the target. The shape of the radiation pattern and the relative amplitudes obtained in the air were compared with those obtained in a slow medium (water). This slow medium was characterized with the wave velocity and the attenuation factor of the GPR signal.

GPR backscattering applied to urban shallow geology: GPR application in seismic microzonation

This paper describes the application of ground penetrating radar (GPR) in the study of the shallow geology of urban environments, applying the survey in Barcelona city. The objective is to determine the existence of clusters of granular materials that could be related to paleochannles or subterranean streams. The method proposed for this study is to evaluate the signal to noise ratio (SNR) in radar data, caused by the backscattered energy as consequence of small targets. Laboratory tests and previous works indicate that variations of the SNR allow the detection of these sedimentary structures. The results will be used in the design of a detailed grid of points where soil vibrations must be measured. The combined methodologies allowed the construction of precise seismic microzonation maps of the city.

GPR assessment of the modernist Santa Creu i Sant Pau hospital in Barcelona: GPR application in cultural heritage

The Santa Creu and Sant Pau hospital is a monumental ensemble built in Barcelona between 1901 and 1930 on the site where six small medieval hospitals were founded in 1401. It is a complex of buildings which represent the art nouveau architecture in Barcelona. It was declared World Heritage site by the UNESCO in 1997. In the previous eight decades of clinical activity, modernist buildings have experienced numerous architectural modifications that have affected both their structures and ornamental elements. The deterioration of the buildings became apparent in 2004. In this situation, in 2006 a master plan to assess the state of the hospital evidenced the critical state of many of its pavilions, and therefore a complete rehabilitation project for the entire ensemble was approved. The assessment of some specific structural elements, that is spandrels and columns, located in the Pharmacy pavilion and in the hospital church, has been performed and evidenced the presence of metallic elements, sub-structures and, in some cases, corroded iron elements, embedded in the masonry structure. A detailed description of these embedded elements and some damaged areas inside the columns was performed through a non-destructive prospection with ground penetrating radar. A 1.6 GHz nominal center frequency antenna was used in this assessment.

GPR survey in heritage structures in Chile. The case study of the Museum of Contemporary art in Valdivia: GPR survey in the MCA (Valdivia, Chile)

The Museum of Contemporary Art of Valdivia is located in an area characterized by high seismic vulnerability. The building and the surrounding area were truly affected by the 1960 earthquake. The edification, built in 1855, is next to the River Valdivia, which undergoes high fluctuation in the salinity. Moreover, the structure suffers of lack of maintenance and the visual inspection denotes different types of damage in different zones. Previous to the rehabilitation, the structure was evaluated with Ground penetrating radar (GPR). The two main objectives of the survey were determining structural arrangement of walls, floor and roof, and to detect possible damage. Visual inspection indicated that some of pathologies were sunken layers, cracks and zones affected by high water content. Many of the damage could be caused by the seismic hazards in the area, and the humid climate. The GPR assessment was carried out with a shielded 800 MHz centre frequency antenna, covering all the different zones of the building.

Ground Penetrating Radar Applications in Seismic Microzonation

The work presents a methodology that combines a first GPR survey and a subsequent measurement of seismic ambient noise vibration. The GPR signal characteristics used in this analysis are two: the amplitude of the background noise in the A-scans, and the frequency content of the received signal. The background noise could be consequence of three main sources: clutter as consequence of external reflections, electronic noise and energy randomly scattered in the medium. The first source could be identify in the GPR B-scans and usually produce anomalies similar to those caused by reflections in the targets inside the medium. The second source produce a continuous noise in the A-scans characterized because the average value is approximately constant. The third source introduces noise in the A-scans but its amplitude depends on the randomly backscattered energy. As consequence, the amplitude of this noise could be used in order to identify sudden changes in the shallow geology, always depending on the grain size distribution. The frequency content depends strongly on the water content. The analysis of the spectrum, combined with the analysis of the B-scans and the backscattering noise in the A-scans could be used in order to locate active subterranean streams.