Columns are one of the most usual supporting structures in a large number of cultural heritage buildings. However, it is difficult to obtain accurate information about their inner structure. Non-destructive testing (NDT) methodologies are usually applied, but results depend on the complexity of the column. Non-flat external surfaces and unknown and irregular internal materials complicate the interpretation of data. This work presents the study of one column by using ground-penetrating radar (GPR) combined with seismic tomography, under laboratory conditions, in order to obtain the maximum information about the structure. This column belongs to a "Modernista" building from Barcelona (Spain). These columns are built with irregular and fragmented clay bricks and mortar. The internal irregular and complex structure causes complicated 2D images, evidencing the existence of many different targets. However, 3D images provide valuable information about the presence and the state of an internal tube and show, in addition, that the column is made of uneven and broken bricks. GPR images present high correlation with seismic data and endoscopy observation carried out in situ. In conclusion, the final result of the study provides information and 3D images of damaged areas and inner structures. Comparing the different methods to the real structure of the column, the potential and limitations of GPR were evaluated.
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 paper 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 final result, predominant 2 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.
The assessment of cultural heritage requires high-resolution and non-destructive methodologies. Ground-penetrating radar is widely applied in the inspection of historical buildings. However, some structures with curved surfaces make the radar data acquisition process difficult and consequently the following data interpretation. This paper describes a case study concerning a circular and buried Greek monument. This monument is a magnificent tomb buried with irregular stones. However, its structure and the internal stones arrangement are unknown. Therefore, a radar survey was carried out to achieve two main objectives: (i) identification of hidden elements and arrangement of the stones and (ii) detection of specific zones where further restoration and maintenance should be recommended. The methodology for the radar data acquisition involves the use of a laser scan in order to define accurately each radar line, covering all the internal surface of the tomb. Radar data processing was developed by converting Cartesian coordinates into polar coordinates. This procedure allows defining better the internal anomalies, improving the radar data interpretation. The main results of the survey were three: (i) the presence of a hidden target buried in the corridor access to the tomb; (ii) the description of the internal structure of the walls of the tomb, defining the stones arrangement and the position and depth to the keystone; and (iii) the existence of delimited zones where the signal is highly attenuated, probably due to a high salt content.internal damage, is also decisive in restoration and conservation processes. The inner information is usually gathered by means of non-destructive surveys in many cases.Ground-penetrating radar (GPR) is a non-destructive geophysical surveying technique that is nowadays applied to structures assessment. The methodology lays in the emission of highfrequency electromagnetic pulses that travel trough the medium. The changes in the electromagnetic properties of the medium are discontinuities, which are identified by the reflection of the energy. As a consequence, the technique provides images where the depth to the discontinuity is determined by the two-way travel time of the reflected waves on the A-scans. The projection on the surface is defined by the coordinates of the position of the receiver antenna in the B-scans. The knowledge of the depth and position of buried targets is fundamental in archaeological applications. Hence, the method has been widely used in this field during the last decades (Neubauer et al.
Abstract:The analysis of clutter in A-scans produced by energy randomly scattered in some specific geological structures, provides information about changes in the shallow sedimentary geology. The A-scans are composed by the coherent energy received from reflections on electromagnetic discontinuities and the incoherent waves from the scattering in small heterogeneities. The reflected waves are attenuated as consequence of absorption, geometrical spreading and losses due to reflections and scattering. Therefore, the amplitude of those waves diminishes and at certain two-way travel times becomes on the same magnitude as the background noise in the radargram, mainly produced by the scattering. The amplitude of the mean background noise is higher when the dispersion of the energy increases. Then, the mean amplitude measured in a properly selected time window is a measurement of the amount of the scattered energy and, therefore, a measurement of the increase of scatterers in the ground. This paper presents a simple processing that allows determining the Mean Amplitude of Incoherent Energy (MAEI) for each A-scan, which is represented in front of the position of the trace. This procedure is tested in a field study, in a city built on a sedimentary basin. The basin is crossed by a large number of hidden subterranean streams and paleochannels. The sedimentary structures due to alluvial deposits produce an amount of the random backscattering of the energy that is measured in a time window. The results are compared along the entire radar line, allowing the location of streams and paleochannels. Numerical models were also used in order to compare the synthetic traces with the field radargrams and to test the proposed processing methodology. The results underscore the amount of the MAEI over the streams and also the existence of a surrounding zone where the amplitude is increasing from the average value to the maximum obtained over the structure. Simulations show that this zone does not correspond to any particular geological change but is consequence of the path of the antenna that receives the scattered energy before arriving to the alluvial deposits.
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