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- Volume 10, Issue 1, 2012
Near Surface Geophysics - Volume 10, Issue 1, 2012
Volume 10, Issue 1, 2012
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Processing stepped frequency continuous wave GPR systems to obtain maximum value from archaeological data sets
Authors Jacopo Sala and Neil LinfordABSTRACTStepped frequency continuous wave ground penetrating radar (GPR) systems allow highly detailed data sets to be collected across a wide bandwidth using a multi‐element array antenna, such as the 3D‐radar GeoScope system. Although the final presentation of the results is similar to time‐domain systems, the correct processing of the initial frequency domain data acquired in the field is essential to obtain the maximum information from the site. Processing begins with the identification of a suitable frequency window for conversion to the time‐domain using an inverse Fourier transform and demonstrates how the use of a varying frequency window with depth can improve the final results. Various methods for background subtraction to minimize the loss of data quality and cope with the strong surface reflection experienced by an air‐launched antenna are also explored. The geometry of the air‐launched antenna is also considered with specific regard to developing a three‐dimensional migration model that accounts for the variation in a signal response expected from such a system. This paper explores these themes with relation to data from archaeological sites and also considers the data processing challenges presented by high density (e.g., 0.075 m x 0.075 m) multi‐hectare surveys.
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From pseudo‐3D to full‐resolution GPR imaging of a complex Roman site
Authors Alexandre Novo, Henrique Lorenzo, Fernando I. Rial and Mercedes SollaABSTRACTThis case history demonstrates how an extra effort applied to data acquisition can benefit the interpretation of 3D GPR data over a Roman site of great complexity situated in a semi‐urban area. Two surveys (during May 2007 and July 2008) were accomplished after Roman wall remains had been found, by chance, during soil removal work at Agro da Ponte (Lugo, NW Spain). The same 250 MHz antenna was used for both of them. The first campaign covered the entire area using a pseudo‐3D strategy (GPR lines spaced by 0.5 m), which was only enough for defining areas of interest and some archaeological features. Based on these results, the next campaign focused on a smaller area and was based on an ultra‐dense grid strategy (GPR lines spaced by 0.1 m), which eventually revealed full‐resolution images of walls, apses and chambers of a Roman villa.
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GPR prospecting of cylindrical structures in cultural heritage applications: a review of geometric issues
Authors Luigia Nuzzo and Tatiana QuartaABSTRACTCultural heritage diagnostics often involves the application of geophysical methods like ground‐penetrating radar (GPR) along non‐planar surfaces. Standard processing algorithms are based on the hypothesis of half‐space geometry. In the investigation of cylindrical structures like columns with a small radius, this basic assumption is strongly violated and the application of conventional algorithms may be inappropriate. Although frequently overlooked, this fact implies that appropriate algorithms should be developed to take into account the additional data complexity arising from the unusual acquisition setting and exceptional care should be used in interpreting the radargrams. The first part of this paper offers a systematic review of problems related to geometric issues (air waves, multiple reflections, shape of the various traveltime curves) performing a theoretical analysis of major differences between cylindrical and half‐space models. The second part shows, through numerical simulations and real data examples, the inappropriateness of the half‐space model in the processing and interpretation of GPR surveys along cylindrical structures. The numerical simulations help visualizing the differences in reflection/diffraction features from buried and above‐surface objects and errors arising from the use of inappropriate models. The real data examples offer a tangible demonstration of the benefits obtained from exploiting the correct cylindrical model and the additional information gained from later arrivals, such as multiple reflections, in constraining the estimation of electrical parameters (propagation velocity) related to constitutive and health states of the structure.
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Ground‐penetrating radar survey at the Roman town of Mariana (Corsica), complemented with fluxgate gradiometer data and old and recent excavation results
Authors Lieven Verdonck, Frank Vermeulen, Cristina Corsi and Roald DocterABSTRACTThis paper presents the results of a GPR survey carried out at the Roman town of Mariana (Corsica, France). Excavations (1959–1965 and 2000–2007) yielded a Roman street with houses and shops, an early mediaeval cathedral and a mediaeval bishop’s palace. When compared with the hypothetical town limits derived from aerial photography, old cadastral maps and the location of two cemeteries, the excavations are in an eccentric position. The principal aim of the geophysical survey was to shed more light on the Early Imperial town centre (1st–2nd century AD), which was to be found further north. The results from a fluxgate gradiometer survey demonstrated the presence of buildings with an orientation corresponding to the excavated street. In the GPR data, an orthogonal street system became noticeable and most of the buildings can be identified as private dwellings. In several parts of the town, there are indications for more than one occupation phase. For example, in the north‐western insula of the main survey area, the GPR results show a large building complex with deep foundations, as well as shallow walls of poor construction quality in the courtyard of this building. A trial excavation confirmed this dichotomy: it revealed solid walls with brick facings originating in the 1st or 2nd century AD, as opposed to alignments of loose boulders, not older than the 3rd century. On the basis of the GPR results, two small excavation trenches from the 1930s, one of which contained the remains of a bathhouse, can now be located exactly.
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GPR investigation in different archaeological sites in Tuscany (Italy). Analysis and comparison of the obtained results
Authors S. Piro and S. CampanaAbstractA Ground‐penetrating Radar (GPR) survey can enhance the quantity and quality of information when applied to archaeological prospection. The potential of the GPR method lies both in its relevance to a wide range of site conditions and the complementary nature of the data in comparison with other geophysical methods.
The areas described in this paper were ‘detected’ by the Laboratory for Landscape Archaeology and Remote Sensing of University of Siena, during aerial prospection between 2001–2005. Analysis of the aerial photographs allowed interpretation of the Aiali, Castellina and Pava sites, province of Grosseto and Siena (Tuscany, Central Italy). These sites are related to quite a limited chronological range between late Roman and the early mediaeval period.
All sites were studied through a multimethodological project based on the integration of field‐walking and digital global position system (DGPS) surveys combined with different geophysical investigations such as: differential magnetics, ground penetrating radar (GPR) and automatic resistivity profiler (ARP).
This paper demonstrates the effectiveness of GPR over the indicated sites characterized by differences in the soil condition and hypothesized archaeological features. With this method a high‐resolution data acquisition was adopted with the aim of reconstructing the location, depth and shape of the archaeological structures in the selected areas. Signal processing and the time‐slice representation technique were used for the analysis of the collected data.
Archaeological excavations and interpretations were then conducted systematically after completing the geophysical surveys (from 2006–2009), which confirmed the location and shape of most of the individualized structures. The obtained results demonstrate the accuracy with which GPR data can be matched to excavation data and the improvement in target definition.
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A multidisciplinary analysis of the Crypt of the Holy Spirit in Monopoli (southern Italy)
Authors Giovanni Leucci, Nicola Masini, Raffaele Persico, Giovanni Quarta and Carmen DolceABSTRACTIntegrated geophysical surveys were undertaken in the Crypt of the Holy Spirit located in the Monopoli territory (Apulia region, southern Italy). The crypt is a remarkable example of a hypogeal church, sculptured in the rock. It is dated between the 12th and 13th century and underwent several architectural changes during the centuries. Geophysical surveys were performed in order to obtain information about the structural conditions of the crypt and to investigate the probable existence of features of archaeological interest (such as tombs) inside the crypt. Surveys were integrated into a comprehensive project for the restoration and the valorization of the crypt. Seismic (sonic and ultrasonic) and ground‐penetrating radar (GPR) methods were used. GPR investigations provided information on possible buried remains. Some of the anomalies were emphasized and enhanced by microwave inversion tomography. Seismic refraction tomography along with GPR was performed on the hypogeal vault of the crypt, to detect possible internal fractures.
The columns of the crypt were investigated by ultrasonic and GPR techniques that demonstrated the presence of cracks and deteriorations. Finally, GPR prospection was performed on a wall in the presbytery. The GPR results allowed us to characterize a detachment phenomenon and to indentify a previous door opening.
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The shrine of Edward the Confessor: a study in multi‐frequency GPR investigation
By Erica UtsiABSTRACTA series of systematic ground‐penetrating radar (GPR) investigations is being carried out at Westminster Abbey. Following an earlier investigation into the 13th century Cosmati mosaic in the Sanctuary, GPR surveys were commissioned in order to locate the site of a large royal tomb. Antennas of 400 MHz and 4 GHz frequency were used in the area of the High Altar steps and on the floor of the current Shrine Chapel. The investigation uncovered evidence of the target tomb, a number of other graves, some previously unknown and at least one buried object. The paper briefly discusses the frequencies selected, their respective contributions and illustrates how these have resulted in a better understanding of the archaeological and historical evidence.
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Ground‐penetrating radar resolution in cultural heritage applications
Authors Vega Pérez‐Gracia, Ramón González‐Drigo and Roger SalaABSTRACTHigh‐resolution methods are required in the non‐destructive study of historical buildings, archaeological sites and cultural heritage structures in general. Ground‐penetrating radar (GPR) is widely used in such studies. However, radar images of archaeological sites and cultural heritage architecture tend to be unclear and are difficult to interpret. Knowledge of the antenna’s behaviour and the optimum expected resolution could help to interpret radar data and define a limit of the real resolution in field surveys. Laboratory tests could provide information about the optimum resolution for each specific antenna. The main goal of this study was to obtain experimental data under lab conditions and thus define the optimum resolution expected in radar surveys. This could then be considered as a limit in field surveys. All of the lab tests were performed using homogeneous media (water and sand) to avoid interference and clutter. As a result, simple images were obtained. The optimum resolution was defined from radar data acquired under these favourable, controlled conditions. In all cases, resolution was defined as the distance (considered as a function of wavelength) between two targets (in the vertical or horizontal axis) that were needed to obtain separate anomalies. All the optimum values were considered to define the limits of the survey resolution in the different cases presented in this paper. In each case, the optimum values were compared with the result and with the radar images to define the best approach to the expected resolution in architectural heritage surveys. To conclude, we define some experimental values of the optimum resolution, described in terms of wavelength. These experimental data are likely to provide a better approach than theoretical estimations.
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Instability analysis of the Villa Arianna site at Castellammare di Stabia (Naples)
More LessABSTRACTThe detection of fractures using ground‐penetrating radar is usually obtained by analysing the attributes of reflected and scattered energy. In this paper, I test advanced processing of the GPR direct ground wave in order to successfully detect and map near‐vertical fractures that generally affect structures located in unstable areas. I show that the advanced processing applied to the GPR time window of a direct ground wave (embedding the early reflection arrivals) overcomes the problem that often occurs when detecting near‐vertical distensive fractures on the basis of reflected and scattered energy. For the validation of the processing, I use a synthetic model simulating an oblique fracture in a homogeneous and stratified host material. The theoretical radargrams show that an oblique vertical fracture induces variations in the ground direct wave, in terms of both two‐way traveltime and shape. Taking into account such evidences, I relate the fracture to the attributes of the delay time of the ground direct wave and the spectral analysis of a time window embedding the direct waves.
I test the advanced processing on real data acquired in the Villa Arianna archaeological complex (Naples) by analysing three kinds of floors composed of heterogeneous material, concrete and ancient mosaic. The last two floor types are affected by extensive cracks, which are visible on the floor. The analysis of attributes of actual data indicates that both attributes are effective for the detection of fractures in an homogeneous near‐surface layer and that the spectral analysis is more effective than the delay time of the ground direct wave for an heterogeneous layer. The selected GPR attributes are quickly and easily applicable in the detection and monitoring of near‐vertical distensive fractures because they can be calculated using GPR data acquired with a system deployed in reflection and continuous mode.
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Volumes & issues
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Volume 22 (2024)
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Volume 21 (2023)
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Volume 20 (2022)
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Volume 19 (2021)
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Volume 18 (2020)
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Volume 17 (2019)
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Volume 16 (2018)
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Volume 15 (2017)
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Volume 14 (2015 - 2016)
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Volume 13 (2015)
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Volume 12 (2013 - 2014)
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Volume 11 (2013)
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Volume 10 (2012)
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Volume 9 (2011)
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Volume 8 (2010)
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Volume 7 (2009)
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Volume 6 (2008)
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Volume 5 (2007)
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Volume 4 (2006)
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Volume 3 (2005)
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Volume 2 (2004)
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Volume 1 (2003)