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- Volume 5, Issue 6, 2007
Near Surface Geophysics - Volume 5, Issue 6, 2007
Volume 5, Issue 6, 2007
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Efficient ERT measuring and inversion strategies for 3D imaging of buried antiquities
Authors N.G. Papadopoulos, P. Tsourlos, G.N. Tsokas and A. SarrisABSTRACTThe optimum processing technique (2D vs. 3D inversion) to interpret and visualize parallel and/or orthogonal two‐dimensional surface Electrical Resistivity Tomography data collected from archaeological sites is examined in this work. A simple modification of a standard resistance‐meter geophysical instrument was implemented in order to collect parallel two‐dimensional sections along the X‐, Y‐ or XY‐direction in a relatively short time, employing a pole–pole array.
The sensitivity analysis showed that the distance between the parallel 2D lines must be smaller or, at the most, equal to the basic inter‐electrode spacing in order to produce reliable 3D resistivity images of the subsurface. This was confirmed by modelling and inversion of both synthetic and real data.
Direct comparisons of the quasi‐3D images, resulting from a posteriori combination of the inverted 2D sections, with the full 3D inverted resistivity models indicated the superiority of the 3D inversion algorithm in the reconstruction of buried archaeological structures, even in complex archaeological sites. Due to the inherent three‐dimensionality of many archaeological targets, quasi‐3D images suffer from artefacts. The combination of a single survey‐direction with a full 3D processing and interpretation scheme is adequate to image the 3D subsurface resistivity variation in detail. Furthermore, the implementation of a quasi‐Newton Jacobian matrix update technique reduced the processing time by one‐half without any significant loss of accuracy and resolution.
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TDEM magnetic viscosity prospecting using a Slingram coil configuration
Authors Julien Thiesson, Alain Tabbagh and Sébastien FlageulABSTRACTSoil magnetic properties are significantly modified by pedogenesis and human activity; consequently their study is an important source of information about soil structure and history. In addition to measurements of magnetic susceptibility, now commonly practised, magnetic viscosity measurements can be undertaken as a routine survey method if well‐matched instruments that are easy to use in the field and that offer a sufficient depth of investigation, exist.
A theoretical study, supported by field tests of a prototype, demonstrates that a Slingram perpendicular coil configuration with a one‐metre inter‐coil spacing allows the required one‐metre depth of investigation to be reached for both 1D and 3D models. The sensitivity of the instrument response to the viscosity is also very high over the whole measurement range (10μs to 1ms), while the sensitivity to electrical conductivity is reduced to a minimum.
A first experiment at the Roman site of Vieil‐Évreux illustrates how viscosity measurements can complement magnetic field and susceptibility measurements for a more accurate identification of an archaeological feature.
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Time‐lapse electrical resistivity tomography to delineate mud structures in archaeological prospections
Authors G. Ranieri, L. Sharpe, A. Trogu and C. PigaABSTRACTArchaeological sites in rural areas are often characterized by structural remains that are made of mud or raw brick, and that produce a very small contrast in physical characteristics, as the surrounding materials of both anthropogenic and natural origins have similar lithological and mineralogical properties. The main measurable differences are due to changes in compaction and porosity and, as a consequence, in permeability, which influence the hydrological behaviour of the subsurface materials.
In this paper, we present some applications based on Electrical Resistivity Tomography (ERT) surveys that are routinely applied in archaeological prospection, carried out over two different weather conditions. Measurement of the changing resistivity, corresponding to different moisture conditions in the subsoil, allowed us to identify archaeological strata and structures surrounded by natural, undisturbed soil.
A number of tests carried out on a physical model permitted a further definition of the acquisition parameters and methodologies to be used to secure the best results. The field surveys were carried out in the necropolis of “Pill’e Matta” (IV BC ‐ V AD), located in the suburbs of the metropolitan area of Cagliari, in the southern part of Sardinia, and in a Punic and Roman site near the village of Terralba, located in the Campidano plane, in the western part of Sardinia. The results show that time‐lapse prospecting can increase the resistivity contrast between archaeological structures and soil.
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GPR surveying over burial mounds: correcting for topography and the tilt of the GPR antenna
Authors Dean Goodman, Hiromichi Hongo, Noriaki Higashi, Hiromichi Inaoka and Yasushi NishimuraABSTRACTA new method for static corrections of radargrams, which accounts for the tilt that the GPR antenna encounters on sites with topography, has been developed. The method uses the average electromagnetic wave velocity and assumed antenna tilt according to the first derivative of the topography profile, in order to adjust the normal ray projecting from the GPR antenna into its correct subsurface location. Static tilt corrections for radargrams with under sampled scan locations can be effectively adjusted using scan sweeping across small angles. Radargrams that are corrected for topography and antenna tilt show that changes in the imaged locations of subsurface structures can be significant, resulting in improved focusing and more accurate positioning of reflections.
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Extended comparison of different GPR systems and antenna configurations at the Roman site of Carnuntum
More LessABSTRACTTo evaluate the suitability of ground‐penetrating radar (GPR) systems for archaeological prospecting, three different GPR systems with several antenna configurations were tested on one test area at the Roman site of Carnuntum, under identical environmental conditions, in 2004. Due to developments in hardware and software within the last year, these measurements where repeated and compared with the previous results. The data processing considers the needs of the different systems and antennas and aims to produce depth slices for best interpretation from an archaeological point of view. The results show advantages and disadvantages of the different systems and antennas, the need to process the data differently and the necessity to increase the power of the antennas.
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Verification and topographic correction of GPR data in three dimensions
Authors Jürg Leckebusch and Jürg RychenerABSTRACTTwo sites in a Roman town allowed a direct comparison between GPR interpretation and excavation results in three dimensions. All the reflection features mapped by the geophysical survey were identified in the subsequent archaeological excavation. Some of these anomalies were shown to be walls preserved as a single layer of stone or completely robbed out. However, a number of walls that were later excavated were either not detected at all by the GPR or not imaged to their full depth. The reason for this is the limited penetration depth of the radar energy due to attenuation, which can be visually estimated from the data. In addition, the presentation of the GPR results was enhanced when the depth information was coded into the map of the interpretation. Finally a new processing step called topographic correction was developed to compensate for the antenna tilt and elevation in three dimensions. The differences and advantages of this method compared to 2D tilt corrections are discussed. Apart from migration, this topographic correction significantly improves the geometrical 3D accuracy and interpretation of the GPR data.
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Beyond image analysis in processing archaeomagnetic geophysical data: case studies of chamber tombs with dromos
Authors S. Piro, L. Sambuelli, A. Godio and R. TaorminaABSTRACTWe analysed the effectiveness of three methods for interpreting magnetic maps in the third dimension, i.e. the depth. The possible synergies among the information obtained by each method have also been examined. The approach is based on a preliminary evaluation of the depth of the causative body according to the solution of Euler’s equation; the application of a two‐dimensional cross‐correlation technique enables the estimation of the spatial orientation, the shape and the susceptibility contrast of the causative bodies. The preliminary interpretation allows the input parameters and the necessary constraints (e.g. susceptibility contrast, maximum depth of the magnetic targets) to be tuned for the subsequent 3D modelling and inversion. We adopt the strategies for the quantitative interpretation, i.e. depths and shape, of the archaeological remains in the Sabine Necropolis of Colle del Forno, Montelibretti, Rome. The approach is shown to be effective for imaging the dromos and cavities of the tombs of this necropolis.
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Geophysical investigations at the Himera archaeological site, northern Sicily
Authors P. Capizzi, P.L. Cosentino, G. Fiandaca, R. Martorana, P. Messina and S. VassalloABSTRACTIn this paper we present a geophysical survey that was carried out as a research activity during field trips to the Himera archaeological site, where relics of a large Greek settlement are still buried, effected by a joint group of archaeologists and geophysicists during an educational project. Two‐dimensional (2D) resistivity and ground‐penetrating radar (GPR) surveys were performed to locate buried archaeological structures at this archaeological site. The results of the GPR surveys show some anomalies characterized by semi‐hyperbolic shapes. In some restricted areas of the town, two grids of parallel GPR profiles were acquired while time‐ and depth‐slices were calculated to identify the extension of the buried structures. In fact, the shapes of many anomalies recognized in the slices seem to describe the distribution of the relics (i.e. walls, roads, etc.).
A 2D electrical resistivity profile showed clear anomalies, which were connected with wall‐like relics. This was also obtained by using on‐site calibration on partially uncovered structures, while the low background resistivity values were interpreted as representing river deposits having different moisture characteristics. Recommendations were also given to the archaeological site‐excavators, following which some of the results were confirmed by subsequent archaeological excavations.
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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)