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- Volume 6, Issue 4, 2008
Near Surface Geophysics - Volume 6, Issue 4, 2008
Volume 6, Issue 4, 2008
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Potential and pitfalls of GPR traveltime tomography for ancient buildings investigation: experiments on a small‐scalem real and synthetic calcarenitic block
Authors L. Nuzzo, T. Quarta, M.B. Galati, M. Fedi and B. GarofaloABSTRACTEven if its use is not widespread in the archaeological community, GPR tomography is a viable tool in the maintenance of Cultural Heritage and for the diagnosis of internal defects in masonry, originating either at the building stage or later because of normal decay or natural disasters. Two‐dimensional GPR traveltime tomography aims to obtain information on the distribution of the dielectric constant on a section of the investigated medium from the picked direct arrival traveltimes between sources and receivers. This paper shows the results of a GPR tomographic experiment on a calcarenitic stone block with an empty central hole, using 1000 MHz as transmitter and 1800 MHz as receiver antennas. The original aims of this work were firstly, to assess the usefulness and limits of very basic tomographic tools, accessible also to the non academic community, in the limited case of locating voids in small‐scale structures (pillars or columns) and secondly, to identify possible pitfalls due to acquisition/ processing procedures or to inadequacy of the inversion algorithm. We examine some problems encountered in data acquisition and we propose a method to estimate the effective bandwidth of antenna pairs of different nominal frequencies and to estimate the zero time correction. The experiment shows that picking the first arrivals is a very delicate operation when the airwaves interfere with the transmitted ones and that using the wrong picked traveltimes in the inversion could lead to inconsistencies or to strong reconstruction artefacts. Finite‐difference numerical modelling is helpful both for identifying the correct arrivals to be picked and for exploring the dependence of the tomographic inversion on cell size, geometry of transmitters and receivers and initial model. The inversion results show a strong dependence on the angular ray coverage. The general improvement observed by increasing the illumination directions confirms the opportunity of using, whenever all sides are accessible, as in the case of columns or pillars, both parallel and orthogonal antenna positioning. In the presence of strong velocity gradients, as in this case, even using the best acquisition configuration for transmitters and receivers, the straight‐ray tomography based on the SIRT algorithm can only detect the anomaly but is unable to resolve adequately its geometry and dielectric constant. Although more time‐demanding, curved‐ray tomography or more sophisticated algorithms are therefore necessary for a better characterization of internal defects in most problems of structural assessment.
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Investigation of underground cavities in a two‐layer model using the refraction seismic method
Authors Tihomir Engelsfeld, Franjo Šumanovac and Nenad PavinABSTRACTThis paper presents a new approach to the investigation of underground cavities. Our technique is based on the refraction seismic method. We have studied a two‐dimensional, two‐layer geological model. In our model, the lower seismic velocity layer is situated above the higher seismic velocity layer, with a circular cavity positioned within the upper layer. We have investigated the influence exerted by the cavity on the first arrivals of seismic waves. The obtained traveltimes are solutions of the eikonal equation and are presented using the time‐distance graph. All refracted waves encountering the cavity have to circumvent it, as it represents an impediment to the propagation of seismic energy. This circumvention causes delays in the first arrivals of the seismic waves at the surface as compared to traveltimes with no cavity present. These delays create a characteristic shape of the time‐distance graph, characterized by the peak point in which the plot line has a discontinuous change. Using this graph and analysing the delays of the first arrivals, we have derived expressions for determining both the position and size of the circular cavity. The practical application of the derived relations has been tested on a model test site built in a natural rock setting. This simple method indicates the presence of the cavity. The accuracy of the calculated cavity parameters: the horizontal position , the depth and cavity radius depends on the geophone spacing. For geophone spacing equal to or less than the cavity radius, the accuracy of the method is shown to be acceptable. The maximal estimated error is equal to a half geophone spacing. Finally, we have demonstrated that this method is also applicable in the detection of non‐circular cavities.
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Geotechnical analysis and 4D GPR measurements for the assessment of the risk of sinkholes occurring in a Polish mining area
Authors Henryk Marcak, Tomislaw Golebiowski and Sylwia Tomecka‐SuchonABSTRACTThe study presented in this paper concerns georadar investigations at a selected former coal mining site in Upper Silesia (Poland) where the risk of sinkhole appearance is high. The results of 3D GPR surveys obtained in three measurement sessions in December 1997, October 2006 and March 2007 were interpreted. The 4D interpretation, i.e., a time‐space analysis, allowed for the identification of loose zones in the ground and fractured zones of the rock mass, which might be a source of sinkhole creation. After the first measurement session, on the basis of the GPR survey results, a dangerous, fractured zone in the ground was localized. This zone was confirmed by a borehole. Between the second and third sessions, a large sinkhole appeared on site, as predicted on the basis of georadar investigations. The geomechanical analyses presented in this paper explain the development of the fractured zones over the earlier mining excavations. Such zones accumulate water and high contrasts of dielectrical permittivity appear, allowing the use of the GPR method for the location of these zones.
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A new broadband radiomagnetotelluric instrument: applications to near surface investigations
More LessABSTRACTA new radiomagnetotelluric (RMT) instrument has been developed that can record time series of electric and magnetic fields in the frequency range from 10 kHz to 1 MHz. Military and civilian radio stations broadcasting in this frequency range are used as transmitters. Transfer functions (e.g., apparent resistivities and phases as a function of frequency) are determined by spectral analysis using a newly developed processing software. The transfer functions presented here are the first ones ever at frequencies above 300 kHz and they can lead to a better resolution of the shallow structure. The new RMT instrument enables quick and efficient mapping of shallow structures and supports tensor measurements thereby allowing a 2D or 3D interpretation of field data. The new RMT system was tested in the laboratory and then applied successfully in the field to several environmental and engineering problems. Case studies from applications in St. Petersburg, Russia and in Uzin, Ukraine will be presented. The observed apparent resistivities and phases are reliable and they show smooth frequency dependence. The data were inverted for 2D conductivity models under the assumption that the strike direction was known beforehand.
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Seismic characterization of an Alpine site
Authors Laura Valentina Socco, Daniele Boiero, Cesare Comina, Sebastiano Foti and Roger WisénABSTRACTSite amplification of earthquake ground motion can arise from local stratigraphic and morphological conditions. In this respect, a detailed geo‐mechanical 2D model is needed, in particular for peculiar situations such as the ones in Alpine valleys. This paper reports a site characterization case history for seismic response analysis, where a combination of both body and surface wave seismic techniques has been used. In particular, it is shown that synergies and mutual cross‐checking of information lead to a reliable 2D model for site amplification studies.
The evaluation of the deep structure and the location of the seismic bedrock have been obtained through seismic reflection and combined active‐passive surface wave analysis, while the shear‐wave velocity profile of the sediments has been obtained by downhole measurements and surface wave tests. Furthermore, a detailed compressional wave velocity model has been retrieved by tomographic inversion of the first arrivals picked on seismic reflection records. This model is useful for geological interpretation as well as for quality control of the other methods. Surface wave analysis has also been performed on the ground roll of the seismic reflection records. The large amount of profile oriented data has made it possible to reconstruct lateral variations of the shear‐wave velocity in the soil deposit. The final pseudo‐2D shear‐wave velocity model is the product of an innovative inversion approach, based on the integration of the Monte Carlo and laterally constrained least‐squares techniques. In the laterally constrained inversion the shear‐wave velocity profiles obtained from downhole tests and data from passive surface wave measurements have been used to constrain the inversion. This has greatly improved the final solution.
This case history underlines the necessity of combining different seismic techniques to derive an accurate and reliable 2D model for site amplification studies. Through several different and independent analyses of the same seismic records, the cost effectiveness of the whole survey is optimized and a full exploitation of the information contained in the reflection seismic data set is achieved.
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A multicomponent GPR survey in Marambaia Isthmus
Authors J. M. Travassos, S. André and J. E. MusaABSTRACTThis work deals with a multicomponent ground‐penetrating radar data set obtained on Quaternary unconsolidated deposits of eolian, alluvial, lacustrine and marine sands with a choice of two cross‐dipole and two co‐polar antenna configurations. The geophysical response is characterized by strong reflectors observed in both co‐ and cross‐polarized 100 MHz data sets down to a two‐way time (TWT) of 400 ns. With this data set we delineate a given horizon using its overall polarization direction, which is different from that defined by the position of the antennas and of profile direction. That horizon, whose existence was not previously known, represents a more complex situation than an isolated body, e.g., a buried metallic pipe, a situation well studied elsewhere.
The polarizing horizon is characterized by linear polarization and a reasonable uniformity of an Alford rotation angle that maximizes the co‐polar responses. With the modal Alford angle we produce improved images corresponding to the two co‐polar components and a conspicuous improvement in the continuity of the reflectors across the polarizing horizon. All goes as if the antennas were oriented parallel and perpendicular to the main polarizing direction of the structure. In addition, the two co‐polar components display a good degree of independence from each other as revealed by their eigenvalues. Our results illustrate how a data set collected with less than optimal field geometry can still satisfactorily reveal the subsurface.
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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)