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- Volume 5, Issue 5, 2007
Near Surface Geophysics - Volume 5, Issue 5, 2007
Volume 5, Issue 5, 2007
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GPR investigations for the study and the restoration of the rose window of Troia Cathedral (southern Italy)
Authors Nicola Masini, Luigia Nuzzo and Enzo RizzoABSTRACTThe development of cracks and distortions caused by past seismic events compromised the integrity of the rose window of Troia Cathedral, one of the most precious Romanesque monuments in southern Italy. Ground‐penetrating radar (GPR) using high‐frequency antennae (mainly 1500 MHz) was selected from among various non‐destructive testing methods for its high‐resolution imaging to scan the internal structure of the various architectural elements of the wheel window: the decimetre‐diameter columns constituting the rays, the ring decorated with intersecting arched ribwork and the surrounding circular ashlar curb. GPR was employed in the classical continuous reflection mode, moving the antennae manually along the architectural elements and paying exceptional care in the acquisition and processing stages to avoid positioning errors. Indeed, the challenging aspects of this case study were the geometrical complexity and small dimensions of the structural elements, causing many logistic/coupling problems. In spite of this, through proper interpretation techniques, based on signal analysis (presence of reflections and diffractions, velocity and attenuation variations) and correlation with features detected by visual inspection of the external surfaces, the GPR survey provided useful information on the internal structure of the rose window, detecting fractures and the boundaries of previously restored parts and locating hidden metallic components connecting the architectural elements. Information on the internal structure and spatial distribution of metallic junctions was essential for gaining insight into building techniques in order to discriminate between restoration strategies which may require either total or partial dismantling of the rose window. GPR results provided crucial evidence in favour of one of the (conflicting) hypotheses about the original building techniques, leading to the selection of partial dismantling as the most suitable restoration strategy. Analysis of measurements revealed the potential of GPR in the field of cultural heritage restoration, even in those cases characterized by complex geometry, structural brittleness and logistic difficulties, such as that discussed in this paper.
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Application of ground‐penetrating radar in detecting water leakage from artificial sandy ground
Authors Soon Jee Seol, Toshiyuki Yokota, Yuji Mitsuhata, Hyoung‐Seok Kwon and Toshihiro UchidaABSTRACTGround‐penetrating radar (GPR) surveys are frequently used to delineate the distribution of subsurface water. This paper demonstrates that GPR is suitable for detecting water leakage from underground construction areas because it produces detailed images of changes in water‐level that are related to water leaks. For this experiment, we used a test pit filled with river sand and a buried mortar wall that divides the pit into two sections. To simulate water leaks, we drilled holes through the mortar wall at a depth of 1.15 m and maintained constant water levels of 1.4 m and 0.4 m below the surface on each side of the wall. Water‐levels were controlled by pump‐extraction and injection using water‐control wells. Variations in water‐level under dynamic hydraulic conditions caused by water leakage through the holes were delineated by dense GPR single‐fold profiling measurements. Wide‐angle reflection and refraction (WARR) measurements were also performed to obtain velocity information and to support the interpretation of profiling data. To image minor changes in waterlevel accurately, we took minor topographic changes into account and applied finite‐difference method migration and time‐variant depth conversion to the GPR single‐fold profiling data. We were able to construct a three‐dimensional water‐level map using the water‐table depths extracted from migrated depth sections. Our results clearly demonstrate a rising trend in water‐level approaching the mortar wall, indicating water leakage from the drilled holes. In addition, the WARR data confirmed that the upward trend in water‐level continues to the mortar wall; it was not possible to image this trend on the single‐fold profiling section within 0.5–1 m of the mortar wall because of interference from measuring equipment buried close to the wall.
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Geophysical surveying an active well catchment in the presence of significant topography and anthropogenic disturbances
Authors Hendrik Paasche, Jens Tronicke, Hansruedi Maurer, Alan G. Green, Esben Auken and Fritz StaufferABSTRACTTraditional approaches to surveying well catchments usually involve investigations at two quite different spatial scales: very high resolution borehole‐based studies and low resolution tracer and pump tests. In recent years, various researchers have attempted to fill the wide gap between the two scales of acquired data by using a variety of geophysical tools that have high‐to‐medium resolution capabilities. Although geophysical surveying may be relatively straightforward at many locations, there are numerous regions where topography, the presence of ubiquitous metal objects and/or a lack of suitably placed boreholes cause the acquisition, processing, inversion and interpretation of certain types of geophysical data to be extremely challenging. It was necessary for us to address such problems in an investigation of an active well catchment near Zurich, Switzerland. The principal goals of our project were to determine the geometries and physical properties of the shallow sedimentary layers, with emphasis on a key water‐bearing gravel unit. To achieve these goals, four different geophysical methods were employed. In a first step, frequency‐domain electromagnetic measurements allowed the locations of buried metal pipes to be established. A 2D tomographic seismic refraction survey then provided information on the interface between the low‐velocity surface loamy topsoil and underlying high‐velocity morainal material. The high electrical resistivity of the important gravel unit allowed its upper boundary to be outlined in 2D models derived from geoelectric data and its lower boundary to be delineated in models based on 1D linked inversions of geoelectric and transient electromagnetic data.
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Plus�minus method to solve large�amplitude near�surface static corrections†
By Ralph BridleABSTRACTThe surface conditions in Saudi Arabia demonstrate significant variability ranging from flat gravel plains to sand�dunes, sabkhas (salt flats) and desert wadis (washouts). Saudi Aramco has used a single�layer velocity model from surface to datum for most of the prospective areas for both 2D and 3D seismic data. Source gathers have been interpreted by picking the first arrivals to obtain a linear fit for the direct and refractor arrivals using the intercept�time method. This creates depth and velocity control input for a multiple‐layer model of the near‐surface, from which static corrections to datum are calculated. On 2D lines with very large amplitude near‐surface anomalies, the plusminus technique has been applied to solve local areas. The implementation of the plus‐minus technique utilizes a variable near‐surface velocity derived from either upholes or the direct‐arrival velocity determined from source gathers. With this method, multiple source‐pairs build up effective spreads. Then by compiling high fold of delay‐times at a location, a statistical average is taken where the high fold reduces the significance of picking errors. The model is built utilizing multiple refraction layers and is referenced to the datum, thus tying it to the regional model.
The example 2D seismic line has a static model calculated from the single‐layer velocity model. The intercept‐time method was applied locally over a near‐surface anomaly caused by unconsoli‐dated material at the foot of a cliff. Further analysis of the near‐surface involved the plus‐minus method to solve the high amplitude time shifts.
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Near‐surface characterization of a geotechnical site in north‐east Missouri using shear‐wave velocity measurements
Authors Ahmed Ismail and Neil AndersonShear‐wave velocity (Vs) as a function of soil stiffness is an essential parameter in geotechnical characterization of the subsurface. In this study, multichannel analysis of surface wave (MASW) and downhole methods were used to map the shear‐wave velocity‐structure and depth to the bedrock surface at a 125m × 125m geotechnical site in Missouri. The main objective was to assess the suitability of the site for constructing a large, heavy building. The acquired multichannel surface wave data were inverted to provide 1D shear‐wave velocity profile corresponding to each shot gather. These 1D velocity profiles were interpolated and contoured to generate a suite of 2D shear‐wave velocity sections. Integrating the shear‐wave velocity data from the MASW method with the downhole velocity data and the available borehole lithologic information enabled us to map shear‐wave velocity‐structure to a depth on the order of 20m. The bedrock surface, which is dissected by a significant cut‐and‐fill valley, was imaged. The results suggest that the study site will require special consideration prior to construction. The results also demonstrate the successful use of MASW methods, when integrated with downhole velocity measurements and borehole lithologic information, in the characterization of the near surface at the geotechnical sites.
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Seismic tomography on the castle hill in Quedlinburg
Authors Th. Schicht, U. Lindner, J. Heckner, G. Strobel and I. RappsilberABSTRACTThe collegiate church of Saint Servatius (part of the UNESCO world heritage list) and other buildings on the castle hill in Quedlinburg show evidence of frequent damage caused by foundation subsidence. The geological structure consists of sandstone of differing hardness from the Cretaceous period. Geotechnical investigations to date have provided information about the underground structure at various points, but no comprehensive 3D spatial model of the entire area was heretofore available. Therefore, the non‐destructive seismic tomography method was used. On different locations around and on top of the castle hill, elastic waves were generated with a weight drop. On the other hillside the arrivals of the seismic waves were recorded by geophones. The result was a tomographic model of the 3D distribution of P‐wave velocities. The results showed a zonally differentiated seismic velocity scheme. Considerable differences in the rock strength were shown. Areas with slower seismic velocities indicate lower rock quality and therefore geotechnically weak zones. The areas of our investigations, which were identified as deconsolidated rock, correspond to damage to the buildings above.
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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)