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- Volume 4, Issue 4, 2006
Near Surface Geophysics - Volume 4, Issue 4, 2006
Volume 4, Issue 4, 2006
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Subsurface water‐content identification in a crypt using GPR and comparison with microclimatic conditions
Authors Giovanni Leucci, Rosella Cataldo and Giorgio De NunzioABSTRACTThe effects of climate, pollution and human negligence cause severe and sometimes irreversible damage to buildings and monuments of cultural interest. It is well known that the presence of water and/or moisture content in a porous material is the initial cause of deterioration. In a previous paper, the authors reported an integrated study on a building of cultural importance, namely the crypt of the Cattedrale di Otranto in Apulia, Italy, based on non‐destructive integrated biological and physical surveys. The method described was able to identify the ‘internal’ factors responsible for deterioration. It was discovered that the distribution of moisture in the stone depended mainly on adverse environmental conditions, and the presence of wet buried structures in the ground.
The first aim of the present study was to identify subsurface water‐content in this same crypt using a ground‐penetrating radar (GPR) technique, and to compare these results with those of the previous microclimatic survey. In particular, the existence of underground discontinuities was verified; we located them and analysed their influence. Moreover, by means of velocity analysis, we obtained a quantitative estimate of the volumetric water‐content under the pavement of the crypt. This finding completes the results of the previous research, as it indicates the causes of the deterioration in the crypt and provides significant information, on the basis of which, effective decisions can be made for safeguarding the historic building.
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Karst investigations using microgravity and MASW; Application to Orléans, France
Authors Nicole Debeglia, Adnand Bitri and Pierre ThierryABSTRACTAn integrated geophysical approach to detecting and characterizing karst structures in an urban environment was applied experimentally to partially explored karst conduits located in Orléans, France. Microgravity was performed to detect voids, in conjunction with multichannel analysis of surface waves (MASW) for the purpose of identifying areas of mechanical weakness.
Microgravity detected negative anomalies corresponding to known conduits and succeeded in identifying the probable extensions of this network in unexplored areas. Control boreholes located on these extensions encountered several levels of water‐saturated voids, probably belonging to the shallowest part of the karst system, overlying the main conduits. Buried urban networks, accurately located by ground‐penetrating radar (GPR), were shown to have no significant gravity effect. Simulations using the compact inversion approach to characterize the size and density of environmental disturbances confirmed this conclusion. In this context, the gravity method has been shown to be suitable for detecting near‐surface (<25 m deep) karst features.
The MASW method, which analyses Rayleigh‐wave propagation, can determine the mechanical behaviour of superficial formations and serve as an indicator for subsurface heterogeneities such as voids or fractures. At the Orléans site, MASW provided evidence of perturbed zones superimposed on gravity anomalies, characterized by the appearance of several dispersion modes, velocity inversions and the attenuation of seismic markers. One of these features was characterized by low velocities and was interpreted as an area of mechanical weakness, confirmed by pressure measurements in the boreholes.
Repeated gravity measurements, or time‐lapse microgravity, were conducted on the anomalous areas to ascertain gravity reproducibility and detect possible temporal variations due to subsurface mass redistribution that may indicate site instability. A two‐year experiment revealed low‐amplitude gravity changes that were recorded in the two sensitive zones. However, their significance is still unclear and these changes need to be validated by further repeat experiments.
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Analysis of multi‐offset GPR data: a case study in a coarse‐grained gravel aquifer
Authors A. Becht, E. Appel and P. DietrichABSTRACTMulti‐offset ground‐penetrating radar (GPR) data were collected in a coarse‐grained gravel aquifer located in a glacial delta environment within dipping foresets (at the Tettnang aquifer test site, SW Germany). We apply prestack processing techniques and normal‐moveout (NMO) velocity analysis in preparation for stacking. The estimation of propagation velocities is of considerable importance for converting time‐domain radargrams into depth‐sections and for an interpretation in terms of petrophysical properties. In our case, interval velocity determination is difficult because reflector dip angles are variable, and stacking and NMO velocities differ significantly since the antenna offset is comparable to the reflector depth. Using a synthetic two‐layer model, we systematically study possible errors in interval velocity determination. We compare the cases of decreasing velocity with depth (typical for ground‐penetrating (GPR) surveys of aquifers) and increasing velocity with depth (typical for seismic surveys). In the case of decreasing velocity with depth, the discrepancy between stacking and NMO velocity is considerable and, consequently, interval velocities calculated with the Dix equation or the more accurate 2D NMO approximation for dipping layers show unacceptably high errors. The errors are much smaller in comparable models with increasing velocity with depth. Thus, the small‐offset approximation inherent in the NMO concept is not appropriate for the first case. Consequently, it is necessary to model the true common‐midpoint (CMP) raypaths to determine realistic interval velocities from our GPR field data.
Stacked GPR sections of multi‐offset data show significant improvements in imaging of reflectors and depth of investigation compared with a standard constant‐offset section. This allows a more reliable interpretation of geological structures. Simultaneous acquisition of four channels, which is possible with some commercial GPR systems, yields significantly better results than constant‐offset standard acquisition without increasing the efforts for data acquisition. Major reflectors in the GPR section can be correlated with distinct contrasts of porosity, represented by the base of open‐framework gravels and sand beds within poorly sorted gravels.
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Velocity spectra and seismic‐signal identification for surface‐wave analysis
Authors G. Dal Moro, E. Forte, M. Pipan and M. SuganABSTRACTRayleigh‐wave dispersion is observed every time acoustic‐impedance stratification occurs, and its analysis is suitable for vertical shear‐wave profile reconstruction. Accurate dispersion‐curve identification is essential in order to properly determine the shear‐wave velocity distribution of a medium. Data sets characterized by several events generate complex velocity spectra that can lead to possible misinterpretations. We analyse a real data set by taking into account theoretical dispersion curves and synthetic data obtained from numerical simulations in order to avoid possible pitfalls that could arise from the complex trends exhibited in the (frequency–wavenumber) and (velocity–frequency) domains. In the domain in particular, we show that reflection events and their multiples generate coherences that could be misinterpreted because of their similarity to typical higher‐mode dispersion curves. Another observed signal is interpreted in terms of guided waves and related phenomena. The results of the fundamental‐mode dispersion curve inversion performed via genetic algorithms indicate a sedimentary cover stratification that simple reflection analysis cannot reveal.
The present case study highlights the importance of a synergic approach, based on integrated synthetic and field data analysis, for correct interpretation of all the wavefield components in the velocity spectrum.
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Fresnel reflection coefficients for GPR‐AVA analysis and detection of seawater and NAPL contaminants
Authors José M. Carcione, Davide Gei, Marco A.B. Botelho, Ana Osella and Matias de la VegaABSTRACTWe obtain the transverse electric (TE) and transverse magnetic (TM) Fresnel reflection coefficients for different interfaces in the subsoil: air/fresh‐water, air/seawater, fresh‐water/seawater, air/NAPL (non‐aqueous phase liquid), NAPL/water and water/NAPL. We consider a range of NAPL saturations, where the complementary fluid is water with 0.65 ppt (parts per thousand) of NaCl. The common feature is that the TM mode (parallel polarization) has a negative anomaly and the TE mode (perpendicular polarization) has a positive anomaly. For the cases studied in this work, pseudo‐Brewster angles appear beyond 40° for the air/NAPL and NAPL/water interfaces and at near offsets (below 40°) for the water/NAPL interface. Pseudo‐critical angles are present for the water/NAPL interface. Besides the reflection strength, the phase angle can be used to discriminate between low‐ and high‐conductivity NAPL, when the properties of the upper medium are known. A wavenumber–frequency domain method is used to compute the reflection coefficient and phase angle from synthetic radargrams. This method and the curves can be used to interpret the amplitude variations with angle (AVA) of reflection events in radargrams obtained with ground‐penetrating radar (GPR).
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Detecting brine zones in salt deposits with the ground penetrating radar (GPR) for safety assessments of underground waste disposals
Authors Jan Igel, Gerhard Kurz and Rüdiger SchulzABSTRACTThe presence of brine is one of the principal hazards of underground waste disposal. A clear reflection at a brine reservoir would be expected when carrying out ground‐penetrating radar (GPR) measurements because of the high contrast in the electrical conductivity between dry rock‐salt and brine. However, this is not the case as many previous measurements have shown, and as is also demonstrated by some of our GPR measurements.
The lack of a distinct reflection at higher frequencies can be explained by the presence of a gradient zone leading to a frequency‐dependent reflection coefficient. Therefore, measurements at two frequencies (25 and 100 MHz) were carried out in a potash salt‐mine with a massive brine reservoir. When comparing the energy distributions of these measurements, the area of the brine reservoir shows a frequency‐dependent reflection coefficient as opposed to areas without brine. Several finite‐difference forward calculations using different frequencies and models deduced from drill‐cores from the area of the brine reservoir are presented. They demonstrate that brine zones are detectable by comparing different frequencies if they are not enclosed by a sharp interface but by a gradual increase in the brine content.
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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)