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- Volume 2, Issue 2, 2004
Near Surface Geophysics - Volume 2, Issue 2, 2004
Volume 2, Issue 2, 2004
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Combined high‐resolution magnetics and GPR surveys of the roman town of Flavia Solva
Authors S. Seren, A. Eder‐Hinterleitner, W. Neubauer and S. GrohABSTRACTHigh‐resolution magnetics and ground‐penetrating radar (GPR) prospecting methods are both employed over large areas (125 000 and 45 000 m2) at the Roman town of Flavia Solva in Austria. High‐resolution magnetics using a multisensor caesium gradiometer were used in a survey in a raster of 0.5 x 0.125 m. GPR measurements using a PulseEKKO 1000 device applying 450 and 900 MHz antennae were carried out in a raster of 0.5 x 0.05 m. The collected data are evaluated and visualized in grey scale, using the self‐developed software, APMAG and APRADAR. In addition, the small prospected areas are combined automatically into one coherent image and transformed into global coordinate system (geo‐referencing) for archaeological interpretation in the Geographical Information System (GIS). The magnetic prospecting shows very clearly the extent of the Roman town of Flavia Solva. The Roman streets and walls appear in the magnetics as negative anomalies (black). The positive magnetic anomalies could be due to brick walls, which did not show up as structures in the GPR. On the other hand, GPR provides a lot of detailed three‐dimensional information about the archaeological structures.
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Finite‐difference modelling for tunnel seismology
Authors Guido Kneib and Alice LeykamABSTRACTThe diagonal differencing algorithm, which takes spatial derivatives along the diagonals of the finite‐difference grid, is well suited to solve the elastodynamic equations for tunnel seismic modelling because it is accurate and stable at strong low‐order discontinuities of the elastic medium. Spatial high‐order finite‐difference operators increase the accuracy and efficiency of the algorithm except at steps in the elastic properties, where these operators fail. A solution is to use high‐order finite differences in most of the grid but to employ two‐point second‐order operators at low‐order discontinuities. This keeps memory requirements and computational effort on a level that allows 3D simulations on moderately priced PC hardware. Measurements with the commercial tunnel seismic prediction method yield complex seismic common‐receiver gathers with long direct‐wave pulses. Finite‐difference simulations in two and three dimensions reveal that these long pulses are due to P‐wave reverberations in and around the water‐filled shothole. Direct S‐waves originate close to the detonation point by conversion at the shothole–rock boundary. A dispersive, low‐frequency guided wave reverberates with the average S‐wave velocity inside the excavation damaged zone around the tunnel. The guided wave creates multiple body waves inside the undisturbed rock as it reverberates. It carries useful information about the degree and depth of the rock excavation damage but the guided wave often overlaps tunnel surface‐wave modes and may be difficult to extract from seismograms. Receivers should be buried deeper than the extent of the excavation damaged zone so that the guided waves and tunnel surface waves, which circulate around the tunnel, do not dominate seismic records and hide direct S‐waves, reflections or other late arrivals.
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Geophysical investigation of antique iron furnaces: insights from modelling magnetic and VLF data
Authors Mathieu Darnet, Pascal Sailhac and Guy MarquisABSTRACTMagnetic and very low frequency electromagnetics (VLF) surveys at an Antique iron mine near Saales (France) revealed several large magnetic anomalies (of the order of a few hundred nT), strong electrical conductivity contrasts (10–1000 Ωm) and out‐of‐quadrant VLF phases (φ >90°).To define both the location and the vertical extent of the underground artefacts, we interpret our magnetic data with Euler deconvolution and a 2D wavelet transform that combines maps of the total field and its gradient. We also explain out‐of‐quadrant VLF phases using simple 3D EM forward modelling. A joint interpretation of magnetic, VLF and topography data enables us to locate iron‐ore exploitation trenches, Antique iron furnaces and their surrounding slags at depths between 1 and 3 m, in good agreement with results obtained during a recent road construction.
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Study of saltwater intrusion using 1D, 2D and 3D resistivity surveys in the coastal depressions at the eastern part of Matruh area, Egypt
Authors K. Gemail, A. Samir, C. Oelsner, S.E. Mousa and S. IbrahimABSTRACTSaltwater intrusions into shallow aquifers near the coast are an important environmental problem along the whole coastal strip of the western desert of Egypt. This phenomenon was studied in two selected coastal depressions on the eastern side of the Matruh area by a geoelectrical survey which comprised several 1D sounding points as well as 2D and 3D resistivity‐imaging surveys.
For the 2D and 3D field surveys, pole‐dipole (both forward and reverse) and pole‐pole arrays were used, respectively. The spacing between adjacent electrodes was 10 m for the 2D resistivity surveys. A 5 × 5 grid of electrodes was used for the 3D survey. The spacing between electrodes was 10 or 20 m, depending on the expected depth of freshwater in each respective area. The choice of electrode spacing was based on the results of interpretation of the depth of the freshwater–saltwater boundaries, as derived from 1D resistivity soundings using a Schlumberger array. The sounding points were distributed over the study areas and the soundings were conducted prior to the 2D and 3D survey.
The data sets were inverted using the smoothness‐constrained least‐squares method. The inversion results indicate that the subsurface resistivity distributions in the whole area are highly inhomogeneous and change rapidly within a short distance. However, from the resulting models, it was possible to correlate the ranges of resistivity with subsurface geological information available from shallow boreholes. The horizontal and vertical sections presented illustrate the configuration of the subsurface conditions that would have been, most probably, insufficiently accurate in the case of 1D modelling.
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Propagation of errors due to incorrect positions of sources and detectors in wave‐field tomography
Authors P.L. Cosentino and V. SanfratelloABSTRACTTomographic data processed by 2D inversion programs can produce fairly large distortions due to incorrect source and/or detector positions. This problem is very serious in high‐frequency electromagnetic tomography (GPR), due to the dimensions of the transmitter and receiver antennae. The errors can even be larger when coupled antennae are used (receiver and transmitter inside the same box) whose positions are not clearly known. Similar errors can be involved in seismic tomography, for instance when the mechanical connection between transducers and sample is defective.
In this paper the problem has been studied using synthetic data which were calculated for different acquisition geometries. Synthetic data have been distorted assuming a position error in the whole set of sensors, transmitters or receivers. Tests have been carried out using a homogeneous, isotropic medium and various simple inhomogeneous models, characterized by different geometric boundaries.
The responses for all the models have been calculated (both forward and inverse problems) assuming straight raypaths, also in the case of velocity variations. The calculated traveltimes were processed using an LSQR algorithm implemented in an iterative inversion program, containing two damping factors to reduce total and local velocity contrasts. The inversion results have been reproduced graphically and the analysis of residuals has been carried out. Finally, propagation of these residuals, both in time and velocity, is discussed.
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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)