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- Volume 1, Issue 1, 2003
Near Surface Geophysics - Volume 1, Issue 1, 2003
Volume 1, Issue 1, 2003
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Georadar and seismic investigations over the Glacier de la Girose (French Alps)
Authors G. Sénéchal, D. Rousset, A.‐L. Salomé and J.‐R. GrassoABSTRACTSeveral ground penetrating radar (GPR) acquisitions have been performed on a temperate glacier (glacier de la Girose, southern French Alps) in order to determine the ability of this method in terms of penetration and resolution. The final goal of these experiments is to constrain the structural context on an area where 3‐C wide‐band seismic recorders have been deployed for icequake analysis. Radar data have been recorded using 50 MHz antennae in order to get information about the glacier thickness, and 250 and 500 MHz frequency antennae have been used for shallow high resolution structural images. For comparison, thickness of the glacier has been independently evaluated from seismic data. Local determination of the electromagnetic wave velocities has been obtained from a CMP acquisition. Several hyperbolas can be picked up to 1600 ns, the strongest one being interpreted as the reflection from the ice‐bedrock interface, at 1150 ns (around 90 m depth). This result is well correlated to the seismic data which show on the unmigrated time section a strong dipping reflection between 50 (West) to 70 ms (East) (between 90 and 120 m after migration and depth conversion).
Where the radar profiles cross crevasses, the unmigrated sections obtained with 250 and 500 MHz antennae show shallow diffraction patterns associated with low amplitude strip. On the migrated sections, one can observe deeper diffraction points, probably generated by rock material or water feeders. The high resolution data provided with radar allow to distinguish several periodic arrivals between 1150 and 1600 ns. These are interpreted as multiple reflections due to a thin water layer separating the base of the glacier and the bedrock. The lower resolution of seismic data doesn't allow to detect this thin layer.
In terms of resolution, deep radar resolution is one order of magnitude better than the seismic one and shallow radar resolution is two orders of magnitude more accurate than seismics. Shallow radar appears as the only efficient tool for imaging the shallow glacier structures.
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The use of electrical conductivity mapping in the definition of an aquifer vulnerability index
Authors Reinhard Kirsch, Klaus‐Peter Sengpiel and Wolfgang VossABSTRACTThere is an increasing demand for vulnerability maps which show the degree of exposure of aquifers against pollution. Parameters shown in these maps are generally cation exchange capacity or clay content of the near surface layers, or quantities related to the infiltration time of surface water. Instead of determining these parameters by means of drillings and geological investigation, we propose the use of geophysical techniques for vulnerability mapping and for the interpolation between drillings. A geophysical quantity which is closely related to these parameters is the electrical conductivity. In this paper the relation between clay content and electrical conductivity is shown and used for vulnerability mapping. It is demonstrated that ground and airborne electromagnetic (EM) techniques are suited for large scale data acquisition.
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Resistivity imaging to determine clay cover and permeable units at an ex‐industrial site
Authors Anna Vickery and Bruce A HobbsABSTRACTA previous geophysical and invasive site investigation over an ex‐industrial site near the Forth Estuary, Edinburgh, identified a network of subsurface oil‐pipes and shallow contaminants. A subsequent resistivity survey has now been conducted to determine possible contaminant migration pathways. Resistivity images measured along 28 profiles have been inverted to obtain resistivity models and their collation has enabled a pseudo three‐dimensional resistivity image of the site to be constructed. Shallow subsurface resistivities indicate that the majority of the site is covered by a layer of clay, approximately 5‐10 m thick, typically regarded as sufficient for containing contaminants within the near surface. However in one region the clay layer is shown to be thin and underlain by permeable sandstone thus presenting a possible pathway for downward migration of fluids. Resistivity values at greater depths infer the existence of several sandstone units and faults that may aid the transportation of contaminants away from the site. There is particular concern that pathways may exist that allow the transportation of contaminants towards the Forth Estuary. A revision of the local geology for the site to include the faulted sandstone has been suggested.
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New logging tool for measuring azimuth of the ground‐water flow
Authors Stanislav Mares, Tomas Henzl, Leo Woltaer, Karl‐Norbert Lux and Uwe StumpABSTRACTIn hydrogeological research for ground water as well as in many contamination studies, the direction of the subhorizontal ground water flow is an important information influencing further decisions in exploitation of the ground water resources or in application of remediation measures.
New construction of the logging tool for determining azimuth of the ground water flow in a single well and its laboratory and field testing is presented. The sophisticated logging tool ‐ combination of an electronic compass with a special circular photometric sensor detecting the time changes of the tracer plume in the measuring slot ‐ offers the information on direction of subhorizontal ground water flow in a single well for velocities ranging between 10‐6 to 10‐1 m/s. The diameter of the tool is 76 mm, length 1.12 m and weight 13 kg. The tool is operating together with current logging systems.
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High resolution GPR imaging and joint characterization in limestone
Authors M. Pipan, E. Forte, F. Guangyou and I. FinettiABSTRACTWe focus on the application of Ground Penetrating Radar (GPR) to evaluate limestone characteristics of interest in environmental and engineering studies, and in particular: a) to image joints, bedding planes and cavities; b) to improve accuracy and resolution of the method; c) to evaluate joint/bedding planes characteristics which affect the radar response with particular reference to thickness, sedimentary infilling, water/clay content and spatial frequency. The work is based on experiments carried out close to road cuts and cavities, where the exposed rock face allows calibration and validation of results. The test‐sites are located in NE‐Italy and are part of the Peri‐Adriatic carbonate Platform. The rocks in the area of study date back to the Paleocene and are mainly peritidal regressive limestone sequences consisting of more than 90% carbonates. Joints and bedding planes surfaces, as results from direct inspection at the rock face, are sufficiently smooth to neglect roughness effects. We demonstrate that imaging can be improved by non‐conventional data acquisition paradigms, and in particular by multi‐offset (linear multi‐fold, LMF) methods. Multi‐offset/multi‐azimuth (azimuthal multi‐fold, AMF) techniques were exploited to select optimum grid orientation and offset range for LMF application. Multi‐fold velocity analysis on CMP gathers shows rather constant wave propagation velocities. They slightly vary between different test‐sites (between 10 cm/ns and 12 cm/ns) primarily due to macro/microscopic characteristics of limestone, such as spatial frequency of joints and porosity respectively, which affect water content in the rock mass. Constant velocities allowed application of post‐stack time migration algorithms. Such algorithms attained imaging accuracy below 3% in the reconstruction of the detectable discontinuities at most test sites. A Kirchhoff algorithm proved to be the optimum solution as it effectively handled the steep dips (up to 70°) that characterize most of the examined sets of joints. Enhanced data quality further results from the application of original processing techniques, such as, in particular, Hough Transform based coherent noise/background removal and Wavelet Transform based instantaneous parameters computation and analysis. Maximum penetration depth at the examined test sites ranges between 15 m for 250 MHz (central frequency) bow‐tie shielded antennas and 23 m for 50 MHz unshielded resistively loaded linear dipoles. As for resolution, antennas in the range of 200‐250 MHz apparently provide effective discrimination of joints/bedding planes spaced not less than 40 cm, and seem therefore the adequate choice at the examined test sites. Low frequency antennas, in the range of 50 to 100 MHz (central frequency), provide a maximum 55% increment in penetration depth at the expenses of a substantially diminished resolution (around 20% of that provided by 200‐250 MHz antennas). The comparison of modelling results to multi‐fold data allows discrimination of radar response from joints filled by air and clayey deposits. Such results were validated by geological evidence at the exposed rock face.
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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)