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- Volume 1, Issue 4, 2003
Near Surface Geophysics - Volume 1, Issue 4, 2003
Volume 1, Issue 4, 2003
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Cone penetration logging for the direct detection of LNAPLs contamination
Authors Jiri Kvapil and Stanislav MaresABSTRACTLight non‐aqueous phase liquids (LNAPLs), such as crude oil and by‐product fuels, represent a common source of organic contamination in the environment. Successful remediation of contaminated sites requires knowledge of the LNAPL distribution in the subsurface. The delineation and monitoring of the LNAPL plume carried out using fluid sampling in monitoring wells can differ from the reality because the residual LNAPLs are trapped in the pores of the soil. Surface geophysical techniques, such as GPR and ERT, can be used only for indirect LNAPL detection. This study suggests the use of cone penetration logging (a combination of cone penetration tests and geophysical logging) as a tool for detecting total hydrocarbon content (residual and movable) in shallow unconsolidated soil materials.
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Investigation of a fractured limestone cliff (Chartreuse Massif, France) using seismic tomography and ground‐penetrating radar
ABSTRACTEvaluating the stability state of a rock slope is a complex problem, mainly due to the lack of knowledge of the real state of the rock mass. Geophysical methods appear to be useful for investigating the deep discontinuity pattern, which may be poorly interpreted from surface observations. However, they have seldom been applied on steep rock slopes. The aim of this study is to test seismic tomography and ground‐penetrating radar on near‐vertical cliffs, and assess the quality of information that they can provide when investigating the characteristics of the fracture pattern inside the massif.
The test site is located in the Chartreuse massif, 20 km north‐west of Grenoble, France. It is a 15 m high limestone cliff, characterized by one main near‐vertical discontinuity set, including some wide open fractures.
Seismic tomography has been conducted between the vertical free surface and the plateau above, along three parallel profiles. Results show strong velocity gradients, from 800 to 3500 m/s. Some triangular low‐velocity zones can be correlated with field observation of open fractures, but different tests on synthetic models and on real data show that the method is too sensitive to such heterogeneous conditions to provide accurate information on the fracture pattern.
Ground‐penetrating radar surveys have also been acquired along vertical profiles on the cliff. We used three different antennae with centre frequencies of 35, 120 and 500 MHz. The penetration depth varied from 10 m (500 MHz) to about 20 m (35 MHz). The main reflectors are near‐vertical and most of them can be correlated with fractures observed on the site. The reflectivity varies strongly along one single reflector, indicating changes in the aperture and/or filling characteristics.
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Moisture assessment with small‐scale geophysics‐the Interurban project
Authors Martin Müller, Oliver Mohnke, Jürgen Schmalholz and Ugur YaramanciABSTRACTUrban soils are subject to strong variations in environmental conditions, such as water flow, solute transport and heat budget. The INTERURBAN research project has been set up to investigate the dynamics of water and solutes at urban locations with particular attention to spatial heterogeneity, organic soil substance, and soil‐biological transformation processes in the unsaturated zone. The objectives of INTERURBAN also define the tasks for geophysics: the development of non‐invasive procedures to map water distribution, water dynamics and important structural parameters at small scales. This paper focuses on the development of a geophysical methodology as a tool for soil research. Approaches to adapting geophysical methods for particular objectives in the framework of the project have been successful. Soil moisture could be derived from ground‐penetrating radar (GPR) in combination with timedomain reflectometry (TDR), high‐resolution direct‐current geoelectrics (DC on a dm scale) could be realized and a newly developed direct‐push spectral induced‐polarization (SIP) probe delivered high‐quality data. The resistivities and phases derived from the direct‐push SIP data correlate very well with the water content and decay times derived from nuclear magnetic resonance (NMR) measurements performed in the laboratory on a core from the same site. For DC geoelectrics and SIP, the layout of the sensors was a customized high‐resolution multi‐electrode configuration (a comb of electrodes with an electrode spacing of 5 cm) and the influence of the short electrode spacing (line electrode) was corrected for. For GPR metal plates were buried at a known depth to allow monitoring of the traveltime by means of water content. Furthermore, the ground wave has been used to determine the shallow water content which correlates well with TDR measurements. For surface NMR (SNMR), smaller loops and excitation intensities were used to enhance the resolution of the method for shallow depths.
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Seismic reflection imaging of an ultrashallow interface from a P‐wave data set with a poor signal‐to‐noise ratio
Authors Roberto Balia and Enrico GavaudoABSTRACTIt is well known that pitfalls are commonly encountered in the acquisition and processing of shallow reflection data. Although they can often lead to misinterpretations, the obsession with these difficulties can generate an excessively pessimistic attitude and ultimately lead to rejecting data that contain genuine reflections. The authors revisited a data set acquired during an ultrashallow P‐wave reflection experiment conducted in December 1996 on a subsurface model characterized by one main, complex interface. The model had been purpose‐built to control acquisition and processing decisions, and to contribute to clarifying at least some of the critical aspects inherent in the ultrashallow seismic reflection method. For the experiment, the acquisition geometry was purposely designed regardless of the known characteristics of the model, considering only a target depth of less than ten metres. The data, which had been acquired with CMP techniques, were generally characterized by a poor signal‐to‐noise ratio and had been considered completely useless. Initially, inexperience in acquiring and, above all, processing ultrashallow reflection data, in conjunction with mise‐valuations, had led to disappointing failures. Velocity analysis on reflection data had proved very difficult and standard processing sequences were inadequate. However, in this case a good result was obtained by modelling the near‐surface velocity using direct‐wave traveltimes analysis and by adopting a simple but segregated processing sequence, which proved necessary due to significant velocity gradients and large relative variations of the reflector depth. Time‐to‐depth conversion afforded a more than acceptable result.
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Cone penetration logging for the direct detection of LNAPLs contamination
More LessABSTRACTIt is becoming increasingly important to carry out efficient geophysical investigations into the hydrogeological conditions in larger esker areas. These investigations are aimed at mapping the prerequisites for groundwater or mineral take‐out or conditions for artificial infiltration. Today most of these investigations are carried out based solely on a few geo‐technical borings, giving only a small amount of information about the subsurface conditions.
We show that the combination of ground‐penetrating radar (GPR) and continuous vertical electrical sounding (CVES) measurements will, in a relatively fast manner, cover larger areas of investigation, and thus provide a large quantity of information about the subsurface. The combination of GPR and CVES will give detailed information about the top layers (down to 10–20 m), and will also provide good characteristics of layers down to 50–60 m.
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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)