bluuebunny/crossref_metadata_2025_split huggingface dataset:
Assessing reliability of 2D resistivity imaging in mountain permafrost studies using the depth of investigation index method
Authors: Marescot L. , Loke M.H. , Chapellier D. , Delaloye R. , Lambiel C. , Reynard E. 
Published: August 2009

ABSTRACT2D electrical resistivity tomography has been applied within a mountain permafrost environment to assist in ice location. In the context of climate change, a warming process could partially thaw this permafrost and thereby increase the risk of slope instabilities. The extent and location of permafrost are therefore of considerable interest to civil engineers.The most challenging aspect of resistivity surveys on mountain permafrost concerns the surface layer, which mainly consists of large blocks with air voids. To overcome the very poor electrical contact, long steel stakes and sponges soaked in salt water are used as electrodes. Nevertheless, only a weak current can be injected. Another challenging aspect is the high resistivity contrast between frozen and unfrozen material, which makes inversion and interpretation difficult and problematic. In order to assess whether features at depth, indicated by the data, are real or are artefacts of the inversion process, a special inversion algorithm was applied to process depth of investigation (DOI) index maps. This method carries out two inversions of the same data set using different values of the reference resistivity. The two inversions give the same resistivity values in areas where the data contain information about the resistivity of the subsurface. On the other hand, the final result depends on the reference resistivity in areas where the data do not constrain the model.As can be deduced from field data from the Swiss Alps and the Jura Mountains, this methodology prevents over‐interpretations or misinterpretations of inversion results in mountain permafrost studies. From the DOI calculations, it is evident that little reliable information on the bedrock under the massive ice can be obtained and that the resistivity within the high resistivity zones cannot be determined accurately. The DOI map also helps to explain the occurrence of erratic and non‐geological structures at depth and indicates to what depth an inverted resistivity profile can provide results.
Properties and effects of measurement errors on 2D resistivity imaging surveying
Authors: Zhou Bing , Dahlin Torleif 
Published: August 2009

ABSTRACTElectrode spacing errors and errors correlated with the magnitude of the observed potential are two key factors that affect the data quality for DC resistivity imaging measurements. This paper investigates the properties and effects of these two kinds of error on 2D resistivity imaging or inversion for practical applications. By analytic analysis and numerical simulations, the off‐line and in‐line electrode spacing errors were quantitatively estimated for all common electrode arrays (pole‐pole, pole‐dipole, pole‐bipole, Wenner, Schlumberger, dipole‐dipole, γ‐array, Wenner‐β) in 2D resistivity imaging surveys. Meanwhile, the spreading patterns of the spacing errors in the pseudosection and the possible artefacts in the imaging (inverted model) are evaluated. We show that the magnitude of the spacing errors are quite different with these arrays, being largest for dipole‐dipole, Wenner‐β and γ‐array surveys, for which a 10% in‐line spacing error may cause twice as large an error (>20%) in the observed resistance or apparent resistivity, which in turn will produce some artefacts in the inverted model. The observed potential errors obtained with the reciprocity principle and collected from different sites and with different electrode arrays, were analysed to show the properties of the potential error caused by many aspects in the field. Using logarithmic plots and error pseudosections, we found that with different electrode arrays and at different sites the potential errors demonstrate a general property, which may be regarded as a negative‐power function of potential reading. Power net transients, background telluric variation and instrument malfunction are possible sources that may cause the large errors present as outliers deviating from this function. We reaffirm the fact that the outliers are often correlated with high contact resistances for some of the electrodes used in a measurement, but this may also be caused by an unsatisfactory connection between the electrode and the cable due to, for example, dirt or oxide on the connectors. These outliers are often the main part of the errors affecting the imaging results. Furthermore, a robust inversion and a smoothness‐constrained inversion were applied to the investigation of the effects of the measurement errors. Using two real data sets, we show that the smoothness‐constrained least‐squares inversion is much more sensitive to the potential errors than the robust inversion, but the two inversion schemes produce very similar models with a high data quality. Artefacts or indefinite parts in the inverted models correlate with the distribution zones of the outliers in the potential error pseudosection.
GPR abilities for the detection and characterisation of open fractures in a salt mine
Authors: Grégoire Colette , Halleux Lucien , Lukas Volker 
Published: August 2009

ABSTRACTGround‐penetrating radar is widely used for mapping fractures in resistive rocks such as salt. In operating mines, fractures are often induced by the mining activity itself. Knowledge of the position and the characteristics of the fractures, and their evolution in space and time, are very important for mine safety. In the salt mine discussed in this paper, radar profiles are carried out daily to detect fractures in the roof of the galleries. For this study, radar measurements are carried out using two different systems at two different sites in the mine. Dynamic and static data are obtained at various antenna–roof distances. A supplementary profile is registered at 500 MHz. At the two sites, 15 boreholes are available and the fracture openings are measured. Most of the fractures observed in the boreholes can be observed on the radar profiles, except for the very thin fractures and the fractures of complex fracture systems, which cannot be resolved. The detectability of the fractures is analysed for the dynamic as well as the static data. Fractures larger than 10 mm can be detected at least up to a depth of 2 m (length of the boreholes). For fractures smaller than 10 mm, the detectability depends on the depth into the salt and the antenna–roof distance. These results may be site specific. In order to characterize the fracture opening, the frequency content of the radar reflection is analysed. Synthetic signals show that in order to obtain reliable results, the analysis has to be performed over a broad frequency range and not at one frequency. For the real data, a reference signal is required and some corrections have to be applied to the reflected wave to take into account the propagation, the geometry and the reflectivity of the roof. If the signal quality is good, the estimation of the openings by inversion is mostly satisfactory. For signals where this is not the case, the differences from the real openings can be explained by a spatial variation of the fractures, irregularities of the opening, the presence of fractures very close to each other or by the quality of the reference signal. It can also be due to the method itself, which is less efficient in some opening ranges. Although the estimation of the fracture opening is not perfect, the uncertainty about the opening is reduced. A statistical analysis should further improve the results.
Cone penetration logging for the direct detection of LNAPLs contamination
Authors: Kvapil Jiri , Mares Stanislav 
Published: August 2009

ABSTRACTLight 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.
Moisture assessment with small‐scale geophysics‐the Interurban project
Authors: Müller Martin , Mohnke Oliver , Schmalholz Jürgen , Yaramanci Ugur 
Published: August 2009

ABSTRACTUrban 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.
Cone penetration logging for the direct detection of LNAPLs contamination
Authors: Aaltonen Jaana 
Published: August 2009

ABSTRACTIt 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.
GPR using an array antenna for landmine detection
Authors: Sato Motoyuki , Hamada Yusuke , Feng Xuan , Kong Fan‐Nian , Zeng Zhaofa , Fang Guangyou 
Published: August 2009

ABSTRACTWe propose a new ground‐penetrating radar (GPR) methodology for buried land‐mine detection. A GPR system using an antenna array was used for common‐midpoint (CMP) data acquisition. This system uses an array of ten Vivaldi antennae and operates from 30 MHz to 6 GHz. The moveout of the CMP data was corrected and the data were stacked to suppress ground clutter. The system was tested in the laboratory and we showed that it images buried mine‐like targets with a high resolution. A model of a plastic land‐mine of Type‐72 was buried in soil consisting of a mixture of gravel and crushed rocks and could be detected by the proposed technique. CMP processing applied to the array signal effectively suppresses the clutter from rough ground surfaces and inhomogeneous soil. We found that the migration algorithm is effective in suppressing ground clutter, but it is even more effective if it is combined with the CMP technique.
Material response analysis of georadar reflection data
Authors: Fokkema Jacob T. , Slob Evert C. , Fokkema Emile , Beekman Sicco 
Published: August 2009

ABSTRACTUnderstanding the nature of the ground‐penetrating radar (GPR) reflection response in terms of the dielectric permittivity and conductivity contrast is an important factor in interpretation. This is the reason for the present study, which aims to carry out an experiment in a controlled fashion, so that the relationship between measurements and the relevant physical parameters can be established. Accurate modelling of the subsurface parameter distribution is hampered by lack of both knowledge of the source signature and an appropriate model. We have therefore constructed a controlled environment in which to carry out measurements that can be modelled with sufficient accuracy in a 1D setting, under the constraint that subsurface conditions can be changed without disturbing the overburden. We present the results of such measurements, an analysis of the data and an interpretation based upon a 1D model, which is sufficiently realistic to accommodate sophisticated physical parameter models.
A comparison of segmentation techniques for target extraction in ground‐penetrating radar data
Authors: Shihab S. , Al‐Nuaimy W. 
Published: August 2009

ABSTRACTIn a typical GPR survey, only a small fraction of the collected data actually represent useful data (i.e. target data), whereas the majority of the data is considered redundant. The first of the post‐processing stages, which relies heavily on a skilled operator, involves indicating those areas that may contain targets and suppressing others. Consequently, this process consumes considerable amounts of time and effort, apart from the fact that the existence of the human factor at this critical stage invariably introduces inconsistency and error into the interpretation. In this paper, automatic detection and segmentation techniques for GPR data are discussed and compared. The techniques rely on the computation of certain features from which a neural network is then able to arrive at a decision whether to classify the data segments in question as targets or otherwise. The first technique is based on extracting statistical features from A‐scan segments while the second technique computes statistical features from B‐scan regions. In the third technique, some regional properties of B‐scan segments are used to achieve discrimination not only between targets and non‐targets, but also between hyperbolic‐shaped and non‐hyperbolic‐shaped targets. All the techniques were tested on different types of GPR data collected from a variety of sites, and they proved to be very efficient in forming a robust automatic technique for data reduction and segmentation. In addition, these techniques are carried out in near real‐time enabling on‐site processing and interpretation of collected data.
Combined high‐resolution magnetics and GPR surveys of the roman town of Flavia Solva
Authors: Seren S. , Eder‐Hinterleitner A. , Neubauer W. , Groh S. 
Published: August 2009

ABSTRACTHigh‐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.
Study of saltwater intrusion using 1D, 2D and 3D resistivity surveys in the coastal depressions at the eastern part of Matruh area, Egypt
Authors: Gemail K. , Samir A. , Oelsner C. , Mousa S.E. , Ibrahim S. 
Published: August 2009

ABSTRACTSaltwater 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.
Using airborne electromagnetics surveys to investigate the hydrogeology of an area near Nyborg, Denmark
Authors: Smith Richard S. , O’Connell Michael D. , Poulsen Lene Hjelm 
Published: August 2009

ABSTRACTA time‐domain airborne electromagnetic survey was flown in the County of Fyn, Denmark, immediately west of the city of Nyborg as a quick and efficient means of providing resistivity information for the development of a hydrogeological model. The bedrock geology is typified by a thick limestone overlain by a sequence of marine marl and a Quaternary moraine of variable thickness and composition. The limestone is the largest aquifer within the area and is used to supply water for Nyborg. The upper 100 m or so of the limestone generally contain fresh water and below that the groundwater is saline. The boundary between the resistive fresh water and the conductive saline water was the target of the airborne electromagnetic survey.Man‐made structures can strongly impact electromagnetic measurements; however, they can be minimized using two tactics: (1) masking the results around large power lines, and (2) non‐linear filtering of any perturbations in the interpretation that are the consequence of these or other structures. The latter strategy is possible because of the dense areal coverage provided by the airborne survey, and is particularly useful when a regional overview is required.Resistivity sections derived from the airborne electromagnetic data indicate a resistive zone of till, diluvial sand and freshwater limestone of variable thickness above a deeper conductive zone of saline water. A conductive layer within the moraine, assumed to be clay or marl, is also occasionally indicated. We used a two‐layer model to estimate the depth to the freshwater/saline‐water interface. Where ground soundings have been made, the depth estimates from the ground and airborne data agree. The depth estimates from the airborne data can be used to infer the thickness of the freshwater aquifer, either locally or regionally.
Advanced signal processing tools for dispersive waves
Authors: Mars J. I. , Glangeaud F. , Mari J. L. 
Published: August 2009

ABSTRACTTwo field examples are presented, showing the advantages of using multicomponent sensors for surface‐wave studies. Multicomponent sensors allow the use of specific signal‐processing tools such as the multicomponent singular value decomposition filter and the multicomponent polarization filter, which are both very efficient at separating surface waves from the other waves that comprise a seismic field record.Firstly, some signal‐processing tools for studying surface waves are described. The various filters range from classical to advanced techniques. For processing single‐component data, the filters are the filter and filters based on singular value decomposition and on spectral matrix decomposition. For processing multicomponent data, the filters are the 4C‐singular value decomposition filter and the classical or high‐order polarization filter.Secondly, processing sequences that can be applied to the field data are described and the single‐component processing sequence and the multicomponent processing sequence are compared. Two field examples are presented. The first data set is a land seismic data recording on 2C sensors. The second data set was obtained from a marine acquisition with OBS (4 components). The results obtained illustrate the advantages of using multicomponent filters. The efficiency of the 4C‐SVD filter and the high‐order statistic polarization filter is demonstrated.
Imaging scattered seismic surface waves
Authors: Campman X. , van Wijk K. , Riyanti C.D. , Scales J. , Herman G. 
Published: August 2009

ABSTRACTSurface‐wave analysis is a key tool for seismologists, ranging from near‐surface characterization in geotechnical applications to global seismology. Even in exploration seismology, where surface waves are regarded as a kind of noise, the fact that they typically represent the bulk of the recorded energy makes an understanding of surface‐wave propagation important. On the other hand, the heterogeneity of the near surface can make such analyses difficult since the heterogeneity is responsible for scattering and mode conversion.Here, we show how multichannel seismic records of scattered surface waves can be used to obtain spatial images of the heterogeneity. We discuss both data processing and imaging and illustrate our method on laboratory‐scale data. Further, synthetic examples show that we can locate individual scatterers accurately, even when many scatterers produce interfering surface waves. Our laboratory results show that the method has the potential to locate near‐surface heterogeneities in the field.