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- Volume 11, Issue 4, 2013
Near Surface Geophysics - Volume 11, Issue 4, 2013
Volume 11, Issue 4, 2013
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Gravity anomaly modelling of multiple geological sources having differing strike lengths and arbitrary density contrast variations
Authors V. Chakravarthi and B. RamammaABSTRACTA method to compute gravity anomalies of multiple geological sources having variable but finite strike lengths with arbitrary density contrast variations is presented. An ensemble of variable but finite strike limited vertical prisms among which density contrasts vary randomly both with depth and lateral position describes the anomalous source(s). The proposed method has the advantage that it can be applied to compute the gravity anomalies of a variety of geologic sources at both local and crustal scale levels. The applicability of the method is demonstrated with synthetic and real field examples. In the synthetic example, the geology resembles that of a typical rifted sedimentary basin, where thick sectioned sediments with arbitrarily varying density contrasts are concealed under a thick pile of flood basalt. For the real field example, the method is applied to calculate the gravity anomalies of a crustal scale structure across the suture zone between the Superior and Churchill structural provinces in the Canadian Shield. Modelling of real field anomalies reveals that i) the Churchill block is much thicker and ii) the western boundary of the base of the fold belt within the Superior block is shallower than reported in previous studies.
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Detection of soil and groundwater domestic pollution by the electrical resistivity method in the West Bank, Palestine
Authors Asal Sirhan and Mohammad HamidiABSTRACTThe environmental assessment of domestic pollution is required to minimize the risk of soil and groundwater contamination. Septic storages (localized sources) may affect the surrounding soil and permeable chalky formations by the intrusion of waste water within the formations. The aim was to develop a clearer understanding of waste water leakage due to numerous waste septic storages implemented within chalk formations at different towns and villages in the West Bank, Palestine. The possession of such information may be used to maintain not only sustainable development but also the protection of the environment. Since waste water is a good electrical conductor, the Electrical Resistivity Tomography (ERT) technique was employed for its ability in determining lateral and vertical electrical resistivity variations of possible infiltration zones of waste water. Furthermore, extracted vertical electrical soundings (VES) provide indications about the nature of the geological features that can affect the behaviour of leakage. Geo‐electrical surveys were carried out to investigate their utility in detecting and examining the effect of pollution. The two‐dimensional (2D) inverted models obtained by many electrode array configurations; Schlumberger, Wenner and dipole‐dipole were consistent. Interpretation of Direct Current (DC) geoelectrical resistivity field data demonstrates the existence of low‐apparent electrical resistivity zones ( < 15 ·m). A trend of decreasing resistivity is observed towards the horizontal direction of the existing storages, indicating the location of pollution plumes in soil and chalky formations. In addition, the heterogeneity of the shallow subsurface can be considered as the main reason for the fluids infiltration. The relative percentage of the resistivity is well documented, the value is minimized as a significant difference appears i.e., the relative percentage reaches ‐82% at the upper surface and it reaches to ‐44% where the septic tanks are located. These reliable results are considered as a key role in monitoring the environmental impacts of waste water on a groundwater system and high‐conductivity zones. Therefore, further geophysical prospecting projects for environmental issues can be realized by this technique.
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Investigation of the Azraq sedimentary basin, Jordan using integrated geoelectrical and electromagnetic techniques
Authors P. Yogeshwar, B. Tezkan and A. HaroonABSTRACTThe Eastern Mediterranean has been used as a passageway for human migration from Africa to the Middle East, the Balkans and Europe. The Azraq basin in the eastern desert of Jordan has been a major spot for human settlements since the middle Pleistocene. The former lake in the basin centre has developed to a hyper‐saline alluvial mudflat, the Qa’ Al Azraq. In the mudflat thick sequences of alluvial sediments are deposited. Such sediment successions are promising archives used for reconstructing a paleoclimate.
In order to identify geological structures and to derive suitable borehole locations in the area for a paleoclimatical reconstruction, the Transient Electromagnetic (TEM) and the Electrical Resistivity Tomography (ERT) methods were utilized. Two transects were investigated from the edge of the basin to the basin centre, crossing three geological formations. The data sets of both methods are interpreted by 1D and 2D inversion algorithms and appraised by inversion statistics. Previously uncertain geological boundaries are determined from geoelectrical models along both transects. Furthermore, a transition zone from fresh to saline groundwater is clearly detected.
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Data repeatability and acquisition techniques for time‐domain spectral induced polarization
Authors A. Gazoty, G. Fiandaca, J. Pedersen, E. Auken and A.V. ChristiansenABSTRACTThe Time Domain Induced Polarization (TDIP) technique is widely used in applied geophysics, particularly for environmental issues, for instance for delineating landfills or detecting leachate percolation. Because the reliability of IP data remains an issue at the field scale, this paper deals with the factors controlling data quality and compares different arrays and acquisition parameters for optimal collection of data in the field.
The first part focuses on repeatability experiments carried out in the former Hørløkke landfill (Denmark), in order to infer the degree of which a signal can be reproduced over time. Results show a good repeatability, with on average less than 10% of difference in raw data. Also, from the results it is inferred that the paramount parameter controlling repeatability is the IP signal level; a value of 2 mV is a sufficient threshold to ensure repeatability within 10% of data difference, although system dependant.
The second part focuses on survey design and underlines the importance of keeping the geometrical factor low. This points to the choice of a relevant measurement protocol, which depends on the threshold of the geometrical factor, again depending on expected chargeability and resistivity, threshold voltage and injected current. Furthermore, acquisition parameters such as the duration of the pulse injection and data sampling have a significant effect on both the signal‐to‐noise ratio and resolution. A comprehensive comparison between three protocols, the gradient array, the linear grid and the dipole‐dipole array, is shown and the choice of an acquisition sequence is discussed.
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Real‐time imaging and data analysis for shallow seismic data using a cloud‐computing portal
Authors Z. Heilmann, G.P. Deidda, G. Satta and E. BonomiABSTRACTIn‐field real‐time data processing is still today a crucial factor that could definitively boost the success and spread of shallow seismic reflection and multichannel ground‐penetrating radar (GPR) methods in the near‐surface geoscience community, as it allows efficient data acquisition and cost‐effective results, especially for modern surveys generating large volumes of data in a short time. To help fulfil this need, we present a cloud‐computing solution combining the powerful computational capabilities of a cloud infrastructure with a subsurface imaging workflow based on a parallelized grid version of the Common‐Reflection‐Surface (CRS) stack, a macro‐velocity model independent imaging method that is very suitable for real‐time imaging, as its data‐driven implementation avoids time‐consuming human interaction in prestack velocity analysis. Our portal is accessible from the field by any mobile computer having a wireless data connection. The user‐friendly web‐browser interface allows, already during acquisition, uploading of the recorded data to remote computing facilities, where a quality control (QC) data analysis report is automatically produced. When the number of uploaded shot records is large enough to produce a subsurface image, stacking, velocity model determination and prestack time migration can be performed fast and highly automated, optionally after applying some preprocessing (e.g., gain, trace balancing and filtering) using the preprocessing toolbox of the portal. To demonstrate the use of the presented system, we simulate in‐field data processing for an already published shallow seismic data set and compare our results with the original ones. The data set, consisting in seismic P‐wave data, was collected to image Palaeozoic bedrock at the Flumendosa River Delta, Sardinia (Italy), with a data acquisition set‐up driven by the expected bedrock depth, which, turning out wrong, prevented a detailed velocity analysis and a good time migration. Using the QC data analysis repotrs to run the imaging routines of our cloud portal we produced within less than an half an hour stacked and migrated sections very close to those published in the original work. These results, if obtained in the field, would have allowed an immediate update of the experimental set‐up. Therefore, we are optimistic that the proposed cloud‐computing solution (or similar systems) can boost the spread of shallow seismic reflection and multi‐offset GPR surveys in near‐surface investigations, similar to what happened in the field of electrical and electromagnetic surveying after suitable real‐time imaging and data analysis systems emerged.
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Automatic filtering of ERT monitoring data in mountain permafrost
Authors Etienne Rosset, Christin Hilbich, Sina Schneider and Christian HauckABSTRACTContinuous monitoring of Electrical Resistivity Tomography (ERT) surveys can be a powerful tool for all kind of long‐term applications in the field of hydrogeophysics and cold‐region geophysics due to its high sensitivity to changes in water and ice content of the near subsurface. However, the large amount of data often calls for autonomous data processing schemes. In this study, a new filter algorithm designed to automatically detect and delete measurement errors from multiple ERT monitoring data is presented. Three successive filter steps were developed in order to eliminate technical errors, overall high‐value outliers and relative outliers within single data levels. The filter procedure is site‐independent and was tested on four different mountain permafrost sites in the Swiss Alps, representing various landforms (talus slope, rock plateau, rock glacier, bedrock). The filter performance is assessed by analysing the effect of the filter procedure on the mean apparent resistivity and on the resulting data misfit of the inversion and both, after the entire filter procedure as well as after each individual filter step. The new filter procedure is expected to yield rapid and high‐quality filtering for monitoring applications. In our study, the procedure is developed to support early detection of electrical resistivity changes associated with freezing and thawing events in permafrost conditions. The filter is applied to 128 ERT data sets from permafrost monitoring stations in Switzerland, including a four year long (2005–2008) ERT monitoring data set from the high‐mountain permafrost monitoring station Stockhorn, which serves as an illustrating example.
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Determining hydrological and soil mechanical parameters from multichannel surface‐wave analysis across the Alpine Fault at Inchbonnie, New Zealand
Authors L.A. Konstantaki, S. Carpentier, F. Garofalo, P. Bergamo and L.V. SoccoABSTRACTCombining S‐wave data, resulting from surface‐wave dispersion analysis with P‐wave tomographic data, is a valuable tool to improve the understanding of near‐surface soil properties and allows the estimation of soil mechanical parameters and the determination of the depth of the water table. To achieve this combination of methods in a complex fault zone setting, active‐source seismic data were acquired at Inchbonnie, New Zealand across the Alpine Fault. This is a major transpressional strike‐slip fault that has generated magnitude > 7.8 earthquakes in the past. In this study, we focus on the surface‐wave component of these data, to determine elastic parameters for the shallow (~60 m) subsurface as well as the depth of the water table. We achieve this by combining S‐wave velocity models from surface‐wave dispersion curve inversion and P‐wave velocity models obtained from traveltime tomographic inversion in a previous study. The surface‐wave dispersion curve inversion is done by means of a laterally constrained inversion algorithm.
As a result, we are able to obtain elastic parameters and map the water table and the geology around the Alpine Fault at Inchbonnie, New Zealand. The Alpine Fault itself appears as a relatively sharp lateral discontinuity in all investigated parameters.
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GPR investigations at an Inca‐Spanish site in Argentina
Authors Néstor Bonomo, Ana Osella and Norma RattoABSTRACTWe present the results of a GPR prospecting carried out at the archaeological site of Batungasta, in the NW region of Argentina. This site was established by the Incas in the 14th century and was later occupied during the Spaniard conquest. Environmental information obtained at this arid zone led us to postulate that parts of the site buildings had been seriously damaged and buried by seasonal flood events as well as aeolian sedimentation. Therefore, we considered applying the GPR method to investigate the existence of buried remains, their architectural characteristics and to provide further evidence about the flows that possibly affected them. This method seemed suitable to attain these goals since it usually provides a good detection capacity for architectural structures and layers in dry environments, as well as good quality maps of the soil. Seven sectors of the site were surveyed using a GPR constant‐offset methodology and 500 MHz antennas. The analysis of the data allowed recognizing electromagnetic patterns that could be associated to the existence of walls. In six of the seven sectors, enclosures formed by rock‐earth and earth walls were detected and mapped, confirming Incaic‐Spanish and Spanish structures, respectively. A sedimentary layer produced by a large flood that covered the structures was detected and mapped, providing evidence of this kind of event and its influence on the site. The maps of this layer were also useful to indirectly visualize the 3D shapes of the discovered walls, which were unclear in many sections of the data and to detect additional anthropogenic structures with very bad preservation conditions. These structures had not been previously identified from analysis of the data sections due to the confusing aspect of the reflections produced at their irregular boundaries.
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2D and 3D imaging of a buried prehistoric canoe using GPR attributes: a case study
Authors Wenke Zhao, Gang Tian, Bangbing Wang, Emanuele Forte, Michele Pipan, Jinxin Lin, Zhanjie Shi and Xuejing LiABSTRACTWe apply Ground Penetrating Radar (GPR) to detect a prehistoric canoe at the Maoshan site, Zhejiang Province, China. A complex attribute analysis of the GPR data allows enhancing the precision in target detection and provides more details about the canoe and the burial environment. The burial depth of the bottom interface of the prehistoric canoe is detected and the integrity of the whole canoe is assessed through a GPR survey. Difficulties in the application of dense sampling of 2D and pseudo 3D GPR data originate from micro‐topographical disturbance that specifically affects the pseudo 3D investigation results obtained from high‐frequency antennas. Data processing and advanced imaging techniques can only remove part of such effects. The research demonstrates that GPR can successfully image wooden cultural relics buried in the shallow subsurface with ultra, high‐density trace spacing and high‐frequency antennas even in totally saturated clay‐rich soils based on 2D profiles and pseudo 3D methodologies, characterized by tight (cm) cross‐line/in‐line spacing.
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Application of alternative seismic‐stacking techniques to ringing noise removal from GPR data
More LessABSTRACTRinging is a common type of coherent noise that degrades the quality of ground‐penetrating radar data. Conventional ringing attenuation or a background removal algorithm works by generating an average ensemble that is then subtracted from all traces in a GPR section. This study presents four alternative algorithms, usually used in stacking‐seismic reflection data, for generating such an ensemble. These algorithms include median stack, diversity stack, alpha‐trimmed stack and smart stack. The traditional algorithm and the four alternative algorithms are tested using two real GPR data sets. The outcomes of the five background removal methods are compared to each other and to the original data using statistical analyses and visual inspection. The results show that all the four alternative techniques are more efficient in background removal than the conventional technique, with the smart ensemble yielding the best results followed by the alpha‐trimmed ensemble.
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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)