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- Volume 15, Issue 1, 2017
Near Surface Geophysics - Volume 15, Issue 1, 2017
Volume 15, Issue 1, 2017
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The 3D Autojuggie: automating acquisition of 3D near‐surface seismic reflection data
Authors Brian E. Miller, Steven D. Sloan, Georgios P. Tsoflias and Don W. SteeplesABSTRACTNew instrumentation, known as the 3D Autojuggie, has been developed that allows acquisition of three‐dimensional shallow seismic reflection data in a cost‐effective manner. The 3D Autojuggie is capable of simultaneously planting 220 geophones with 0.5‐m spacing in the inline and crossline directions in less than 1 minute. Likewise, all 220 geophones can be picked up in the same amount of time so that the receiver grid can be moved and redeployed on the order of a few minutes without disconnecting cables and seismographs. This significantly reduces the time required to roll geo‐phones and cables and the number of crew members needed for three‐dimensional high‐resolution seismic surveying. While efficiency of three‐dimensional surveying is demonstrated here, the use of the Autojuggie will be limited in rough terrain and in dry or rocky top soil.
This study presents two field tests of the 3D Autojuggie. The first one was conducted over soil using conventional geophones with spikes, and the second one was conducted over hard pavement using geophones mounted on base plates. Each dataset is compared to corresponding walkaway lines of conventional manually placed control geophones. Data acquired using the 3D Autojuggie are directly comparable and nearly indistinguishable to hand‐planted geophones. Field experiments resulted in a minimum of triple the acquisition efficiency increase compared with conventional geophone deployment methods. This work demonstrates that the 3D Autojuggie can facilitate the acquisition of high‐resolution three‐dimensional seismic data, making possible routine ultra‐shallow seismic imaging.
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Facies discrimination with electrical resistivity tomography using a probabilistic methodology: effect of sensitivity and regularisation
Authors Thomas Hermans and James IrvingABSTRACTElectrical resistivity tomography has become a standard geophysical method in the field of hydrogeology, as it has the potential to provide important information regarding the spatial distribution of facies. However, inverted electrical resistivity tomography images tend to be grossly smoothed versions of reality because of the regularisation of the inverse problem. In this study, we use a probabilistic methodology based upon co‐located measurements to assess the utility of electrical resistivity tomography to identify hydrofacies in alluvial aquifers. With this methodology, electrical resistivity tomography images are interpreted in terms of the probability of belonging to pre‐defined hydrofacies. We first analyse through a synthetic study the ability of electrical resistivity tomography to discriminate between different facies. As electrical resistivity tomography data suffer from a loss of sensitivity with depth, we find that low‐sensitivity regions are more affected by misclassification. To counteract this effect, we adapt the probabilistic framework to include the spatially varying data sensitivity. We then apply our learning to a field case. For the latter, we consider two different regularisation procedures. In contrast to the data sensitivity that affects the facies probability to a limited amount, the regularisation can affect the probability maps more considerably because it has a strong influence on the spatial distribution of inverted resistivity. We find that a regularisation strategy based on the most realistic prior information tends to offer the most reliable discrimination of facies. Our results confirm the ability of electrical resistivity tomography surveys, when properly designed, to detect facies variations in alluvial aquifers. The method can be easily extended to other contexts.
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A cost‐effective 3D electrical resistivity imaging approach applied to dike investigation
Authors Clara Jodry, Sèrgio Palma Lopes, Yannick Fargier, Philippe Côte and Martin SanchezABSTRACTAlthough DC electrical resistivity imaging is widely applied to dike investigation, either rapid 2D or high‐resolution 3D approaches fail to address actual needs. An intermediate electrical resistivity imaging approach referred to as “3D‐“ is introduced in this paper. The methodology is based on existing tools, and it offers useful and sufficiently reliable 3D images of the investigated structure within a cost‐effective and flexible procedure. The survey design, the model discretisation, and the thorough integration of a priori information are the main phases of this procedure. To demonstrate the benefits and limitations of this approach, it is applied to an existing stretch of embankment levee along the Loire River. A numerical study was carried out both on synthetic and real data to assess the 3D imaging capability of the approach and the influence of prior information on the inversion outputs. The important role of a priori information is shown to be even more essential here. The results demonstrate the efficiency and versatility of the 3D‐ approach for reliable and cost‐effective investigations of long dikes.
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VEMI: a flexible interface for 3D tomographic inversion of time‐ and frequency‐domain electrical data in EIDORS
Authors Giorgio De Donno and Ettore CardarelliABSTRACTThe aim of this work is to present a MATLAB interface called Versatile interface for Electrical data Modelling and Inversion included in the EIDORS software. The interface is able to invert 2D and 3D electrical data acquired in the time or the frequency domain, both for cylindrical and prismatic geometries. Therefore, it can be a flexible tool for inversion of both real‐ and complex‐valued resistivity tomography data acquired in the laboratory and in the field. The forward solver has been proved to be stable and accurate through a comparison with an analytical solution, whereas the inverse solution, achieved through a Gauss‐Newton routine with an optimised damping inner loop, can be performed with the help of other useful tools incorporating a priori information in the inversion process. The reliability of the Versatile interface for Electrical data Modelling and Inversion has been tested through a 3D laboratory example, where both time‐ and frequency‐domain data and two 3D field example with time‐domain data were acquired. The 3D laboratory example, simulating a shallow aquifer contaminated by a chlorinated solvent (hydrofluoroether), demonstrated the reliability of the Versatile interface for Electrical data Modelling and Inversion to detect the contaminant pathway within the physical model. Hydrofluoroether is clearly visible on phase and chargeability models, where the highest phase values are located underneath the spilling point, even though it remains undistinguishable in the resistivity and amplitude ones. Through the combined analysis of the inverted chargeability and phase models, we can reduce the degree of uncertainty in the interpretation of geophysical models. These results were also validated through comparison with the respective synthetic models simulated in a previous paper by the same authors. Real‐world tests have been performed on a closed landfill where few a priori information are available about the original design and on an industrial site contaminated by chlorinated solvents. In the former case, we reconstruct a three‐layer configuration (covering, waste and bottom liner), where the effective layering inferred from the resistivity model is confirmed by the chargeability one. In the latter case, we detect the chlorinated solvents within the deeper aquifer through a combined analysis of the resistivity and chargeability models, where the very high resistivity values are associated with high chargeability.
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Seasonal saline intrusion monitoring of a shallow coastal aquifer using time‐lapse DC resistivity traversing
Authors Eva Sutter and Malcolm InghamABSTRACTLong‐term time‐lapse DC resistivity measurements along six 145‐ to 275‐m‐long transects have been carried out to image seasonal changes in the resistivity distribution within a shallow coastal aquifer on the Kapiti Coast, New Zealand. Two‐monthly repetition of the measurements shows a continuous decrease in the bulk resistivity at depth over an extended horizontal portion of the profile lines when coming into the drier summer season. This is most distinct in more rural locations and less pronounced along urban profiles, and is interpreted to be the result of a decrease in the fluid resistivity resulting from lower freshwater pressure that allows saltwater to intrude. Coming into the wetter winter season, the bulk resistivity at depth increases again, as freshwater pressure in the aquifer increases. A 24‐hour tidal cycle monitoring along one of the northern profiles, observations of seawater level stages, and tidal ranges show that the observed resistivity changes are seasonally driven rather than tidal overprints. Time‐lapse monitoring using electrical resistivity tomography has thus proven itself as a suitable tool for imaging both the saline interface shape and extent, and the changes in the subsurface resistivity distribution in an unconfined coastal, shallow, sand aquifer over a seasonal time frame. This significantly adds to the knowledge about the dynamic behaviour near the saline interface in coastal aquifers and can be used for groundwater assessment in this and other similar geological and hydrological environments around the world.
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Spectral time‐domain induced polarisation and magnetic surveying – an efficient tool for characterisation of solid waste deposits in developing countries
ABSTRACTTime‐domain induced polarisation and magnetic data were acquired to map and characterise the decommissioned and un‐engineered municipal solid waste disposal site of the Kwame Nkrumah University of Science and Technology, located in the Kumasi Metropolis of Ghana. In this survey, 13 induced polarisation profiles 500–800 m long and 26 magnetic profiles 400–800 m long were acquired. In addition, two boreholes were drilled to help in the interpretation of the geophysical data. The study was carried out with the aim of determining the risk posed by the waste deposit to the quality of the soil and the ground water system, which is the main potable water supply for the Secondary School, the University Teaching Hospital and the Veterinary School, situated within the catchment area of the site. Full‐decay 2‐D time‐domain induced polarisation inversions in terms of Cole–Cole parameters were used for interpreting the induced polarisation data. The chargeability, resistivity and normalised charge‐ability distributions, together with the magnetic results, aided in a full characterisation of the site geology, the waste and the associated pollution plume. In particular, clear contrasts in resistivity and the polarisation parameters were found between the saprolite layer and the granitic bedrock, which are the main lithological units of the area. Furthermore, it was found that the Kwame Nkrumah University of Science and Technology waste deposit is characterised by a low‐chargeability and low‐resistivity signature and that the low‐resistivity area spreads out from the waste deposit into the permeable saprolite layer, indicating the presence of a leachate plume. A fracture zone in the granitic bedrock beneath the waste deposit, which is a potential conduit for leachate contamination of the ground water system, was also identified. The study thus provides the information needed for assessing the future impact of the waste on the water quality in the area and for designing risk‐mitigation actions.
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Physical characterisation of soils recovered from the ANZAC battlefield
Authors Amir Orangi and Guillermo A. NarsilioABSTRACTAs part of a unique tri‐nation study to commemorate the centenary of the iconic First World War ANZAC battlefield, this paper summarises the results of a laboratory experimental investigation of some geotechnical and geophysical characteristics of soil recovered from this field in Gallipoli, Turkey. The geophysical characterisation of these samples comprises the determination of the dielectric properties, namely real, imaginary and dielectric dispersion, over a frequency band between 200 MHz and 6 GHz. This characterisation is performed at the samples’ in situ state as well as in a controlled range of water content from oven‐dried to saturation. Specific frequencies pertaining to certain in situ geophysical applications and satellite surveys have been targeted as reference frequencies to study the dielectric data. Moreover, attenuation coefficients related to ground‐penetrating radar applications are also estimated at 200 MHz for a range of water contents. Microstructure and mineralogy were examined by environmental scanning electron microscopy and X‐ray diffraction techniques, respectively. Given the historical and archaeological significance of the ANZAC battlefield, these results can be used for future feasibility studies, planning, and result interpretation of geophysical investigations, including choosing the most appropriate time of the year and geophysical prospection tools, in this restricted‐access study area.
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Using ground‐penetrating radar to explore the cemented soil horizon in an arid region in Iran
ABSTRACTPetrosalic horizon (cemented by soluble salts) is identified as a root‐restricting layer in the arid and semi‐arid regions. The knowledge of the variability of cemented layers within the soil profiles provides valuable information to decision‐makers for agricultural and/or engineering activities. This study was conducted to evaluate the potential of ground‐penetrating radar in detecting petrosalic horizon within the soil pedon in an arid area (Kerman Province, southeastern Iran). The measurements were performed using an impulse ground‐penetrating radar system with a centre frequency of 250 MHz along ten parallel transects of 100‐m length and 10‐m distance between two consecutive transects at two sites. The results of the soil stratigraphy indicated the presence of petrosalic horizons at 12‐ to 36‐cm depth. The ground‐penetrating radar images showed patterns corresponding to these soil horizons. The interpreted radargrams were quite consistent with the findings of the soil pedons. It is, therefore, suggested that ground‐penetrating radar could be employed to detect the boundaries between different soil horizons in the arid areas with various degrees of conductivity. This, in turn, helps differentiate soil‐mapping units and improve the reliability and accuracy of digital soil maps. The results confirmed that ground‐penetrating radar could be used as a rapid, efficient and non‐destructive tool providing highresolution and continuous visualisation of the soil horizons to detect the presence and depth of petro‐salic horizons. Such information could be crucial in making decisions in agricultural practices (e.g., creating gardens) and engineering activities (e.g., constructing roads and buildings).
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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)