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- Volume 11, Issue 6, 2013
Near Surface Geophysics - Volume 11, Issue 6, 2013
Volume 11, Issue 6, 2013
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Electrical resistivity imaging of unknown bridge foundations
Authors R. Arjwech, M.E. Everett, J.‐L. Briaud, S. Hurlebaus, Z. Medina‐Cetina, S. Tucker and N. YousefpourABSTRACTUnknown bridge foundations pose a significant safety risk due to stream scour and erosion. Records from older structures may be non‐existent, incomplete or incorrect. We evaluate 2D and 3D electrical resistivity imaging (ERI) as a means to reliably identify the depth of unknown bridge foundations. A survey procedure is described for mixed terrain/water environments in the presence of rough terrain. Some electrodes are installed on the stream banks while others are adapted for underwater use. Tests were conducted at five field sites, including three roadway bridges, a geotechnical test site and a railway bridge, containing drilled shafts and spread footings of both known and unknown depth extent. The 2D data acquisition was carried out in the dipole‐dipole configuration. The 2D ERI method resolved the shape and depth extent of the larger bridge foundations but, with less accuracy, the shape and depth extent of the smaller foundations. The 3D ERI method is time‐consuming and does not add sufficient additional value over 2D ERI to become a practical tool for unknown bridge foundation investigations. The 2D ERI method is a cost‐effective geophysical method that is relatively easy to use by bridge engineers.
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Artificial neural network and statistical models for predicting the basic geotechnical properties of soils from electrical measurements
More LessABSTRACTThe ability to predict the geotechnical properties of subsurface soils using non‐invasive geophysical measurements can be undeniably useful to the geotechnical engineer. Using laboratory data, we assess the potential of artificial neural networks to investigate the relations between geotechnical and electrical parameters characterizing a variety of soils. The geotechnical parameters are: fines content, mean grain size, mean pore size and the specific surface area. The electrical parameters obtained from low‐frequency electrical measurements (4 Hz) include the resistivity amplitude, phase shift and the loss tangent. Relations that can be used to predict the geotechnical parameters of a soil given its electrical parameters are developed. The predictive capabilities of the neural networks are compared with traditional multivariate regression models. The performances of the neural network and regression models in predicting (a) the geotechnical parameter given the same electrical parameters as inputs and (b) the electrical parameters given the same geotechnical parameters as inputs are compared. In both cases, the neural network outperforms the multivariate regression as the neural network is able to capture and model the non‐linear and complex relationships among the variables. The relative importance of the geotechnical parameters on the overall electrical conduction was examined using the neural networks. The results indicate that mean grain size and fines content are the two geotechnical parameters that influence phase‐shift values the most; fines content and mean pore size influence the resistivity amplitude the most, whilst fines content and mean grain size influence the loss tangent the most. It was observed that of the four geotechnical parameters, the mean grain size influences the measured resistivity values the least.
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Towards geophysical and geotechnical integration for quick‐clay mapping in Norway
ABSTRACTQuick clay is a known hazard in formerly‐glaciated coastal areas in e.g., Norway, Sweden and Canada. In this paper, we review the physical properties of quick clays in order to find a suitable, integrated and multi‐disciplinary approach to improve our possibilities to accurately identify the occurrence of quick clay and map its extent both vertically and laterally. As no single geophysical method yields optimal information, one should combine a variety of geophysical methods with geotechnical data (in situ measurements using Cone Penetration Testing (CPTU), Seismic CPTU (SCPTU) and Resistivity CPTU (RCPTU); laboratory tests) for an in‐depth quick‐clay assessment at a given site. In this respect, geophysical data are used to fill the gaps between geotechnical boreholes providing ground‐truth. Such an integrated and multi‐disciplinary approach brings us closer to 2D or pseudo‐3D site characterization for quick clays and as such, an improved assessment of the potential hazard they pose. The integrated approach is applied in practice on two Norwegian quick‐clay sites. The first site, Hvittingfoss, was remediated against potential landslides in 2008 whereas the second one, Rissa, was the scene of a major quick‐clay landslide in 1978, quick clays being still present over a large area. The collected data and preliminary site characterizations illustrate the high diversity as well as the complexity and clearly emphasize the need for higher resolution, careful imaging and calibration of the data in order to accomplish the assessment of a quick‐clay hazard.
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Using marine resistivity to map geotechnical properties: a case study in support of dredging the Panama Canal
Authors Dale F. Rucker and Gillian E. NoonanABSTRACTThe distribution of subbottom geotechnical strength properties within the Panama Canal are needed to help with the Canal’s expansion. Core data already exist in the Canal, including lithological/stratigraphical descriptions and qualitative measurements of rock hardness. These data have been acquired within the Canal during previous expansion activities conducted over the past 60 years. Alone, the core data can be used to estimate rock hardness at unsampled locations using geostatistical methods. However, to help reduce uncertainty in the interpolation of rock hardness, a spatially continuous electrical resistivity survey was conducted to provide a better means of bridging information between cores. Although no direct causative link between rock hardness and resistivity exists, it was thought that the resistivity would be dependent upon jointly influencing parameters that comprise the geome‐chanical attributes of the rock, in this case porosity. For example, tuff generally had lower hardness and lower resistivity values compared to andesite and differences in porosity of these rock types would help explain the trend. When considering the resistivity in this geologic context, the spatial interpolation of rock hardness showed better agreement with measured data at sampled locations compared to methods that did not consider any geological context (including kriging of core data or a polynomial regression model between resistivity and rock hardness). Additionally, it is believed that full three‐dimensional inverse modelling of the resistivity data helped to correctly resolve the location of low‐resistivity features that could have been detected as off‐line effects in two‐dimensional processing algorithms. With these results, it is anticipated that the costs of dredging could be reduced by the simple fact that necessary resources can be anticipated for some of the harder rock types.
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A comparison study of different image appraisal tools for electrical resistivity tomography
Authors David Caterina, Jean Beaujean, Tanguy Robert and Frédéric NguyenABSTRACTTo date, few studies offer a quantitative comparison of the performance of image appraisal tools. Moreover, there is no commonly accepted methodology to handle them even though it is a crucial aspect for reliable interpretation of geophysical images. In this study, we compare quantitatively different image appraisal indicators to detect artefacts, estimate depth of investigation, address parameters resolution and appraise ERT‐derived geometry. Among existing image appraisal tools, we focus on the model resolution matrix (), the cumulative sensitivity matrix () and the depth of investigation index () that are regularly used in the literature. They are first compared with numerical models representing different geological situations in terms of heterogeneity and scale and then used on field data sets. The numerical benchmark shows that indicators based on and are the most appropriate to appraise ERT images in terms of the exactitude of inverted parameters, providing mainly qualitative information. In parallel, we test two different edge detection algorithms – Watershed’s and Canny’s algorithms – on the numerical models to identify the geometry of electrical structures in ERT images. From the results obtained, Canny’s algorithm seems to be the most reliable to help practitioners in the interpretation of buried structures.
On this basis, we propose a methodology to appraise field ERT images. First, numerical benchmark models representing simplified cases of field ERT images are built using available a priori information. Then, ERT images are produced for these benchmark models (all simulated acquisition and inversion parameters being the same). The comparison between the numerical benchmark models and their corresponding ERT images gives the errors on inverted parameters. These discrepancies are then evaluated against the appraisal indicators ( and ) allowing the definition of threshold values. The final step consists in applying the threshold values on the field ERT images and to validate the results with a posteriori knowledge. The developed approach is tested successfully on two field data sets providing important information on the reliability of the location of a contamination source and on the geometry of a fractured zone. However, quantitative use of these indicators remains a difficult task depending mainly on the confidence level desired by the user. Further research is thus needed to develop new appraisal indicators more suited for a quantitative use and to improve the quality of inversion itself.
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Mapping of quick clay using geoelectrical imaging and CPTU‐resistivity
Authors Torleif Dahlin, Hjördis Löfroth, David Schälin and Pascal SuerABSTRACTQuick clay has a major impact on landslide risk and it is therefore of considerable interest to map its presence and extent. In Sweden, quick clay has been involved in most landslides in soft clay with serious consequences. The predominant method for detection of quick clay in Sweden has been to take undisturbed samples and to perform fall‐cone tests on the clay in its undisturbed and remoulded state. Originally deposited in saltwater in a marine environment, the salt maintains the stability of the clay. When the salt is leached out, the clay can become quick. When salt is leached from clay of marine origin the resistivity increases. In this study the intention was to calibrate electrical imaging with cone penetration tests with resistivity measurements (CPTU‐R) and measurement of the total penetration resistance, i.e. the total rod friction, together with both geotechnical and chemical analyses on specimens in the laboratory.
The results show that electrical imaging can be used for separation of leached soil volumes in marine clays that may form quick clay, from those where the salt content remains too high for this. In the dry crust and thin weathered zone at the top, the resistivity is high but the clay is non‐quick. Also soils with less clay content will have higher resistivity without being quick. The technique may thus be used as a screening tool in order to delimit areas where further investigations are needed from areas that do not require more attention. This has a potential of saving significant resources if used in a relatively early stage of the survey process. It can also increase the overall quality and reliability of the survey results.
The induced polarization (IP) results are consistent and seem to be geologically realistic, and appear to contain additional information to the resistivity that is related to material or electrochemical properties, although it is not clear how due to lack of sufficiently detailed reference data.
The electrical imaging gives a general picture of the variation in resistivity along soil sections. The CPTU‐R gives variations at depth with very high vertical resolution in one specific location. There is generally good agreement between the models based on electrical imaging and the CPTU‐R. The CPTU‐R results may be used to calibrate the electrical imaging results with quick clay estimations based on rod friction.
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Assessment of aggregate resources: an integrated geophysical approach
Authors R. O’Driscoll, G. Stuart, G. Tuckwell and J. SergeantABSTRACTFluvio‐glacial sand and gravel provide an important resource for the construction industry. The efficient and economic extraction of such geologically complex deposits requires a detailed understanding of their grade distribution. This is typically undertaken as point observations over the resource using a borehole drilling programme. We investigate the viability of using measurements of the elastic and electrical properties of the subsurface, derived from geophysical profiles, to determine the spatial variability of aggregate quality.
A refraction seismic, surface wave seismic and resistivity survey was carried out over a fluvioglacial sand and gravel deposit at Scorton, Yorkshire, UK, on agricultural land adjacent to an active quarry. Two 190 m profiles, with borehole control at their extremes, were acquired with a 2 m source and receiver spacing. The P‐ and S‐wave velocities ( and ), together with resistivity and depth distributions were determined down to c. 20 m. The subsurface was divided into four layers: uncon‐solidated sand and gravel; dry consolidated sand and gravel; saturated sand and gravel; and basal clay.
The work flow initially involved processing and interpreting the three geophysical techniques independently. Crossplots of the elastic and electrical parameters were used to distinguish unconsolidated from consolidated material; the water table; increased silt content; and recognize the basal clay. The Vp/Vs ratio proved very powerful at discriminating fine material () from coarse material (Vp/Vs ~ 1.5). Regression analysis of crossplots produced P‐ and S‐wave relationships for the material, while electro‐acoustic relationships were less successful, especially below the water table. The results of the petrophysical analysis were used to set up guided inversions for the resistivity survey. Careful use of the refraction results as an a priori model for the resistivity survey was found to improve the resistivity inversion, though use of a sharp boundary introduced edge effects at the water table.
The results from the integrated analysis were combined with the guided inversions into a final interpretation that enabled silt content, particle size and clay lenses to be mapped, which were not identified in the separately processed data.
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Assessment of ground compaction using multi‐channel analysis of surface wave data and cone penetration tests
Authors A. Bitri, K. Samyn, S. Brûlé and E.H. JavelaudABSTRACTThe construction of a new industrial and commercial area in 2009 at the Givors’ former glass factory area in France involved heavy dynamic compaction work. For the purpose of founding the new buildings, it was necessary to improve the ground mechanical properties of 7–15 m of well‐graded gravel backfill lying on geotechnical bedrock. In order to assess the quality and depth of ground compaction, cone penetration tests are often performed before and after compaction. The method is intrusive and a one‐location test. It requires a substantial amount of time to evaluate a large area and evaluation quality is quite dependent on the operation technique and soil type. In this paper, the quality and extent of ground compaction were evaluated using results from the in situ Multi‐Channel Analysis of Surface Waves (MASW) seismic method and cone penetration tests (CPT). MASW tests were used to determine shear‐wave velocity () profiles before and after compaction and CPT tests were adopted to determine the correlation between and the measured penetration resistance () improvement along profiles. The results of this study show the effectiveness of surface waves for the evaluation of compaction performance and demonstrate the potential of this technique to engineering and environmental problems.
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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)