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- Volume 11, Issue 3, 2013
Near Surface Geophysics - Volume 11, Issue 3, 2013
Volume 11, Issue 3, 2013
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First in situ tests of a new electrostatic resistivity meter
Authors Sébastien Flageul, Michel Dabas, Julien Thiesson, Fayçal Rejiba and Alain TabbaghIf field applications of the electrostatic method are limited to roughly the first ten metres due to the necessity of staying in a low‐induction number domain, the possibilities it opens in urban area surveying, dry hole resistivity logging, non‐destructive testing and laboratory studies of the complex resistivity justify the design of a new multi‐frequency resistivity meter presenting a very low‐input capacitance and high‐phase sensitivity. After a first series of sample measurements in the laboratory, the new resistivity meter was tested in two different field contexts: the mapping of building remains in a Gallo‐Roman archaeological site under a flat meadow and the assessment of the thickness of anthropogenic layers in a town. The first test allowed a direct comparison with previous galvanic resistivity measurements and proved a very good agreement between the magnitude and spatial distribution of electrical resistivity. The second test established its reliable measuring abilities in a disturbed environment.
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Multi‐directional derivation of self‐potential/elevation gradient (Ce) maps – swirl procedure
Authors İnan Ulusoy, Philippe Labazuy and Erkan AydarThe derivation of gradient maps in geophysics, particularly in the field of self‐potential has the potential to improve our understanding on the source of a signal. Self‐potential/elevation gradient maps are beneficial in significantly reducing the topographic effect. Manual calculation of the gradient for large data sets in two‐dimensions is time consuming and highly dependent on the direction of the calculation. Automation of the calculation process has the potential to overcome the time and directional dependency problems. The derivation of gradient maps in the multi‐direction improves the result and array based operators can perform the automatic calculations rapidly. Four different gradient calculation methods based on a new automatic array oriented procedure (swirl procedure) are discussed and tested with artificial and field data sets. These four methods can be simply defined by the number of data contributing to the calculation (full‐swirl or limited‐swirl procedures) and the mathematical operator (maximum value or mean value) used in the calculation. The mean value operator using the full‐swirl procedure gave the most reliable result in terms of gradient range and accuracy. The swirl procedure can effectively perform the self‐potential/elevation gradient calculations and it has a potential use in various applications.
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Ground‐penetrating radar investigation inside a karstified limestone reservoir
Authors Guy Sénéchal, Dominique Rousset and Stéphane GaffetIn this paper, we present ground‐penetrating radar (GPR) investigations performed along a 3.7 km long tunnel located inside a lower Cretaceous limestone massif of south‐eastern France. This fractured massif is mainly characterized by water circulation and karstic structures. This kind of geological formation contains a large part of the fresh underground water resources of the world and is also considered as an analogue of Middle East oil reservoirs. Since tunnel walls are covered by thick reinforced concrete, direct geological observations are impossible.
After some preliminary tests, the entire tunnel was investigated using 250 MHz shielded antennas. Data are generally of very good quality, with reflection time up to 400 ns (down to 18 m under the tunnel floor with a velocity of 9 cm/ns). We correlate the GPR signal along the tunnel with surface geological observations: the upper part of the investigated formation (Bedoulian) displays prominent stratigraphic reflectors while the lower part (Barremian) does not. Numerous diffractions are observed in both formations and can be related to karstic features.
These investigations allow to better constrain the geological context along the tunnel, necessary for future hydrogeological studies. We conclude that this tunnel offers a unique opportunity of performing GPR measurements within a karstified limestone massif.
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Fracture aperture and fill characterization in a limestone quarry using GPR thin‐layer AVA analysis
Authors Aisha A. Kana, L. Jared West and Roger A. ClarkThe detection and characterization of fractures is important in many engineering practices e.g., rock fall assessment, quarry rock quality determination and mine roof and tunnel stability. Fractures may provide suitable contrasts in electrical properties for detection by GPR instruments. Their ability to reflect radar waves and the dependence of this reflectivity on fracture properties (aperture and fill) makes the GPR method a promising tool for rock fracture characterization. Doing so successfully requires quantification of the reflectivity, reflection coefficient, R, of individual fractures and its variation with incidence angle (AVA), or in practice, offset. Measuring the full AVA response using Common Mid Point (CMP) surveys enables simultaneous estimation of fracture aperture and fill permittivity, difficult to achieve with Common Offset (CO) profiling which does not evaluate angle dependence. This paper reports on AVA analysis carried out on CMP data acquired with 500 MHz antennae over Carboniferous Limestone with horizontal bedding plane fractures in Yorkshire, United Kingdom. Using the transverse electric (TE) polarization mode, data were collected at a sampling interval of 0.05 ns for offsets at 0.04 m steps from 0.37–16.05m.
The recorded amplitude is related to the reflection coefficient but is also influenced by other factors we eliminated by making some simplifying assumptions and amplitude corrections. We assume frequency‐independent electrical properties, constant antenna coupling with the ground surface, constant losses associated with transmission losses through any interfaces above the target reflection for the range of offsets considered and a flat homogeneous surface over which measurements are made. Amplitudes were corrected for conductive attenuation, spherical spreading and antenna patterns, which we measured through transillumination surveys across limestone boulders and numerical modelling using the GprMax 3D modelling code. To constrain fracture aperture and fill permittivity, we use the least squares fit of normalized reflection coefficient curves to corrected CMP amplitudes. The analysis allowed characterization of the (bedding plane) fracture fill relative permittivity (~7.8) and aperture (~0.043 m, ~0.2 at 500 MHz). The values obtained are consistent with field observations of fracture fill, corresponding to a mixture of clays and calcite.
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2D elastic full‐waveform imaging of the near‐surface: application to synthetic and physical modelling data sets
More LessStandard seismic methods are generally not well adapted to provide sharp quantitative images of the first few metres of underground. A two‐dimensional full‐waveform inversion of land seismic data, based on frequency‐domain viscoelastic modelling, offers a promising approach to take advantage of the full complexity of seismograms and to simultaneously build 2D images of VP and VS parameters. In order to understand the behaviour of this method in a near‐surface context and anticipate the corresponding field applications, we perform this investigation by applying waveform inversion on a simple layered medium. We first use synthetic data obtained from numerical modelling and then we employ laboratory data obtained by small‐scale physical modelling. We demonstrate that such a near‐surface 2D model can be quantitatively determined even in a realistic situation where the data are dominated by high‐amplitude surface waves. A comparison of results derived for the same medium from ideal synthetic data and noisy experimental data allows detecting anomalies in the reconstruction of velocity models due to the experimental nature of the data used.
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Towards an automatic seismic localization of human footsteps
Authors Sven Heider, Stefan Jetschny and Thomas BohlenToday, many companies or government institutions are required to monitor outdoor areas of office buildings, frontiers or restricted areas. In this paper we present an automated algorithm to monitor and to localize human footsteps with the help of their excited seismic waves. The preliminary goal of the data processing is the calculation of traveltime differences for each receiver pair of an arbitrary array of geophones. The data are filtered with a band‐pass filter, which improves the signal‐to‐noise ratio significantly. A short term average / long term average (sta / lta) picker determines the time of the first breaks. After cutting out time windows around the picked arrival times, the traveltime differences are than calculated for each receiver pair using cross‐correlation. Finally, a grid search algorithm is used to invert the traveltime differences into locations of the footstep. Therefore, the propagation velocity of the Rayleigh wave has to be approximated. In one field measurement reliable and precise localizations were achieved. Footsteps on a fixed position were localized within an area of 30 m x 30 m with less than one metre deviation and paths on a straight line were reconstructed successfully.
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Recursive ant colony optimization: a new technique for the estimation of function parameters from geophysical field data
Authors Deepak K. Gupta, Jai P. Gupta, Yogesh Arora and Uma ShankarWe introduce Recursive Ant Colony Optimization (RACO), a new technique for the estimation of function parameters from field data obtained from various geophysical surveys. RACO is a modified form of the ant colony method (ACO) and can be used to determine the best solution for various geophysical problems. As RACO is an extension of ACO, it simulates the social behaviour of ants, optimizing their path from the nest to the food source. RACO applies ACO recursively, introducing an additional term ‘depth’ that decides the extent of the recursion. The results of each depth contribute towards the construction of models for the following depth and the range of values for each parameter is condensed around the actual solution. The algorithm is tested on a two parameter mathematical function and to further test its efficiency and stability on real world problems, we applied it on a few geophysical inversion problems. We show its application on synthetic data sets for the self‐potential anomaly (noise free and 10% noise) caused in the case of an inclined sheet like a body buried inside the earth, synthetic 1D vertical electrical sounding (VES) geo‐electric data sets for three‐layer and five‐layer earth models, a real 1D VES data set obtained from the Tangasol region, India and P‐wave logs for two sites located in the Krishna‐Godavari (KG) Basin, India. The results generated by RACO inversions are found to be in good agreement with previous studies conducted using these data sets.
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Geophysical assessment and geotechnical investigation of quick‐clay landslides – a Swedish case study
We present a preliminary assessment of the potential utility of various geophysical measurements carried out over a quick‐clay landslide site in south‐west Sweden. The multidisciplinary approach includes active P‐ and S‐wave seismic investigations, including 2D and 3D reflection and refraction surveys, passive single and 3C seismic surveys, electrical resistivity tomography and electromagnetic surveys including controlled‐source and radio‐magnetotellurics, ground‐penetrating radar and potential field studies. The P‐wave and particularly S‐wave reflection seismic data show a high‐resolution image of bedrock topography and the stratigraphy of a 100 m thick sequence of sediments that lies on top, which include lightly consolidated quick‐clays. Of particular interest is the identification of a layer of relatively coarse‐grained material between 10–20 m below the ground surface. Geotechnical investigations indicate that most but not all quick‐clays at the site are located above this layer. Further studies are required to determine the importance of their relationship and whether the coarse‐grained layer may have had a role in triggering quick‐clay landslides in the region. Geoelectrical and electromagnetic methods provide high‐resolution images of the unconsolidated subsurface and particularly the normal and leached clays. Radio‐magnetotelluric methods proved valuable near the river where traditional geoelectrical methods failed to provide sufficient depth coverage. The study shows that geophysical data are able to image major subsurface structures associated with quick‐clay landslides.
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Resistivity logging as a tool for identifying initial weathering in crystalline rocks
Authors Berit Ensted Danielsen and Heine Buus MadsenIn rock engineering construction projects it is common to make core drillings to quantify rock quality using different classification systems such as weathering and rock quality designation. The classifications are done on a metre scale by a geologist on site. However this is a subjective assessment where the human factor comes into play. Important observations might be overlooked or interpreted incorrectly. Therefore a more objective method is needed. The scope of this paper is to show how even low degrees of weathering of rocks lowers the resistivity, an effect that otherwise might be overlooked by the geologist because the core appears unaltered and is thus not reflected in the standard rock quality measures. This was done by means of thin‐section microscopy, point counting and resistivity logging on gneisses and amphibolites from two drill cores done in connection with the construction of the Hallandsås Tunnel, Southern Sweden. The study showed that the resistivity logs can detect even low grades of weathering of amphibolites, which can be important in determining the mechanical properties of the rock. The result suggests that focusing on the transitions between different lithologies is important because the rocks at the lithological contacts are more susceptible to fracturing, water flooding and weathering. This might not be obvious from the regu‐lar geological core description but this is indicated by the resistivity logs.
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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)