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- Volume 1, Issue 3, 2003
Near Surface Geophysics - Volume 1, Issue 3, 2003
Volume 1, Issue 3, 2003
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Properties and effects of measurement errors on 2D resistivity imaging surveying
Authors Bing Zhou and Torleif DahlinABSTRACTElectrode 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.
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Experiments of joint acquisition of seismic refraction and surface wave data
Authors Sebastiano Foti, Luigi Sambuelli, Valentina L. Socco and Claudio StrobbiaABSTRACTDispersive Rayleigh waves, contained in conventional P‐wave refraction records, can be used to determine the shear‐wave velocity profile. In the present paper, the synergies of a cross interpretation of refraction and surface wave data are exploited using data collected at sites where other geotechnical and geophysical information was available.
Selected examples are presented to emphasize the relative advantages and limitations of the two techniques in the cases of hidden layer, velocity inversions and shallow water table.
Surface wave analysis was performed to estimate the experimental dispersion curves in the f–k domain and the results of the dispersion curves inversion were compared with refraction results in terms of velocity profiles.
The experimental results prove that many advantages, in terms of resolution and reliability, can be obtained with joint acquisition and cross interpretation of P‐wave refraction and surface wave data, without a significant increase in testing time.
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Two‐dimensional electrical imaging for detection of hydrocarbon contaminants
More LessABSTRACTThe effects of a long‐term diesel oil pollution due to leakage from buried tanks have been investigated using electrical resistivity tomography. The reliability of 2D electrical resistivity imaging of the subsoil was assessed using a numerical modelling approach that simulated the different behaviour of the contaminated zone. The effects of inversion parameters, such as the damping factor and smoothing matrix, have been studied in order to evaluate the optimal parameters to process real data.
The results of the field test indicated that highly conductive anomalies can be related to the biological degradation of hydrocarbons: geochemical analysis performed on several groundwater samples confirmed the presence of biodegradation activity. Chemical analysis pointed out an anomalous concentration of iron and manganese cations dissolved in the groundwater. Very low values of resistivity can be associated with a marked modification of the cation exchange capacity of the soil mixture due to degradation of these hydrocarbons. Chemical and physical interactions due to hydrocarbon pollution affect the electrical properties of soils and groundwater.
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GPR abilities for the detection and characterisation of open fractures in a salt mine
Authors Colette Grégoire, Lucien Halleux and Volker LukasABSTRACTGround‐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.
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Improving the resolution of shallow seismic sections using eigenimage analysis
More LessABSTRACTThe basic limitation of geophysical techniques used for imaging various shallow localized subsurface inhomogeneities is their spatial resolution, that is, the ability of the methods to detect small‐scale objects. This is especially true for low‐resolution techniques, such as recently developed methods using seismic surface waves. The objective of this work is to improve the spatial resolution and to enhance the detectability of small‐scale objects on seismic sections. For this purpose, a method of eigenimage analysis, based on singular value decomposition of seismic sections, was used. The method effectively separates events with different horizontal coherence. Since shallow subsurface inhomogeneities are characterized by poor horizontal coherence on seismic sections, such a separation can serve as a tool for detecting the inhomogeneous areas in the subsurface.
Application of the method to synthetic and real seismic data shows that it can significantly improve the spatial resolution of the subsurface imaging.
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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)