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- Volume 12, Issue 5, 2014
Near Surface Geophysics - Volume 12, Issue 5, 2014
Volume 12, Issue 5, 2014
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Theoretical approach for the depth of penetration of in situ magnetic susceptibility measurements
More LessABSTRACTWe present a mathematical approach for analysing in‐situ magnetic susceptibility measurements in terms of elementary contributions from different depths within the soil. Our analysis leads to a compact integral formula that relates the actual measurements to the depth profile of magnetic susceptibility within the soil, in such a way that the former is simply a weighted spatial average of the latter. The degree of penetration of a given measurement is then readily quantified by the weighting function of the corresponding instrument. Our results match and complement the experimental determinations of other workers to currently used susceptibility meters. We provide theoretical curves for the depth of penetration of four instruments that have been studied in the literature, and also refine previous estimations of the characteristic depths intrinsic to each instrument.
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Semiautomatic interpretation of microgravity data from subsurface cavities using curvature gradient tensor matrix
Authors Wenna Zhou, Jiyan Li and Xiaojuan DuABSTRACTWe propose an effective technique, based on eigenvalues of the curvature gradient tensor matrix (CGTM), to determine edges and depths of shallow subsurface cavities. The new technique is similar to the tilt angle and tilt‐depth methods in edge detection and depth estimation, respectively. Our improvement to this method is that the vertical derivative of the tilt angle is replaced by an eigenvalue of the CGTM. Zero contours of eigenvalues of the CGTM can be used to outline the edges of causative sources, and the characteristic is similar to the vertical derivative of gravity field. Therefore, we use the smaller of the two eigenvalues to replace the vertical derivative in the numerator of the tilt angle, and this results in an improved edge detection method. In addition, we deduce a new depth estimation method using the point mass model parameters which can be used to estimate the centre depths of shallow causative sources. The new method is tested on a synthetic model with and without noise. It demonstrates that the new method is simple, robust and accurate. Finally, we apply the method on the measured gravity data of a mining subsidence area from Liaoyuan, Northeast of China. The results show a good correspondence with the inversion results of electrical resistivity tomography (ERT) data. Such results can serve as preliminary depth estimates and location of near surface cavities.
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3D complex resistivity tomography on cylindrical models using EIDORS
Authors Giorgio De Donno and Ettore CardarelliABSTRACTComplex resistivity imaging is a relatively new geophysical technique, developed in the last few decades mainly for hydrogeological and environmental applications. The aim of this work is to present an EIDORS application of the 3D complex resistivity tomography on cylindrical laboratory models. EIDORS is an open‐source numerical environment developed with the aim of sharing data and promoting collaboration between groups working in these fields. In spite of being a well‐recognised software for forward modelling and inversion for medical tomographies, EIDORS still needs to be adapted for geophysical purposes. We discuss the role played by the mesh choice and the contact impedances on the accuracy of the finite‐element solution achieved by tetrahedral elements. When a 3D tomography is performed on a standard machine with limited local memory, the dual reconstruction can help to retain a sufficient accuracy without increasing the allocated memory. Although for medical applications on the human body a linear inversion can effectively represent the slight changes in resistivity magnitude, when a subsoil has to be investigated resistivity can vary substantially. Thus we develop an algorithm to add to the non‐linear inversion for complex resistivity data, through the integration of the EIDORS basic functions. The algorithm has been validated through four synthetic examples. The reconstructed models, having a growing degree of complexity, are similar to the true ones. We highlight the role played by phase and resolution to detect the anomalies. When the dipole length is enlarged and the embedded anomalies decrease in size, the reconstruction becomes more difficult. We show that EIDORS could act as a base code for tomographic inversion of frequency‐domain data (and also of time‐domain real‐valued data) for laboratory problems, because of its high flexibility and reliability reached by the forward and inversion routines.
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Detection of landfill cover damage using geophysical methods
Authors Fanny Genelle, Colette Sirieix, Joëlle Riss, Véronique Naudet, Michel Dabas and Philippe BégassatABSTRACTIn closed hazardous waste (i.e. industrial) landfills, impermeable covers are used to seal in the waste so as to minimise water infiltration and the accumulation of leachate inside the waste. In this paper, we present a geophysical study performed on a French landfill where the cover was designed using impermeable (clay) and drainage (sand) layers along with a Geosynthetic Clay Liner (GCL) in between. As observed, the quantity of leachate increases after rain events making the leachate treatment more expensive. This could be due to areas of weakness in the cover that have developed over time or which date back to its installation. Three different geophysical methods were employed and confronted to detect and assess such damage, and, if necessary, to help choose the most suitable remediation of the cover: Automatic Resistivity Profiling (ARP) which enables a rapid mapping of the entire landfill; the self‐potential method (SP) as one of its sources is water drainage; and electrical resistivity tomography (ERT) to obtain 2D resistivity models. Based on the ARP results revealing high lateral heterogeneities of the superficial cover materials, the SP and ERT measurements were conducted on a limited area. A negative SP anomaly was observed at the top of the landfill, where the cover is the thinnest and the GCL the most damaged, suggesting preferential infiltration under the GCL. The 2D ERT profile showed both vertical and horizontal variations. Comparison with manual auger holes showed that the alternating clayey and sandy layers did not conform to plans for the original cover when it was installed at this site about twenty years ago. Variations in electrical resistivity noticed at the depth of the GCL were attributed to possible damage to the GCL and a greater than expected permeability since the GCL is normally very resistive. The results obtained in this study were important to understand the damage to the cover due to the use of different types and thicknesses of cover materials compared to what was originally planned, and deterioration of the GCL.
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Characterization of organic‐contaminated ground by a combination of electromagnetic mapping and direct‐push in situ measurements
Authors Yuji Mitsuhata, Dai Ando, Takehiko Imasato and Kazushige TakagiABSTRACTContamination of soil and groundwater by synthetic volatile organic compounds (VOCs) and hydrocarbons has recently raised public concern. Geophysical techniques are frequently used to characterize contaminated sites and to specify subsurface contaminant plumes in Europe and America, but there have been very few such surveys in Japan. Electromagnetic (EM) induction mapping was applied to investigate a contaminated site on reclaimed land near a harbour in central Japan. The use of EM mapping enabled efficient coverage of a study area in the site and imaging of the subsurface resistivity distribution down to approximately 10 m. In situ direct‐push membrane interface probe (MIP) and electrical conductivity (EC) in situ measurements were also performed as more direct sensing techniques, and the results were compared with soil core samples. The results suggest that the first and second conductive zones mapped by this investigation correspond to clayey soil zones that act as barriers to prevent the infiltration of contaminants. In addition, the in situ MIP measurements and laboratory analyses indicate multiple occurrences of contamination by VOCs and oil. Although EM mapping was not able to clearly specify a contaminant plume, it was demonstrated as a useful technique to delineate the infiltration pathways of contaminants by illustrating the subsurface distributions of clayey zones. In addition, the combination of direct‐push in situ measurements and EM mapping is demonstrated as an essential characterization strategy to verify the interpreted resistivity structure and to determine the relationship between the heterogeneous resistivity and contaminant distribution.
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Estimation of the near surface soil water content during evaporation using air‐launched ground‐penetrating radar
More LessABSTRACTEvaporation is an important process in the global water cycle and its variation affects the near surface soil water content, which is crucial for surface hydrology and climate modelling. Soil evaporation rate is often characterized by two distinct phases, namely, the energy limited phase (stage‐I) and the soil hydraulic limited period (stage‐II). In this paper, a laboratory experiment was conducted using a sand box filled with fine sand, which was subject to evaporation for a period of twenty three days. The setup was equipped with a weighting system to record automatically the weight of the sand box with a constant time‐step. Furthermore, time‐lapse air‐launched ground penetrating radar (GPR) measurements were performed to monitor the evaporation process. The GPR model involves a full‐waveform frequency‐domain solution of Maxwell’s equations for wave propagation in three‐dimensional multilayered media. The accuracy of the full‐waveform GPR forward modelling with respect to three different petrophysical models was investigated. Moreover, full‐waveform inversion of the GPR data was used to estimate the quantitative information, such as near surface soil water content. The two stages of evaporation can be clearly observed in the radargram, which indicates qualitatively that enough information is contained in the GPR data. The full‐waveform GPR inversion allows for accurate estimation of the near surface soil water content during extended evaporation phases, when a wide frequency range of GPR (0.8–5.0 GHz) is taken into account. In addition, the results indicate that the CRIM model may constitute a relevant alternative in solving the frequency‐dependency issue for full waveform GPR modelling.
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Iterative modelling of AEM data based on a priori information from seismic and borehole data
More LessABSTRACTAirborne electromagnetic methods (AEM) have become an important part of groundwater mapping in a wide range of geological settings. However, as for all geophysical methods, the results of the inversions are non‐unique and it is therefore relevant to include a priori information in order to obtain the most realistic geological inversion outcome. Despite the extensive use of AEM, only a few studies describe the effect of including a priori information in large‐scale AEM surveys. In this study, ancillary information from seismic and borehole data are used as a priori information. The basis for the study is a densely spaced airborne transient electromagnetic dataset (SkyTEM) from a 100 km2 area in the western part of Denmark. Six different inversions are performed, and these are formulated as blocky and smooth inversions with different amounts of a priori information in the deepest part of the sections. The use of a priori information has a significant influence on the interpretation of the sections in the lowermost part of the sequence. Furthermore, the middle part of the sections, which are not constrained by the a priori information, show a significant change through the different inversions. Thus, the study shows that the inclusion of a priori information to the deeper part, significantly enhances the understanding of the geology both in the intermediate and deep levels.
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An object oriented approach to automatic classification of archaeological features in magnetic prospection data
Authors Michael Pregesbauer, Immo Trinks and Wolfgang NeubauerABSTRACTMagnetometer prospection is commonly used in archaeology for the non‐invasive detection, mapping and investigation of buried prehistoric sites. The recorded data can contain numerous anomalies caused by archaeological structures in the ground. State‐of‐the‐art geomagnetic data processing results in geo‐referenced maps that traditionally are interpreted within Geographical Information Systems. With the increasing size of surveyed areas, the manual outlining and classification of magnetic anomalies becomes a highly time consuming process. Possibilities for automated classification of the magnetic prospection data prior to the actual archaeological interpretation would considerably enhance the productivity of the archaeological interpretation process regarding magnetic prospection data. Object oriented image processing methods known from remote sensing applications offer a large spectrum of readily available procedures for the automatic and semi‐automatic analysis of raster data sets. Suitable algorithms have been adapted and utilized in order to exemplarily analyse a magnetic archaeological prospection data set with the goal of automatically mapping magnetic features, facilitating further archaeological interpretation. This article shows that the presented semi‐automated classification procedure is able to replace the manual drawing of anomalies based on the expert knowledge and experience of the interpreter to a large degree.
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3D inversion of magnetic data seeking sharp boundaries: a case study for a porphyry copper deposit from Now Chun in central Iran
Authors Maysam Abedi, Ali Gholami and Gholam‐Hossain NorouziABSTRACTThis paper describes an application of 3D inversion of magnetic data to recover a susceptibility model from magnetic anomalies. For this purpose, the subsurface of the desired area of the magnetic anomaly is divided into a mesh with a large number of rectangular prisms with unknown susceptibilities. A Tikhonov cost function with multi‐term regularizers involving boundaries of susceptibility distribution and an edge‐preserving penalty function, as a tool to recover sharp boundaries, was used. Three methods (i.e., the U‐curve, Tikhonov‐curve and L‐curve methods) are applied to determine the optimum regularization parameter during the inversion process. Testing of the applied methods showed that the application of the U‐curve (a well‐known method in applied mathematics) in geophysical inverse problems and Tikhonov‐curve as a proposed technique can be appropriate candidates, like a common L‐curve method, for choosing the optimal regularization parameter. To avoid the natural tendency of magnetic structures to concentrate at the shallow depths in models created by inversion, a depth weighting function derived from information of the depth‐to‐the‐bottom of a generating source was applied. The AN‐EUL technique as a combination of the analytic signal and the Euler deconvolution methods is used to estimate the structural index of causative sources in order to construct an appropriate depth weighting function. Here, it is assumed that there is no remanent magnetization and the observed data are influenced by only the induced magnetization. A case study involving ground based measurements over a porphyry‐Cu deposit located in Kerman providence of Iran, Now Chun deposit, is included. The recovered 3D susceptibility model provided beneficial information for design of the exploration drilling programme. The susceptibility lows in the constructed model, in particular, their depths down to 410 m, coincides with the known locations of copper mineralization.
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The influence on sample preparation on spectral induced polarization of unconsolidated sediments
Authors K. Bairlein, A. Hördt and S. NordsiekABSTRACTSpectral induced polarization (SIP) measurements in the laboratory are in many cases intended to provide representative and comparable results of complex electrical conductivity. This is not invariably the case when using unconsolidated sediments, as the sample preparation influences several SIP‐relevant properties of the samples, including the pore geometry. The pore space is supposed to control the polarization effect and therefore a change in the pore space will change the measured parameters. We analysed the influence of the sample preparation on SIP measurements by testing various filling methods, each defined by a sequence of particular steps, with regard to the reproducibility of the spectra. The measurements were performed on three different sample materials. Variations of the spectra due to different filling methods were obtained, indicating the importance of considering the sample preparation. Methods that improved the reproducibility compared to loose packing of the samples were found, but the most suitable packing procedure depended on the material properties. The mean relaxation times and normalized chargeabilities were obtained from a Debye decomposition. Although unidentified processes caused scattering of the measured parameters, a relation of the IP‐parameters to the porosities was present. A decrease of the porosity reduced both the relaxation times and the normalized chargeabilities.
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Parameter estimation from non‐hyperbolic reflection traveltimes for large aperture common midpoint gathers
Authors Hervé Perroud, Rafael Krummenauer, Martin Tygel and Renato R. LopesABSTRACTAs far as superficial seismic reflection events are concerned, the classical normal moveout (NMO) process, which leads to the construction of seismic zero‐offset (or stacked) sections, encounters several difficulties due to the large data aperture. One of these difficulties is that the hyperbolic approximation of the reflection traveltime in common midpoint (CMP) gathers is no longer valid when offset largely exceeds the target depth. Recently, Fomel and Kazinnik proposed a novel, multi‐parameter, non‐hyperbolic formula for the traveltime of such reflection events. This formula, which will be referred to here as the FK traveltime after the authors, is exact for reflectors whose shape can be described by a hyperbola, and shows promising accuracy for long offsets and/or curved reflectors. It also depends on the same set of parameters as the common reflection surface (CRS) traveltime. In this paper, we propose strategies for estimating these CRS parameters based on the FK traveltime using large aperture data. However, in contrast to traditional CRS processing, and due to their widespread use, only common midpoint (CMP) gathers will be considered for the parameter searches. We will begin with a sensitivity analysis, showing the impact of each parameter on the traveltime. Based on this analysis, we will propose a two‐step estimation strategy, that could lead to improved seismic images, especially for very shallow, high aperture events. We will then highlight, through synthetic examples and discussions, the strengths and limitations of this strategy.
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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)