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- Volume 7, Issue 3, 2009
Near Surface Geophysics - Volume 7, Issue 3, 2009
Volume 7, Issue 3, 2009
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Integration of multidimensional archaeogeophysical data using supervised and unsupervised classification
More LessWhen multiple geophysical methods are used to survey an archaeological site, an integrated approach to interpreting the data is often pursued. The use of supervised and unsupervised classification methods are tested using ground‐penetrating radar, magnetometry and magnetic susceptibility data sets from a site in the American Southwest. Pueblo Escondido was a large prehistoric village associated with the Mogollon culture in southern New Mexico, with peak occupation during the transition between pithouse and pueblo architectural periods (ca. 1280–1290 AD). Image classification has the benefit of producing unambiguous discrete maps and capitalizes on the multivariate relationships between data sets. Theoretically, unsupervised classification could identify new archaeological classes that were not anticipated but no such classes were identified. The K‐means cluster analysis succeeded only in identifying weak, moderate and strong positive anomalies found in the original data sets. Supervised classification utilizing Mahalanobis distance produced much better results. Training sites based on archaeological excavations were used to classify all locations in the survey area, yielding a predictive model of archaeological features in three classes, plus a background class. The result shows features that were not easily identified in the original data sets but are made visible by the multivariate model. The model could be used for guiding future excavations and arguably leads to a better understanding of the site’s subsurface content and spatial organization.
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Application of signal dependent rank‐order mean filter to the removal of noise spikes from 2D electrical resistivity imaging data
Authors Jalal Ferahtia, Nouredine Djarfour, Kamel Baddari and Roger GuérinIt is well‐known that when inverting two‐dimensional (2D) electrical resistivity data, a major source of errors is the presence of noise and in particular noise spikes. The popular median filter is often applied to the removal of single spikes. However, when the signal is highly corrupted with successive spikes, the median filter performance is poor. This paper deals with the use of the signal dependent rank‐order mean filter for the detection and removal of noise spikes from highly corrupted 2D electrical resistivity imaging data. In addition to its computational simplicity, this filter is shown to be extremely robust, even in the presence of very strong noise, especially when it is applied recursively. The signal dependent rank‐order mean filter was tested on 2D synthetic resistivity data contaminated by near‐surface inhomogeneities and the results confirmed efficient removal of the disturbances normally associated with near‐surface inhomogeneities. The signal dependent rank‐order mean filter was also applied to field data and demonstrated its ability to significantly improve the accuracy of the inversion process and to produce good visual results in the inverted electrical sections.
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Imaging near‐surface inhomogeneities using weighted multipath summation
Authors Vladimir Shtivelman, Shemer Keydar and Michael MikenbergImaging of shallow subsurface objects in 3D space can be performed by a spatial summation of seismic waves diffracted by these objects. The summation can be implemented using either a fixed velocity or, alternatively, a recently introduced multipath summation approach. The multipath summation does not require any explicit information on velocities since it is performed for all possible velocity values within a wide specified range. However, the spatial resolution of the images obtained by the multipath summation is usually inferior to those obtained using the fixed velocity approach.
In order to improve the image quality, we propose a new approach to the multipath summation performing the summation with image dependent weights. The ‘optimal’ weights can be obtained from asymptotics of the corresponding path integral (which is a continuous analogue of the multi‐path summation). Application of the weighted multipath summation to synthetic and real data demonstrates that the quality of the imaging can be greatly improved as compared to the unweighted summation and sometimes even to the fixed velocity approach. Moreover, the weighted multipath summation can produce reliable estimates of the summation parameter (velocity above the target object), which in some cases can be used to further improve the image quality.
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Comparison of instruments for geoelectrical soil mapping at the field scale
Authors Robin Gebbers, Erika Lück, Michel Dabas and Horst DomschIn precision agriculture geoelectrical methods have shown their capability to detect spatial variation of important physico‐chemical soil parameters in an efficient way. Nevertheless, relationships between the electrical parameters (electrical conductivity or resistivity) and other soil properties are not always consistent over different fields. This can, to some extent, be due to the characteristics of instruments used for soil mapping. However, a limited amount of research has addressed this issue. In this study, seven instruments for mobile mapping (continuous geoelectrical measurements) available on the market were tested (ARP 03, CM‐138, EM38, EM38‐DD, EM38‐MK2, OhmMapper and Veris 3100). Instruments were employed on a sandy site in north‐east Germany. Measurements were compared to a profile, which has been investigated with a high accuracy reference. Additional investigations were conducted concerning the influences of temperature drift, seasonal variations and soil properties on soil EC. Marked differences between the instruments were found with respect to depth of investigation, accuracy and handling that have to be taken into account when geoelectrical surveys are planned or interpreted. Regarding depth of investigation and robustness of the measurements, ARP 03 and Veris 3100 seem to be the most suitable instruments for precision agriculture.
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Imaging and monitoring tree‐induced subsidence using electrical resistivity imaging
Authors G.M. Jones, N.J. Cassidy, P.A. Thomas, S. Plante and J.K. PringleTree‐induced subsidence is a natural ground movement phenomenon that arises when tree roots absorb water from clay‐rich soils, causing them to shrink. Tree‐induced subsidence is particularly problematic for buildings and structures in close proximity to deciduous trees where changes in seasonal climate can have a significant effect on the degree of relative surface movement. Unfortunately, there is a lack of in‐depth understanding of tree‐induced subsidence dynamics and limited access to affordable/reliable assessment technologies for the victims of such phenomena. This paper presents and discusses the use of 2D electrical resistivity imaging (ERI) as a tool for recognizing tree‐related subsurface desiccation and its associated subsidence. The findings of a two year ERI study in a high‐risk tree‐induced subsidence environment are presented (mature Oak and Willow trees growing in expansive London Clays) where repeat (time‐lapse) resistivity profiles are compared to conventional ground levelling and soil moisture content data. This study confirms that ERI is a valuable tool for the characterization of seasonally varying subsurface moisture distributions and that the technique is capable of defining the architecture and temporal‐spatial variance of an active tree root system. Moreover, the method provides a critical insight into the visualization of tree‐induced subsidence phenomena and reveals promising practical potential for use as a modern, affordable, non‐invasive tree‐induced subsidence assessment tool.
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High‐resolution magnetic gradient and electrical resistivity tomography survey at the Plaka Petrified Forest Park in Lesvos Island, Greece
Authors G. Vargemezis, N. Zouros, P. Tsourlos and I. FikosLesvos Island, in the North Aegean area of Greece, exposes large accumulations of fossilized tree trunks. They are collectively known as the Petrified Forest of Lesvos, a designated protected natural monument. The paper describes the results of a geophysical study that has been carried out in the area of Plaka on the western part of Lesvos in order to investigate the near‐surface geology and detect buried fossilized tree trunks.
In situ and laboratory measurements of magnetic and electrical properties of the trunks have been conducted. Considering the magnetic susceptibility distribution and the contrast between the trunks and the surrounding material, magnetic gradient anomalies of negative signature were expected to reflect the existence of trunks. Electrical resistivity tomography has also been conducted in order to detect buried trunks, based on their resistivity contrast with the pyroclastic surrounding material.
Flow paths of the pyroclastic material that can be related to the tectonic features at the time have been detected. In general, petrified trunks have been detected as high resistive bodies. In one case, where the trunk was buried very close to the surface, the resistivity was lower than the surrounding material.
2D and 3D subsurface resistivity models of the surveyed area have been constructed, pointing out probable locations of buried petrified trunks. Excavations that followed the geophysical survey revealed petrified trunks in most of the cases although some of the excavated resistive targets proved to be surface fracture systems filled with resistive oxides.
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Gravity anomalies of pull‐apart basins having finite strike length with depth dependent density: a ridge regression inversion
More LessAn inversion using ridge regression to estimate simultaneously the parameters of pull‐apart basins having finite strike length (2.5D) and regional gravity background from observed gravity anomalies is presented. A parabolic function is used to describe the density contrast variation with depth within the structure. The algorithm begins with initializing both the regional background and parameters of the basin and subsequently improves them iteratively until the modelled gravity anomalies mimic the observed ones. The applicability and efficacy of the inversion is demonstrated with a set of synthetic gravity anomalies 1) attributable entirely due to a theoretical model, 2) in the presence of pseudorandom noise and 3) in the presence of both pseudorandom noise and regional gravity background. It was found from the analysis of synthetic gravity anomalies that the modelled parameters of the structure closely mimic the true ones even when the gravity anomalies are corrupted with pseudorandom noise. In the presence of both random noise and regional background the estimated parameters deviate only modestly from the assumed ones. Furthermore, the applicability of the algorithm is exemplified with a derived density‐depth model to analyse the Bouguer gravity anomalies observed over the Ranigunj basin, India. The estimated depth of the basin is consistent with the available borehole information. The interpretation of the basin supports the hypothesis that this basin might have been formed as a result of both E‐W kinematics and orthogonal extension rather than simple local extensional tectonics.
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Hydrocarbon contaminated soil: geophysical‐chemical methods for designing remediation strategies
More LessRapid and correct characterizations of contaminant plumes are necessary to plan efficient and economically viable remediations of affected zones. In this context, we have carried out a joint implementation of two geophysical non‐invasive methods and chemical monitoring from wells to characterize an area affected by a hydrocarbon spill caused by a traffic accident involving a transport truck and its trailer. The studied area is situated in Alejo Ledesma, Argentina and has an area of 4350 m2. The geophysical prospecting was carried out through the resistivity and the electromagnetic induction methods. The first method provided precise vertical resistivity sections, which were acquired at representative contaminated/uncontaminated sectors, whereas the second allowed a quick prospecting of the entire area. The chemical procedure consisted of samples obtained from a small number of monitoring wells, which were located at selected positions. We correlated the geophysical interpretations with the chemical data to delimit the zone affected by the spill. The detection and characterization of the contaminated plume by the geophysical methods showed positive results even though a liquid phase was not present at the site. A remediation methodology could be determined from these results. The efficiency of the applied methodology (stabilization/solidification) was also confirmed through these methods.
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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)