Home About us Contact
|
Home About us Contact | ||
|
|
||
Resistivity Tomography (resistivity + tomography)
Kinds of Resistivity Tomography Selected AbstractsKarstic morphologies identified with geophysics around Saulges caves (Mayenne, France)ARCHAEOLOGICAL PROSPECTION, Issue 3 2010Rémi Valois Abstract Geophysical measurements were carried out around Saulges caves that sought to highlight the local karstic morphologies and the impact on preservation of archaeological material within the caves. Electrical resistivity tomographies (ERTs) and apparent conductivity mapping detected a soil cover on the plateaus that ends abruptly over fractured limestone or over a bowl-shaped structure filled with clay soil. Moreover, there is at least one zone of soil accumulation with a basin form with almost no soil cover around this structure. The ERT and seismic refraction tomography (SRT) detected an important anomaly in the valley. Many clues indicate that this anomaly is a karstic conduit filled with water or clay. Therefore, some karstic dissolution zones have been found and only geophysical methods are able to detect such features. Detection of preferential pathways could help to protect prehistoric art within the caves and new karstic morphologies help to better understand this karstic system. Copyright © 2010 John Wiley & Sons, Ltd. [source] Testing etching hypothesis for the shaping of granite dome structures beneath lateritic weathering landsurfaces using ERT methodEARTH SURFACE PROCESSES AND LANDFORMS, Issue 10 2003Anicet Beauvais Abstract An Erratum has been published for this article in Earth Surface Processes and Landforms 28(13) 2003, 1491. Granite domes, boulders and knobs buried within saprolite have been detected beneath lateritic weathering landsurfaces using 2D electrical resistivity tomography (ERT). This technique provides a valuable means of mapping the bedrock topography and the regolith structures underneath landsurfaces, as it is intrinsically very sensitive to the electrical properties of superimposed pedological, hydrological and geological layers, allowing the determination of their relative geometry and spatial relationships. For instance, 2D inverse electrical resistivity models including topographic data permit the de,nition of lithostratigraphic cross-sections. It shows that resistive layers, such as the more or less hardened ferruginous horizons and/or the bedrock, are generally well differentiated from poorly resistive layers, such as saprolite, including water-saturated lenses, as has been corroborated by past and actual borehole observations. The analysis of the 2D geometrical relations between the weathering front, i.e. the bedrock topography, and the erosion surface, i.e. the landsurface topography, documents the weathering and erosion processes governing the development of the landforms and the underlying structures, thus allowing the etching hypothesis to be tested. The in,ltration waters are diverted by bedrock protrusions, which behave as structural thresholds compartmentalizing the saprolite domain, and also the regolith water table, into distinct perched saturated subdomains. The diverted waters are thus accumulated in bedrock troughs, which behave like underground channels where the saprolite production rate may be enhanced, provided that the water drainage is ef,cient. If the landsurface topography controls the runoff dynamics, the actual bedrock topography as depicted by ERT imaging in,uences the hydrodynamics beneath the landsurface. In some way, this may control the actual weathering rate and the shaping of bedrock protrusions as granite domes and knobs within thick saprolite, before their eventual future exposure. Copyright © 2003 John Wiley & Sons, Ltd. [source] Sensitivity of electrical resistivity tomography data to electrode position errorsGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2005Greg A. Oldenborger SUMMARY Limitations of imaging using electrical resistivity tomography (ERT) arise because of the difficulty of quantifying the reliability of tomographic images. A major source of uncertainty in tomographic inversion is data error. Data error due to electrode mislocations is characterized by the sensitivity of electrical potential to both source and receiver positions. This sensitivity is described by a scattering-type equation and, therefore, depends not only on source,receiver separation, but also on the location and magnitude of contrasts in electrical conductivity. At the overlapping scales of near-surface environmental and engineering geophysical surveys, for which electrodes may be close to the target and experiment dimensions may be on the same order as those of the target, errors associated with electrode mislocations can significantly contaminate the ERT data and the reconstructed electrical conductivity. For synthetic experiments, variations in the data due to electrode mislocation are comparable in magnitude to typical experimental noise levels and, in some cases, may overwhelm variations in the data due to changes in material properties. Furthermore, the statistical distribution of electrode mislocation errors can be complicated and multimodal such that bias may be introduced into the ERT data. The resulting perturbations of the reconstructed electrical conductivity field due to electrode mislocations can be significant in magnitude with complex spatial distributions that are dependent both on the model and the experiment. [source] 2D data modelling by electrical resistivity tomography for complex subsurface geologyGEOPHYSICAL PROSPECTING, Issue 2 2006E. Cardarelli ABSTRACT A new tool for two-dimensional apparent-resistivity data modelling and inversion is presented. The study is developed according to the idea that the best way to deal with ill-posedness of geoelectrical inverse problems lies in constructing algorithms which allow a flexible control of the physical and mathematical elements involved in the resolution. The forward problem is solved through a finite-difference algorithm, whose main features are a versatile user-defined discretization of the domain and a new approach to the solution of the inverse Fourier transform. The inversion procedure is based on an iterative smoothness-constrained least-squares algorithm. As mentioned, the code is constructed to ensure flexibility in resolution. This is first achieved by starting the inversion from an arbitrarily defined model. In our approach, a Jacobian matrix is calculated at each iteration, using a generalization of Cohn's network sensitivity theorem. Another versatile feature is the issue of introducing a priori information about the solution. Regions of the domain can be constrained to vary between two limits (the lower and upper bounds) by using inequality constraints. A second possibility is to include the starting model in the objective function used to determine an improved estimate of the unknown parameters and to constrain the solution to the above model. Furthermore, the possibility either of defining a discretization of the domain that exactly fits the underground structures or of refining the mesh of the grid certainly leads to more accurate solutions. Control on the mathematical elements in the inversion algorithm is also allowed. The smoothness matrix can be modified in order to penalize roughness in any one direction. An empirical way of assigning the regularization parameter (damping) is defined, but the user can also decide to assign it manually at each iteration. An appropriate tool was constructed with the purpose of handling the inversion results, for example to correct reconstructed models and to check the effects of such changes on the calculated apparent resistivity. Tests on synthetic and real data, in particular in handling indeterminate cases, show that the flexible approach is a good way to build a detailed picture of the prospected area. [source] Using DC resistivity tomography to detect and characterize mountain permafrostGEOPHYSICAL PROSPECTING, Issue 4 2003Christian Hauck ABSTRACT Direct-current (DC) resistivity tomography has been applied to different mountain permafrost regions. Despite problems with the very high resistivities of the frozen material, plausible results were obtained. Inversions with synthetic data revealed that an appropriate choice of regularization constraints was important, and that a joint analysis of several tomograms computed with different constraints was required to judge the reliability of individual features. The theoretical results were verified with three field experiments conducted in the Swiss and the Italian Alps. At the first site, near Zermatt, Switzerland, the location and the approximate lateral and vertical extent of an ice core within a moraine could be delineated. On the Murtel rock glacier, eastern Swiss Alps, a steeply dipping boundary at its frontal part was observed, and extremely high resistivities of several M, indicated a high ice content. The base of the rock glacier remained unresolved by the DC resistivity measurements, but it could be constrained with transient EM soundings. On another rock glacier near the Stelvio Pass, eastern Italian Alps, DC resistivity tomography allowed delineation of the rock glacier base, and the only moderately high resistivities within the rock glacier body indicated that the ice content must be lower compared with the Murtel rock glacier. [source] Subsurface Imaging of an Abandoned Solid Waste Landfill Site in Norman, OklahomaGROUND WATER MONITORING & REMEDIATION, Issue 2 2006Joseph T. Zume Leachate plume emanating from an old unlined municipal landfill site near the city of Norman, Oklahoma, is discharging into the underlying alluvial aquifer. Subsurface imaging techniques, electrical resistivity tomography and electrical conductivity (EC) logging, were used on the site to detect and map the position of the leachate plume. Anomalous EC zones, delineated with the two methods, correlated with the occurrence of the plume detected by water chemistry analyses from multilevel monitoring wells. Specific conductance, a potential indicator of leachate contamination, ranged from 1861 to 7710 ,S/cm in contaminated zones and from 465 to 2180 ,S/cm in uncontaminated ground water. Results are in agreement with those from earlier studies that the leachate plume emerges from the landfill along preferential pathways. Additionally, there are indications that the leading edge of the plume has migrated, at least, 200 m away from the landfill in the direction of ground water flow. [source] Tracing solute infiltration using a combined method of dye tracer test and electrical resistivity tomography in an undisturbed forest soil profileHYDROLOGICAL PROCESSES, Issue 21 2010Jae Gon Kim Abstract An accurate prediction of solute infiltration in a soil profile is important in the area of environmental science, groundwater and civil engineering. We examined the infiltration pattern and monitored the infiltration process using a combined method of dye tracer test and electrical resistivity tomography (ERT) in an undisturbed field soil (1 m × 1 m). A homogeneous matrix flow was observed in the surface soil (A horizon), but a preferential flow along macropores and residual rock structure was the dominant infiltration pattern in the subsurface soil. Saturated interflow along the slopping boundaries of A and C1 horizons and of an upper sandy layer and a lower thin clay layer in the C horizon was also observed. The result of ERT showed that matrix flow started first in A horizon and then the infiltration was followed by the preferential flows along the sloping interfaces and macropores. The ERT did not show as much detail as the dye-stained image for the preferential flow. However, the area with the higher staining density where preferential flow was dominant showed a relatively lower electrical resistivity. The result of this study indicates that ERT can be applied for the monitoring of solute transportation in the vadose zone. Copyright © 2010 John Wiley & Sons, Ltd. [source] Applicability of electrical resistivity tomography monitoring to coarse blocky and ice-rich permafrost landformsPERMAFROST AND PERIGLACIAL PROCESSES, Issue 3 2009C. Hilbich Abstract The inversion and interpretation of electrical resistivity tomography (ERT) data from coarse blocky and ice-rich permafrost sites are challenging due to strong resistivity contrasts and high contact resistances. To assess temporal changes during ERT monitoring (ERTM), corresponding inversion artefacts have to be separated from true subsurface changes. Appraisal techniques serve to analyse an ERTM data set from a rockglacier, including synthetic modelling, the depth of investigation index technique and the so-called resolution matrix approach. The application of these methods led step by step to the identification of unreliable model regions and thus to the improvement in interpretation of temporal resistivity changes. An important result is that resistivity values of model regions with strong resistivity contrasts and highly resistive features are generally of critical reliability, and resistivity changes within or below the ice core of a rockglacier should therefore not be interpreted as a permafrost signal. Conversely, long-term degradation phenomena in terms of warming of massive ground ice at the permafrost table are detectable by ERTM. Copyright © 2009 John Wiley & Sons, Ltd. [source] Tracing a major Roman road in the area of ancient Helike by resistivity tomographyARCHAEOLOGICAL PROSPECTION, Issue 4 2009G. N. Tsokas Abstract In his journey through Achaea in the second century AD, Pausanias mentioned the destroyed Classical site of Helike on the sea and noted that its ruins were still visible underwater near the southwest shore of the Corinthian Gulf. In 2001, the Helike Project excavators discovered on the coastal plain of Helike southeast of Aigion the first ruins of Classical buildings, buried under lagoon sediments 3,m deep. They also found segments of a major Roman road oriented NW,SE and buried 1,1.50,m deep under the contemporary surface. Pausanias referred to the main road through the Helike plain, which he followed during his visit. According to his description, he saw the submerged ruins of Helike toward the sea north of the Roman road. Tracing the exact location of the road would, therefore, help to locate the lost city. We employed resistivity tomography to explore the areas between trenches where the road had been unearthed and also to investigate its possible extension beyond those locations. Since 2004, resistivity studies performed by grids or single profiles have been carried out at 11 locations. As a result, the ancient road was detected for a length of about 2,km. It was also imaged either in two-dimensional or three-dimensional contexts at all the locations where it was detected. Copyright © 2009 John Wiley & Sons, Ltd. [source] The role of integrated geophysical survey methods in the assessment of archaeological landscapes: the case of PortusARCHAEOLOGICAL PROSPECTION, Issue 3 2009Simon Keay Abstract The regular application of geophysical, geochemical and topographical survey techniques to evaluate archaeological sites is well established as a method for locating, defining and mapping buried archaeological materials. However, it is not always feasible to apply a range of different methods over a particular site or landscape due to constraints in time or funding. This paper addresses the integrated application of a variety of survey techniques over different sites and landscapes in Italy and elsewhere, focusing on the recent results from the ongoing survey and excavations at Portus, the port of Imperial Rome. An integration of methods, including magnetometry, resistance survey, ground-penetrating radar (GPR) and electrical resistivity tomography (ERT) has been used at the site to fulfil a number of different research objectives. Results of the magnetometer survey have successfully recorded the nature and extent of archaeological material over an area of 220,ha, allowing a plan of the port and related structures to be produced and variations in archaeological potential across the entire landscape to be assessed. The integration of several techniques in one area of the site between the Porto di Claudio and the Porto di Traiano has mapped the structural remains of this area of the port prior to and during investigation of the zone through excavation. Current work on the geophysical survey data, using different software programs for the processing of survey data and merging different datasets using geographical information system packages, has allowed the results of the work to be visualized and presented to archaeologists in a comprehensive and unambiguous fashion, facilitating the future management and preservation of the site. In addition ongoing research is using different statistical and visual methods of integration to refine the archaeological interpretation of the study area. Copyright © 2009 John Wiley & Sons, Ltd. [source] GPR, ERT and magnetic investigations inside the Martyrium of St Philip, Hierapolis, TurkeyARCHAEOLOGICAL PROSPECTION, Issue 3 2009Luigia Nuzzo Abstract Hierapolis, Denizli, Turkey, was one of the most important Hellenistic-Roman cities in Asia Minor. Located about 250,km east of Izmir, the area is a UNESCO World Heritage Site because of its magnificent archaeological remains and the white travertine pool formations created by its peculiar geothermal setting. The Italian Archaeological Mission in more than 50 years of activity in Hierapolis has brought to light and restored important vestiges of the ancient city, helping to understand the urban layout in the various epochs of its development. In 2001,2003 geophysical surveys were performed by the University of Lecce in several areas inside the archaeological site of Hierapolis to support the archaeological excavations. This paper reports the results of the integrated geophysical surveys performed in 2003 inside the Martyrium of Saint Philip, a mausoleum built on the place where it is believed that the Apostle was martyred. Ground-penetrating radar (GPR), electrical resistivity tomography (ERT) and magnetic gradient investigations were carried out in the central octagonal room, whereas the accessible lateral rooms were surveyed with GPR and occasionally ERT. The acquisition was performed along a series of closely spaced lines and the processed data were visualized as two-dimensional vertical sections (GPR), map view (magnetic gradiometry), depth slices or three-dimensional volumes (GPR and ERT) to allow an integrated interpretation of the geophysical results. The analysis of the geophysical datasets revealed a series of anomalies in both the central and lateral rooms that could be ascribed to the building foundations and to other possible archaeological structures, probably related to earlier stages of the sacred building and to tombs, as well as other anomalies (voids, fractures) of presumable natural origin. Copyright © 2009 John Wiley & Sons, Ltd. [source] Deep subsurface geophysical prospection at Tell Qarqur, SyriaARCHAEOLOGICAL PROSPECTION, Issue 3 2008Jesse Casana Abstract This paper presents results of recent archaeo-geophysical investigations utilizing low-frequency ground-penetrating radar (GPR) and electrical resistivity tomography (ERT) at the site of Tell Qarqur in western Syria. Like many major tell sites in the Near East, Tell Qarqur possesses archaeological strata more than 30,m in depth and was nearly continuously occupied for 10 000 years. The sheer scale and complexity of tell sites has rendered traditional archaeo-geophysical methods, designed to map cultural features at 1,2,m below the surface and in only two dimensions, relatively ineffective. The instruments, survey strategies, and processing protocols we have utilized at Tell Qarqur have succeeded in documenting architectural features and archaeological stratigraphy in three dimensions and at much greater depths than has been possible previously. Our results offer new perspectives on the organization and development of settlement at Tell Qarqur and highlight the potential of our methods for archaeo-geophysical investigations at deeply stratified sites more generally. Copyright © 2008 John Wiley & Sons, Ltd. [source] Non-destructive electrical resistivity tomography for indoor investigation: the case of Kapnikarea Church in AthensARCHAEOLOGICAL PROSPECTION, Issue 1 2008G. N. Tsokas Abstract The area inside and around the church of Kapnikarea in Athens (Greece), was explored by means of electrical resistivity tomography (ERT). ,Flat base' electrodes were used exclusively because the survey had to be fully non-destructive. The performance of ,flat base' electrodes was satisfactory, leading to inversions of small root mean square (RMS) errors and reliable subsurface images, which were checked against existing borehole logs. High-resistivity anomalies were observed beneath the floor of the church. They are attributed to possible voids, remains of ancient wells, or other man-made structures concealed under the floor of the church. The results show that ,flat base' electrodes provided the advantage of fully non-destructive geoelectrical measurements. They also show that the use of the non-destructive ERT method offers a serious alternative and a complementary method to ground-penetrating radar surveys inside existing monuments. Copyright © 2007 John Wiley & Sons, Ltd. [source] Multi-temporal geophysical survey of a Roman bath complex in Montegrotto Terme (Padova, northern Italy)ARCHAEOLOGICAL PROSPECTION, Issue 3 2007E. Finzi Abstract Between 2000 and 2005, several geophysical surveys were carried out by the staff of the Department of Geosciences, University of Padova, on the site of the Terme Neroniane in Montegrotto, near Padova, covering a whole complex of Roman spa buildings. This work represented a good opportunity to compare the effectiveness of various techniques (ground-penetrating radar, magnetic gradiometry and electrical resistivity tomography) and to seek the reasons for differing results. Depth, contrast between composition of the geological background and building materials, good state of structural preservation, and the lack of settlement following the Roman age supported experimental results. Old and more recent excavations, conducted in parallel with the University of Padova surveys, enabled continual comparisons between geophysical models and evidence from the site. Copyright © 2007 John Wiley & Sons, Ltd. [source] | |||||||||||||