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Near Surface 2006 - 12th EAGE European Meeting of Environmental and Engineering Geophysics
- Conference date: 04 Sep 2006 - 06 Sep 2006
- Location: Helsinki, Finland
- ISBN: 978-90-73781-62-7
- Published: 04 September 2006
1 - 100 of 142 results
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Why Not X in Airborne TEM?
Authors E. Auken, A. V. Christiansen and K. SørensenWe describe the challenges for the realization of a transient EM (TEM) system capable of measuring both the z- and x-components. This involves significant changes to instrumentation as well as to the processing and inversion algorithms.
Geophysical mapping of groundwater resources using TEM involves both delineation of the aquifer structure and the aquifer protection. The demands for the instrumentation and the processing algorithms are quite different for the two tasks. Structural mapping requires large depth penetration and thereby a large transmitter moment while aquifer protection mapping requires early time measurements and extremely accurate determination of the system transfer function. Furthermore, the inversion software must be capable of modelling this system transfer function.
For an airborne TEM system systematic analysis of the resolution capabilities of relatively thin low resistive layers in the upper 30 - 50 m of the subsurface has shown that the resolution is enhanced if not only the transient z-component but also the transient x-component of the induced fields in the Earth is measured and included in the inversion.
The abstract will describe and discuss three major challenges related to the inclusion of the x-component in an airborne TEM system.
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The New 4-Frequency Airborne EM System of JAC
Authors M. Kurimo, I. Suppala, H. Leväniemi and M. LahtiGeological Survey of Finland (GTK) and British Geological Survey (BGS) have jointly (JAC) constructed a new, 4-frequency EM unit installed in a fixed-wing aircraft. The system follows the technique of the previous dual frequency unit of GTK. The frequence range is now from 0,9 to 25 kHz. The four separate coils are installed inside the pods situated at the wing-tips, the separation between transmitter and receiver coils being 21.4 m. The system sensitivity is wide, covering well geological and environmental targets, and the lowest frequency enhances the depth penetration.
This paper describes the technical characteristics of the new system: specifications and noise level, and analyses the half space model responses. A test area survey was performed to find out the possibilities to utilise the data in interpretation. A 3D model based on systematic 1D inversion was compiled successfully.
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Laterally Constrained Inversion of Fixed-Wing Frequency-Domain AEM Data
Authors E. Tartaras and D. BeamishNew high-resolution airborne geophysical surveys of the UK, undertaken with the system developed under the Joint Airborne-geoscience Capability programme, established between the Geological Survey of Finland and the British Geological Survey, will provide large 4-frequency airborne electromagnetic data sets. These data sets will be used to characterise the conductivity distribution of the subsurface for environmental and exploration purposes. To invert these large data sets in a fast and robust manner we have developed “LC1DINV”, a laterally constrained one-dimensional inversion algorithm. This algorithm inverts simultaneously for all observation points along a profile and regularises the inverse problem by requiring that differences between model parameters at adjacent points be small. We use the conjugate gradient method for minimising the data misfit subject to the lateral constraints and a priori model terms. We have inverted 4-frequency data obtained over Suurpelto, a test area in southern Finland, characterised by conductive clays overlying a highly resistive granitic shield. The results show that LC1DINV can successfully locate the depth extent and variations of the clays. Comparison of these results with those obtained with two other types of inversion shows that LC1DINV produces well-defined layer boundaries and laterally smooth cross-sections.
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Laterally Constrained 1D Inversion of Airborne Electromagnetic Data
Authors M. T. Pirttijärvi and J. LerssiThe transformation of airborne electromagnetic (AEM) data into apparent resisitivity and depth is traditionally made separately for each measurement point and does not take into account the neighbouring data or model values or multiple frequencies. Laterally constrained inversion, where the model roughness is minimized together with the data error using a 1-D layered earth model, provides an easy and fast method to transform multi-frequency AEM data into a physically more meaningful form. The method is based on linearised inversion, singular value decomposition and adaptive damping together with Occam type constraining for model roughness. The new inversion method is implemented in the AEMINV program program, the graphical user interface of which is based on the DISLIN graphics library. Thus, it can be compiled and run on a variety of computer platforms without major modifications. The inversion method was found stable and computationally efficient. Because the model is one-dimensional, the results show apparent resistivity and depth although the inversion yields 2-D resistivity pseudo-sections. The AEM inversion using a two-layer model, however, gives a possibility to map separately the overburden conductance and basement resisitivity.
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Post-Tsunami Helicopter-Borne Electromagnetics Along the Coasts of Aceh, Indonesia
Authors B. Siemon, A. Steuer, U. Meyer and H. J. RehliAfter the earthquake and the tsunami-event on December 26, 2004, the Indonesian and German governments decided to set up a project that was dedicated to re-install the public life of the people in the coastal region of Aceh Province, Indonesia. As the tsunami waves caused large scale coastal salt-water intrusions and destroyed thousands of shallow drinking water wells, the focal point was the water assessment in coastal areas of Aceh about nine months after the tsunami.
The target areas are: Banda Aceh / Aceh Besar, the west coast between the towns of Calang (Aceh Jaya) and Meulaboh (Aceh Barat), and Sigli on the north-east coast. A helicopter-borne survey including five-frequency electromagnetics (HEM), magnetics and gamma-ray spectrometry was conducted by the BGR airborne group from August to October 2005. The northernmost survey area, Banda Aceh / Aceh Besar, comprises a 20 km by 50 km wide area that was surveyed within four weeks.
The HEM survey revealed coastal saltwater occurrences caused by the tsunami, deep saltwater occurrences up to several kilometres inland, and several potential fresh-water occurrences. Several requests for information on the geological/hydrogeological situation at sites for planned water wells were successfully evaluated.
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The Tellus Airborne Geophysical Survey of Northern Ireland
Authors D. Beamish and H. LeväniemiThis paper describes an airborne geophysical survey of Northern Ireland that is being conducted over a two year period. Measurements from a fixed-wing aircraft operating at 56 m include magnetic (gradiometer), radiometric and frequency-domain electromagnetic. The survey will complete over 80,000 line-km of coverage in the summer of 2006. The Phase 1 data, described here, comprise ~47,000 line-km obtained across the western and central areas of the province. The nature of acquiring geophysical data, at high resolution (200 m line spacing) in populated areas is distinct from that of other exploration contexts. The survey is being coordinated and conducted alongside a high public profile. The initial Phase 1 Tellus survey results have exceeded expectations. They have excited the interest of the planning, mineral and environmental communities.
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Semi-Automatic Approach to Deculture High Resolution Airborne Data
Authors M. Lahti, R. J. Cuss and A. DonaldThe problem of cultural noise places a severe limitation on the effectiveness of high-resolution airborne magnetic surveys. This paper presents a semi-automatic deculturing approach to enhance data from Northern Ireland surveyed in 2005. The high-resolution data (200m line spacing, 56 m flight elevation) acquired over this highly developed country is affected by cultural noise connected to various man-made sources potentially masking subtle geological information. The semi-automatic approach consists of two steps: 1) Automatic use of Pointer data to locate potential sources of cultural noise e.g. houses. 2) Manual refining of Pointer filtered data to clean remaining cultural noise and minimising geological distortion. The two-step approach allows a quick delivery of Pointer-filtered data where the level of cultural influence is significantly reduced. The final manual cleaning of remaining cultural noise is a slower process that results in high quality data with enhanced geological signature.
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Kimberlite Exploration at Low Magnetic Latitudes. Case Study at Serra da Canastra, Brazil
Authors P. T. L. Menezes and M. D. M. GarciaAirborne magnetic data from Serra da Canastra diamondiferous province is used to identify possible kimberlite signatures. Serra da Canastra is located in the central portion of the Brazilian territory within a low magnetic latitude area.
The interpretation strategy was based on the joint analysis of analytic signal and Euler deconvolution. A selected kimberlite target should have a roughly circular analytical signal anomaly coincident with a depth (0 – 200 m) and structural (N=2)
constrained Euler solution. The proposed approach led to ecognition of previously known pipes and new targets were evidenced.
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Geophysical Investigation of the Nasca Lines
Authors A. Weller, K. Hartsch, S. Rosas and G. ReppchenGeomagnetic mapping and electrical imaging are applied to investigate the soil structures in the vicinity of the geoglyphs in the Nasca desert (Peru) in comparison with areas of undisturbed soil in order to derive conclusions for a conservation of the lines. Both magnetic and geoelectrical method show significant anomalies caused by the lines but also structures of up to now unknown origin in the subsurface.
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New Results on Comparison of Different GPR Systems and Antenna Configurations at the Roman Site Carnuntum
Authors S. Seren, A. Eder-Hinterleitner, P. Melichar and W. NeubauerAbstract
Extensive test measurements were carried out over the last 3 years to evaluate different GPR systems, configurations and post-processing methods for archaeological prospection (Seren et. al. 2005). A new antenna by manufacture GSSI, the increasing of the submitted power of two antennas and improvements in filtering and visualization give new comparably results.
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2D and 3D Resistivity Tomography for the Detection of an Ancient Tunnel in Lavrion Area (Greece)
Authors C. Orfanos, G. Apostolopoulos and K. LeontarakisAt the archaeological site of Bertseko, in Laurion, the entrance of an ancient tunnel was found during archaeological excavations. After approximately 15 m the tunnel remains inaccessible – blocked with filled materials. 2d and 3d resistivity measurements were carried out in order to delineate the direction of the tunnel and to evaluate the effectiveness of resistivity imaging technique to detect such targets. Firstly, eight 2d resistivity profiles were carried out with the pole-pole array in a non-rectangular grid, in order to image a wider area, a greater depth, and to avoid several surface obstacles. Then the most promising area from the previous procedure was outlined and selected for analysis with 3d resistivity tomography (pole-dipole array) for better resolution. The field strategy adopted improves the delineation and the definition of the target and avoids the loss of valuable information with respect to the object under investigation. The integrated results from both 2d pole-pole and 3d pole-dipole profiles provide answers to archaeologist’s questions, delineate with great accuracy the direction of the tunnel and give rise to new aspects for the improvement of subsurface imaging and acquisition strategy.
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Target Detection Using Multi-Sensor Geophysical Surveys - Examples from Archaeological Surveys
Authors I. A. Hill, P. R. Fuller and C. LeechMulti-sensor survey systems can perform archaeomagnetic surveys more cost effectively than existing techniques, while simultaneously providing a denser, precise dataset with the potential for extending interpretation beyond simple imaging. The full benefits of application of existing potential field interpretation techniques to such data is yet to be realised.
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Combined Two-Dimensional Inversion of Four- and Three-Electrode Arrays Resistivity Data
More LessIn this paper, I suggest to use combined inversion of three- and four-electrode arrays data. Two-dimensional (2D) inversion results of Wenner (W), Schlumberger (S), Dipol-Dipol (DD), left- and right side pole-dipole (PD-L and PD-R) synthetic data and their combined inversion results are compared according to resolution and accuracy of the inverted models. Comparison results show that combined inversion of any two arrays data (eg. W+S, W+DD, PD-L+DD) sets give better resolution than the individual inversion of each corresponding arrays data set. However, inversion of PD-L, PD-R and and DD data sets jointly give better accuracy than the single use of any of these electrode arrays and combined inversion of any two electrode arrays.
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EM Induction Frequency Sounding - Estimation of Penetration Depth
Authors E. V. Balkov, M. I. Epov, A. K. Mastein and Y. A. MansteinOne of the most important characteristics of any geophysical instrumentation is the penetration depth. Present paper is devoted to description of the approach for estimation of the penetration depth for frequency sounding method. The approach takes into account the accuracy of the instrumentation and geoelectrical model as well. It is more accurate than traditional one that based on skin depth estimation. The investigations are carried out for two types of target objects for EMI frequency sounding device EMS developed by authors.
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Seismic – Acoustic Research on Identification of Some Archeological Buildings in the Submersible Zones
By A. SorinIn Romania, geophysical methods are normally used to estimate the distribution of cultural relics, before digging. Objects of archeological interest are usually located within a few meters of the surface.
Seismic –acoustic research was made with an specific equipment (“X-Star Full Spectrum Sub-bottom Profiler”)which works with frequency modulation in the range of 2-16 Hz. The vertical resolution of this system is better than 1 decimeter
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Marine Resistivity Measurements for the Foundation of Lavrion Old (1888) French Bridge
Authors G. Apostolopoulos, G. Amolochitis and S. StamatakiMarine resistivity measurements can give valuable information for the sedimentation under the sea bottom if the latter is not too deep. The Schlumberger configuration is preferred with its more efficient signal-to-noise ratio. Pole-Pole can give information which is less accurate in horizontal layering but offers deeper detection and possible positioning of lateral inhomogeneities in the event of great resistivity contrasts.
The old (1888) iron “French Bridge” in Lavrion Port suffers from corrosion by seawater and displacements and needs protection with the plan all to be mounted in a new substructure with the less visible changes. The marine resistivity survey gives valuable information to the engineer like where the old foundation lays, where to position the geotechnical drillings for additional information and outlines the environment of the structure to be built for the protection of the bridge.
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Gravity Survey in the Study of Vulnerability of Old Town
More LessGravity prospecting in urban areas can make possible to define vulnerability maps of territory, which are very useful in prevision and prevention of buildings crash risk especially in old town.
In this paper we show the results of two studies carried out to determine the shallow subsoil properties of two historical sites in Sardinia (Italy) by means the implementation of densities micro zoning map.
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Magnetic Properties of Archaeological Building Materials from Egypt
Authors A. Weller and A. El-BassionyThe magnetic properties of the building material are compared with the parameters of the embedding soil. The X-ray diffraction measurements performed on the selected samples had shown a higher magnetite content in the mud-brick samples compared to the low content in the soil samples. The measurements of the magnetic volume susceptibility for the selected samples had shown a measurable contrast between the mud-brick samples and the soil samples in Saqqara and Qantir.
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First Multi Beam and High Resolution Reflection Seismics Survey on the Tiber River Lower Course
Authors L. Orlando, M. Bernabini, A. Bosman, F. Chiocci and L. MacelloniAn integrated geophysical survey was carried out with the aim to investigate hydrodynamics, sedimentology, stratigraphy, pollution, archaeology and navigability of Tiber River (central Italy) lower course. Multibeam, side scan sonar and seismic reflection (Sparker and Pinger sources) data were acquired for a length of 80 km. The accurate location of the survey and the special processing used for the multibeam data gave us a digital elevation model of the river bed with a maximum resolution (in places) of 0.2 m.
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Focused Inversion of Gradient Magnetic Data from Saqqara North Cemetery, Egypt
Authors A. Weller and A. El-BassionyInversion of magnetic gradient data for archaeological purposes is performed using a focusing functional. The focused inversion of real magnetic gradient data from Saqqara proves to be successful by using the Minimum Gradient Support as a focusing tool. Information about the susceptibility contrast between the archaeological structures and the surrounding soil is found to be critical for the reliability of the inversion.
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First Results from Thermal Tomography in the Study of Wall Structures and Archaeological Layers
Authors C. Piga, G. Ranieri and R. DeianaMasonry walls, especially those of historic buildings, are often difficult to investigate. The current approach consists of analysing small portions of the structure. The techniques employed are often invasive and in any case incapable of providing a complete picture of the variety of problems to be solved. The aim of this work is to test the effectiveness and feasibility of thermal methods suitable for assessing the conditions of masonry.
Different physical models that mimics real conditions has been designed for this purpose. A number of anomalous elements were introduced in the models, to simulate the heterogeneity of the structure.
The same method has been applied in the analysis of thermal imaging to try to reconstruct buried walls. A quasi-real scale model has been constructed also in this case.
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High-Resolution Seismic Reflection to Identify Areas with Subsidence Potential beneath U.S. 50 Highway in Eastern Reno County, Kansas
By R. D. MillerHigh-resolution seismic reflections were used to map the upper 200 m along an approximately 22 km stretch of U.S. 50 highway in Reno County, Kansas, where natural and anthropogenic salt dissolution is known to threaten ground stability. Surface subsidence in this part of Kansas can range from gradual (cm per year) to catastrophic (meter per second), representing a significant risk to public safety. The high signal-to-noise ratio and resolution of these seismic reflection data allowed detection, delineation, and evaluation of several abnormalities in the rock salt layer and overlying Permian sediments. Locations were identified where failure and associated episodes of material collapse into voids left after periodic and localized leaching of the 125 m deep, 40 m thick Permian Hutchinson Salt member were evident. Anhydrite and shale layers several meters thick within the salt are uniquely distinguishable and appear continuous for distances of several kilometers. Over a dozen unique features potentially related to subsidence risk were identified.
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Applicability and Limitations of Impact Echo in Bridge Deck Condition Monitoring
Authors P. Shokouhi, N. Gucunski and A. MaherA010 Applicability and Limitations of Impact Echo in Bridge Deck Condition Monitoring P. Shokouhi * (The University of Texas at El Paso (UTEP)) N. Gucunski (Rutgers University) & A. Maher (Rutgers University) SUMMARY Near Surface 2006 — Helsinki Finland 4 - 6 September 2006 One of the most common problems in concrete bridge decks is a corrosion-induced deck delamination. A number of nondestructive testing (NDT) techniques have been utilized to map delaminated zones in concrete bridge decks (Scott et al. 2003). Impact echo (IE) an ultrasonic seismic method has proved successful in detection of delaminations in concrete bridge decks at
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Target Prioritization in TEM Surveys for Sub-Surface UXO Investigations Using Response Amplitude, Decay Curve Slope, Signal to Noise Ratio, and Spatia
Authors D. Hall, S. MacInnes and J. DickersonTo reduce the number of false-positives in identifying UXOs and related debris for an ongoing Site Investigation, a target prioritization routine was developed based on the likelihood of a metallic source. Numerous parameters were evaluated statistically using over 2000 existing intrusive investigation results. Those showing the best ability to discriminate between metallic and non-metallic sources were incorporated into the scheme, including peak amplitude response, power-law decay slope, signal to noise ratio, and spatial match filter response. Parameters were assigned increasing numeric values based on likelihood of a metallic source, and summed to produce a target rank. Subsequent application to over 6000 new intrusive investigations revealed that the highest ranked 25% of targets were nearly five times more likely to yield a metallic source when investigated than the lowest ranked 25%. These results show that target selection routines in UXO investigations would benefit from using more parameters than just magnitude response, allowing more aggressive target identification and reduced costs by lowering the number of false positives that are investigated. Proposed refinements may further increase predictive capabilities.
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Advancements in 3D Subsurface Modeling Using Refraction Data
More LessRecent advancements in refraction data analysis have increased the ability to image geologic features, provide better presentation, and offer additional benefits from refraction results. The end product from a refraction investigation can be either a 2D image or a full 3D model, as presented here. Full 3D earth models can be used for many purposes after the geophysical survey is complete. That is, subsurface physical property data in 3D model space permits site assessment from a new perspective. For example: structural loading, seismic loading, or construction excavation requirements can be evaluated using 3D model results.
This paper presents an advanced approach to refraction data processing, presentation, and visualization, using the "Geostructural Analysis Package" (GAP). GAP incorporates several numerical modeling processes: discrete element method, particle flow code, finite differencing, and the material point method. These four numerical modeling methods have been combined and optimized for seismic applications. GAP is an innovative tool that allows better data analysis and presentation that can be used to produce 3D volumetric models for further site analysis. A set of model results from a case history are presented to demonstrate the benefit of processing refraction data using this new approach. GAP represents the newest advancement in subsurface modeling using engineering-scale seismic data.
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Using Borehole Radar to Image a Pothole in the Merensky Platinum Orebody
By P. du PisaniThe Merensky orebody (reef) in South Africa is one of the world's most important platinum resources. Potholes (slumps) in the Merensky Reef lead to significant mining losses. This paper describes for the first time how borehole radar can be used to image potholes in the Merensky Reef. Three dimensional visualization of the borehole radar data is used to delineate these structures and to improve the geological model ahead of mining.
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Crosshole Radiowave Imaging (RIM) at Eurajoki Olkiluoto, Finland
Authors E. Heikkinen, A. Korpisalo, T. Jokinen, T. Zhienbaev, A. Shuval-Sergeev, N. Popov and T. AhokasCrosshole radiowave imaging survey was conducted late in 2005 at Eurajoki Olkiluoto in Finland. Work belongs to site characterization process for spent nuclear fuel disposal into Finnish bedrock, which is run by Posiva Ltd. Crosshole radiowave imaging (RIM) was performed as a joint operation of Geological Survey of Finland and FGUNPP Geologorazvedka from Russia, using FARA-MCH tool. Pöyry Environment reviewed the results and compared the material to existing background information. The field work performed successfully between two borehole pairs at surface collar distances of 250-300 m. Recorded amplitude data were processed to resistivity images using crosshole tomographic reconstruction. The results prove that the method can be used in 100-400 m scale between boreholes to delineate and follow sulphide-bearing horizons, which are controlled by ductile and brittle geological processes in the site. Obtained results indicate that the location and geometry of resistive and conductive units in bedrock correlate rather well with data from geological mapping and geophysical logging from boreholes. The obtained projected geometry correlates also rather well with one subset of seismic 3-D reflections, and with geophysical electrical and electromagnetic results from the site.
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Monitoring Leachate Levels in Landfill Sites Using Automated Time-Lapse Electrical Resistivity Tomography (ALERT)
Authors O. Kuras, R. D. Ogilvy, J. Pritchard, P. I. Meldrum, J. E. Chambers, P. B. Wilkinson and D. LalaAutomated time-Lapse Electrical Resistivity Tomography (ALERT) is a concept that aims to provide tomographic images of landfill sites at regular intervals or on demand. The British Geological Survey has developed an ALERT system specifically for the deployment on landfills and has undertaken a research programme focussing on leachate monitoring techniques based on ERT. Simple scale experiments demonstrate the capabilities of the ALERT technique to accurately monitor liquid levels in a porous medium between wells. ALERT data provide additional control and information about inter-well properties and processes, thus contributing to an improved understanding of landfill hydrology.
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Combined Monte Carlo and Gradient Procedures Used to Optimized Solution of the Comprehensive Interpretation of Well Logs
Authors U. Woznicka, J. A. Jarzyna and A. CichyThe comprehensive interpretation of well logs is a mathematical task on a solution of a set of equations. The number of equations is determined by the number of the logs and conditions determining boundary and geological requirements. The common methods (e.g. gradient method, GRAD) are limited to cases when one global minimum occurs in the examined range of variables. Only Monte Carlo method, MC, which consists on numerous solutions obtained by random sampling of possible variables can find the global minimum of the function when other local minima are present. The MC method is time consuming, especially, in application to the well logging interpretation when enormous number of points along the depth profile should be calculated. We propose a complex method of solution combining both MC and GRAD methods. The method has been applied to the Interlog application in the GeoWin system for processing and interpretation of well logs and the field example is presented in the paper. The results confirm that the combined MC-GRAD solutions can give more information than the simple GRAD solution. Presented approach can be treated as an additional tool, especially dedicated for log analysts, for solving a set of logs difficult to interpretation.
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Correctness of the Comprehensive Interpretation of Well Logs Regarding to Geological Conditions and Calculation Methods
Authors J. A. Jarzyna and U. WoznickaA great amount of unknowns resulting from a complicated lithology and from many logs run in a borehole causes that a set of equations in the comprehensive interpretation is great and sometimes indeterminate. In real geological conditions we sometimes get solutions that are different from laboratory estimates of porosity or other parameters. Discrepancies between lab measurements and geological data on one side and results of the comprehensive interpretation of well logs on the other side were the direct reason to examine the method of optimization adopted for solution. The proposed mathematical solution in the form of combined Monte Carlo and Gradient methods works as a tool assuring interpreter that the solution is correct. The general results obtained from Interlog-GeoWin were compared to ULTRA and Petrolog systems and the agreement was satisfied.
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Geoelectric Survey of Hydraulic Experiments at the German Deep Drilling Site KTB at KM-Scale
Authors D. Laass, E. Danckwardt, G. Petzold and R. VoigtOne of the major findings of the German Continental Deep Drilling Program was the evidence of free fluids down to a depth of nearly ten kilometres. Furthermore, a series of hydraulic experiments in the pilot borehole focused on the characterization of transport processes of fluids in crystalline crust were carried out. Because of the significant differences of the electrical conductivity between the investigated fluids (natural free saline fluids and artificially inserted fresh water), geoelectrical methods are qualified for a successful detection in the kilometre scale. The Fluid Pump Test (2002/03 – production of about 22,300 cubic metres of saline formation fluids) or the Fluid Injection Test (2004/05 – injection of about 84,600 cubic metres of fresh water) clearly indicate a decreasing respectively an increasing in electrical resistivity. Inhole-logs in the open hole section (3,850 m – 4,000 m) could give a first indication (very closed to the pilot borehole) of consequence of the producing of saline formation fluids respectively of the fluid inflow displaced by injected fresh water last year. Also in greater distances to the pilot borehole, the time course of the decreasing (2003) and of the increasing of the apparent resistivity (2005) could be measured by geoelectrical combined surface-hole-experiments.
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Active and Passive Seismic at the Unstable Rock Slide of Åknes (Norway)
Authors I. Lecomte, M. Dietrich, M. Roth, O. Méric, C. Delarue and J. S. RonningThe unstable rock slope of Åknes, Western Norway, has been recently identified as a potential threat for people and infrastructure in the inner Storfjord system. The unstable part has an estimated total volume of 30 to 40 million m3 based on recent geological and geophysical mapping. It is moving with a mean rate of about 4 cm/year, but with a rate as high as 15 cm/year in some parts. Such rock slide, when hitting the water in the fjord, could generate a flood wave (tsunami) with severe consequences, as experienced in the past by the local population. During the 2005 field campaign, three seismic measurements/installations were made: 1) a refraction seismic experiment in the center of the slope to map a potential sliding plane, 2) a temporary installation of 10 seismic mini-arrays of 9 channels each for passive monitoring of seismic events and noise measurements, and 3) a permanent installation of a smaller-scale seismic network for long-time monitoring of the site with real-time data access.
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2D Inversion of the First Field Data of a New Radiomagnetotelluric Device in the Frequency Range from 10 kHz to 1 MHz
Authors B. Tezkan, A. Saraev, V. Shuman, P. Georgescu and N. B. ChristensenIn the project RMT-monitoring funded by the European Union, a new radiomagnetotelluric device has been developed which can record time series of electric and magnetic fields in a wide frequency range from 10 kHz to 1 MHz. Transfer functions are determined by spectral analysis using a newly developed processing software. A workbench program has been developed in which the transfer functions are interpreted with conductivity models based on new inversion algorithms.
The developed tensor radiomagnetotelluric device was tested in the laboratory and then applied successfully in the field to several environmental and groundwater problems in Russia, Ukraine, Denmark, Romania, and Germany.
The observed transfer functions in the field are the first ones worldwide in the frequency above 250 kHz which leads to a better resolution of the shallow structure. The new device enables quick and efficient mapping of polluted areas and supports tensor measurements allowing a 2D or 3D interpretation of the field data.
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High-Resolution S-Wave Reflection Survey in Urban Areas Using a Woven Belt Type Land Streamer
By T. InazakiHigh-resolution S-wave reflection surveying has been successfully conducted on the paved surface in large urban areas by means of a Land Streamer, which was originally developed by the author. The Land Streamer is featured by non-stretch woven belt which acts as a towing member but also as a module to mount geophone units. Whereas each geophone unit is placed on the paved surface through a metallic baseplate instead of firm planting to the ground, the Land Streamer tool provides comparatively clean data even on the pavement resistant to traffic noises. Consequently, the tool is capable of expanding the opportunity of S-wave reflection survey in the urban areas where the surface is mostly paved and cultural noise is considerably high. A series of high-resolution S-wave reflection surveys conducted at paved surfaces in large urban areas successfully delineated detailed structures of the surficial layers shallower than 60 m, and proved the wide availability of the tool to engineering, environmental applications, and earthquake disaster prevention.
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Geophysical Surveys in Prospecting of the Granite Host Rock of a Planned Radioactive Waste Disposal Site in Hungary
Authors A. Gulyás, L. Vértesy, A. Madarasi and J. KissA great amount of geophysical work was involved in the programme of final disposal of low- and intermediate-level radioactive waste in Hungary. Among others wide spectrum of geoelectric-electromagnetic methods were applied from the early stages of the project. In this poster we present the results obtained in prospecting the internal structure of the granite body either outcropping in the deep valleys, or buried under 30-60 m thick Quaternary sediments. VLF, Slingram-EM and high-frequency magnetotelluric surveys were performed to extend the detailed but were scarce information of the borehole geoelectric measurements and obtain a spatial view about the resistiv-ity distribution of the granite body, to delineate conductive zones of as possible tectonic zones, blocks etc. Regional gravity, magnetic and magnetotelluric studies were performed to delineate the granite body and for marking out regional structural lines.
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Geophysics in Constructing the Geological Model of Olkiluoto
Authors M. Paananen, T. Lehtonen and T. AhokasThe paper presents some geophysical results that have been useful in constructing the geological model of Olkiluoto. Interpretation of wide-band EM soundings, HLEM, charged potential and single-hole measurements revealed several sulphide-rich conductive zones. These zones often appear to control also the brittle deformation zones.
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Investigation of the EDZ Using High-Resolution GPR with Modulating Frequency
Authors Y. Suyama, M. Toida, K. Masumoto and S. TakeuchiThe EDZ (Excavation Damage or Disturbed Zone) is considered to be a fast transport pathway and provides a boundary condition for transport in bentonite buffers in groundwater scenarios for the geological disposal of radioactive waste. The distribution of the EDZ obtained through in-situ measurements, is important for safety assessments. High-resolution Ground Penetrating Radar (HGPR) with modulating frequency functions as a wide-angle measurement technique. It is one of the advanced techniques that can be used to investigate the EDZ of a radioactive waste disposal tunnel. Verification tests of the HGPR were carried out using a model and in-situ condition in the gallery of Tono mine in central Japan, where comprehensive techniques for investigation, analysis and assessment of the deep geological environment had been developed. In summary, it is shown that the investigation technique was able to detect underground structures with a resolution of less than 0.1m.
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Quality Control and Validation of Detailed Logging Data from Slim Boreholes
Authors E. J. Heikkinen, T. Lehtimäki and T. AhokasBorehole geophysical logging data can provide a direct link between results of large scale geophysical methods and core sample analysis. Also the in situ measurement data will focus the measured response to actual location and to the specific lithology, or overprint of alteration or deformation. This way the logging data serves as unique means to explain and control the results from a large variety of different views and scales, ranging from airborne and ground level mapping to seismic reflection surveys and electromagnetic soundings. The actual parameter information used in model interpretations is only accessible by borehole methods. Further to this, the comparison of different parameters and geological background information can lead to better understanding of processes involved in the subsurface. The presentation deals with practices of quality control and validation of slim hole logging data using different geological and sampling control data. The work includes the sample controlled conversion of measurement data to meaningful physical properties, depth matching of the data to a very good location accuracy, and computing of derivative parameters. Example data set has been produced in site characterization project of Posiva for spent nuclear fuel geological disposal in Finland.
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Geophysical Studies at the Suurpelto Construction Area
Authors T. K. T. Huotari, I. A. Suppala, H. T. Vanhala, T. Valjus, A. E. K. Ojala and J. P. PalmuIn this project geophysical, geological and geotechnical data has been collected from the Suurpelto construction site. The Suurpelto site is locating in the City of Espoo in southern Finland. The main aim of the studies is to investigate the geological history of the site and also use the data for constructional purposes. Airborne, ground as well as laboratory geophysical techniques have been used during this project. AEM measurement is fast method for outlining the highly conductive clay areas. 1D-inversion of 4-frequency AEM data was done using constrains from other Suurpelto datasets. More detailed conductivity structures were achieved from inverted ERT and ground EM data. The laboratory resistivity measurements revealed the highly conductive clays and less conductive gyttja clay sections from the drillhole samples. These sediments are the most difficult for constructional purposes.
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Municipal Waste Dump Geophysical Investigation
Authors M. Bavusi, E. Rizzo, V. Lapenna and S. PiscitelliThe proposed work is about the geophysical survey applied in three Italian waste dump (Savoia di Lucania and Pallareta in Basilicata region, Novellara in Emilia-Romagna Region). The Basilicata landfill are provided of HDPI liner while in the other one a compacted clay stratum is present at the bottom. In each site we carried out active and passive geophysical techniques: electrical resistivity tomographies (ERT), induced polarisation (IP), self potential (SP) and magnetic methods. The ERTs, inverted by using the Res2DInv software, provided information about the geometry of the waste basins where an HDPI liner is present. The IP results show high chargeability nuclei into the refuse indicated the metallic garbage cumuli. The SP signal is negative into the basins and slightly positive in the surrounding area. Moreover, the SP data show strong spikes at the borders of the basins. The SP data of Novellara waste dump (clay stratum coating) show positive values inside the basins. The magnetic method evaluates the dispersion of metallic garbage close to the waste dumps. Finally, the different acquired data allow to sketch the lateral and buried geometry of the waste basins and to infer their leachate-tight, at least where the HDPI liner is present.
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Electrical Resistivity Tomography (ERT) as a Tool for Monitoring Moisture Dy-namics in Soil Filled Containers
Authors O. Mohnke, K. Prokoph and U. YaramanciThe geophysical subproject of the multidisciplinary research group INTERURBAN (DFG 409), formed to investigate the water- and solutedynamics at urban locations with particular attention to spatial heterogeneity and biological transformation processes in the unsaturated zone, aims at the development of a noninvasive conception to map the water distribution and dynamics at small scales. Hydrophobic areas have a major impact for the transport of water and matter through preferential flow in soils. Soil animals, e.g. earthworms, modulate the composition of soil solutions in time and have a species-specific impact on the release pattern of nutrients and pollutants from plants and soil. To assess the possibilities and limitations of 3D ERT in the cm-dm range to monitor the long and short term dynamics of water repellent zones in the presence of soil fauna activity a feasibility study was carried out using soil filled mesocosms (MC) inoculated with earthworms. Results indicate a sufficiently high spatial resolution and sensitivity to reliably monitor in-situ changes of soil moisture on the relevant scales, Earthworm inoculated MC indicate a more homogenous decrease of resistivity throughout the MC whereas in the reference MC the changes are confined to the area below the irrigation zone.
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Criteria of Gas Hazard Assessment in the Areas of Closed Mines of the Wałbrzych Coal District, Southwestern Poland
Authors H. Sechman, M. Dzieniewicz, M. J. Kotarba and A. KorusThe mine closures in the Wałbrzych Coal District, and particularly the cessation of drainage, resulted in the recovery of the groundwater table in Carboniferous multi-aquifer system which, in turn, intensified the flux of coalbed gases into the near-surface zone.
Surface geochemical surveys run in the years 1997-2001 and 2004-2005 in the Wałbrzych District demonstrated general increase in methane (up to maximum 49.6 vol. %) and carbon dioxide (up to maximum 17.4 vol. %) concentrations in soil gas samples collected at depth down to 2 m. The forced influx of methane and carbon dioxide into the near-surface zone measured with the modified, static chmaber method was up to 620 dm3/m2h i 330 dm3/m2h, respectively. Studies on coalbed methane and carbon dioxide flux to the near-surface zone supported by geochemical, geological and mining-engineering criteria, enabled the determination of a three-degree scale of near-surface gas hazard. Recognition of gas hazard mechanisms in the near-surface zone of post-mining areas is significant for restructuring of the mining industry in Poland and prediction of gas hazard caused by coal mine closure.
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Shallow 2D and 3D Seismic Tomography on an Unstable Mountain Slope
Authors H. Horstmeyer, B. Heincke, H. R. Maurer, A. G. Green, H. Willenberg and T. SpillmannAs transport routes and population centres in mountainous areas expand, risks associated with rockfalls grow at an alarming rate. As a consequence, there is an urgent need to delineate mountain slopes susceptible to catastrophic collapse in a safe and non-invasive manner. For this purpose, we have developed a 3-D tomographic seismic refraction technique and applied it to an unstable alpine mountain slope, a significant segment of which is moving at 0.01-0.02 m/year. First-arrivals recorded across the exposed gneissic rock mass have extraordinarily low apparent velocities. Inversion of their traveltimes produces a 3 D tomogram that reveals the presence of a huge volume of very low quality rock with ultra-low to very low P-wave velocities of 500-2700 m/s. Such low values likely result from the ubiquitous presence of dry cracks, fracture zones and faults at a wide variety of scales. They extend to more than 35 m depth over a 200 x 150 m area that encompasses the mobile segment of the slope and a large part of the adjacent stationary rock mass. Although hazards related to the mobile segment have been recognized since the last major rockslides in 1991, those related to the adjacent low quality stationary rock mass have not.
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Geophysical Characterization of the V.F. De Xira-Lisbon Fault System, Portugal
Authors J. P. Carvalho, T. Rabeh, F. Carrilho, J. Cabral and M. MirandaThe Lower Tagus Valley (LTV) has suffered large historical seismic events which originated important material damages and loss of lives. Besides the plate boundary activity, the study area has been recently considered an important seismic source to the seismic hazard of the LTV area. The V.F. Xira-Lisbon, was selected as a priority target for investigation, based upon its near-surface expression on the oil-industry seismic reflection profiles, its significance in the Cenozoic basin structural pattern and the apparent relationship to the regional seismicity, its closeness to Lisbon and its seismic potential. Using aeromagnetic, seismic reflection and seismicity along with geological data, new insights into this fault system have been established. Its deep rooting into the paleozoic basement, the unknown northward and southward prolongations and its connection to relocated seismic epicentres are a few examples. The expected total length of the fault will be more than 90km, increasing the estimated magnitude of the maximum credible earthquake and the seismic hazard of Lisbon significantly. Acquisition of high resolution seismic data will help confirming estimated vertical offsets of the fault in Quaternary times, allowing an improved assessment of the seismic potential of the V.F Xira-Lisbon fault system.
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The Effect of Vibrations from Wind Turbines on Seismically Sensitive Critical Installations
Authors P. Styles, R. England, I. G. Stimpson, S. M. Toon, D. Bowers and M. HayesIn order to meet Kyoto targets, the UK has set the challenge of reducing carbon dioxide emissions by 60% by 2050. Renewable energy, especially wind power, will be an important contributor with a target of 10% of energy from renewables by 2010. The Southern Uplands of Scotland offer a prime wind resource because of a large region of high topography, appropriate wind conditions and 2 Gigawatt of generating capacity is planned. However, the United Kingdom Comprehensive Test Ban Treaty seismic monitoring site (CBTB) is situated at Eskdalemuir in the Scottish Borders. Concern was expressed that vibration from wind farm developments might prejudice the detection capability of this facility. The Ministry of Defence placed a precautionary blanket objection to wind farm developments within 80 km of Eskdalemuir. In order to assess vibration levels a 10 station broadband seismic network and a 4 station infrasound network were established for 6 months at distances out to 20 km from a 26 turbine wind farm situated on very similar geology and topography to Eskdalemuir and the planned wind farm developments. The study has permitted the identification of the principal propagation mode for ground vibrations from wind turbines and enabled their characterisation.
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Progress and Problems in Near Surface Gravity
By S. EloThe paper describes three applications of near-surface gravity: (i) an attempt to estimate in-situ weathering of a dimension stone deposit, (ii) improvements in 3D gravity inversion of overburden thickness and (iii) new results of monitoring the subsidence and density of a landfill. The examples demonstrate the state-of-art in near-surface gravity: mean error of gravity measurements ±0.010 mGal or better, advanced modelling techniques and a demand for highly accurate terrain models.
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MRS Study of Groundwater Flow and its Relationship with Sinkholes Development in the Dead Sea Coastal Area
Authors A. Legchenko, M. Ezersky, C. Camerlynck, A. Al-Zoubi and K. ChalikakisA geophysical survey was performed in the Dead Sea costal area of Israel and Jordan. It was carried out in framework of NATO project SfP N° 981128. The goal was trying to understand the processes of sinkholes development caused by the fresh water flow towards the Dead Sea. For that, the Magnetic Resonance Sounding (MRS) method was applied.
Our results reveal a very heterogeneous aquifer with two orders of the transmissivity variation around the investigated area that could be divided into two parts. In the northern part, the aquifer allows intensive circulation of groundwater. It was identified as a karst aquifer thus validating the salt dissolution mechanism of sinkholes development known from the literature. In the southern part, the subsurface is composed of compact clay-type material with low hydraulic conductivity. Development of the karst was not observed.
Basing on MRS results, we suggest that development of the karst in the northern part have changed the local hydraulic gradient and consequently, smaller amount of water is flowing towards the south. Change of the water path may explain why these sinkholes are slowing down their development while in the northern part they continue growing up.
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Using ERT and CPTR in Geotechnical Investigations
Authors I. M. Vaara and A. MattssonIn this study the applicability of the electric resistivity tomography and the CPTR-bore logging method was studied in geotechnical soil investigations. Also the induced polarisation effect is discussed. Usually the geotechnical soil investigations utilize lots of drillings and the number of them is set in advance. The aim of this study was to show that by using the ERT-measurements the number of the bore loggings could be diminished and they could also be situated in the most relevant places.
The most important conclusions drawn from the results are that the ERT-method is very useful in geotechnical soil investigations. Also the IP effect gives information on the quality of different soil types. The CPTR gives the point like value of the resistivity of the ground which is important in evaluating the corrosion risk for steel structures. Together with ERT –measurements the CPTR loggings can be very informative about the depth of peat and clay layers.
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GPR Investigations in Coastal Sambaqui Archaeological Site in Santa Catarina, South Brazil - Preliminary Results
Authors J. L. Porsani, S. I. Rodrigues, P. C. F. Giannini and P. A. D. DeblasisIn this paper, preliminary results from GPR Investigations are presented, accomplished at Encantada-III coastal sambaqui, in Jaguaruna, Santa Catarina state, South Brazil. This sambaqui consists of sand mound covered by decimetre black sediment layer with carbonatic shells, dated of 4970-4830 years AP (Beta 189713), and could be constructed by societies that inhabited this region during pre-colonial period. GPR profiles show anomalous reflectors, which can be related to targets of archaeological interest. Numeric modelling studies, simulating propagation of electromagnetic wave at coastal sambaquis, presented good accordance with real data obtained in the field. Synthetic model was construct in order to evaluate the performance of GPR method in relation to materials found in that environment, besides helping interpretations of real results. So, the results function as reference to direct excavation activities in burying quest that will be the next step of this searching.
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Mapping Ice-Bonded Permafrost with Electrical Methods in Sisimiut, West Greenland
More LessPermafrost delineation and thickness determination is of great importance in engineering related projects in arctic areas.
In this paper, 2D geoelectrical measurements are applied and evaluated for permafrost mapping in an area in West Greenland.
Multi-electrode resistivity profiles (MEP) have been collected and are compared with borehole information.
It is shown that the permafrost thickness in this case is grossly overestimated by a factor of two to three. The difference between the inverted 2D resistivity sections and the borehole information is explained by macro-anisotropy due to the presence of horizontal ice-lenses in the frozen clay deposits.
It is concluded that where the resistivity method perform well for lateral permafrost mapping, great care should be taken in evaluating permafrost thickness based on 2D resistivity profiles alone. Additional information from boreholes or other geophysical techniques is needed for correct interpretation.
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Electromagnetical Investigation of a Dune Valley in Oostduinkerke (Belgium)
Authors K. Martens and K. WalraevensAlong several profiles, an electromagnetic prospection (horizontal dipole mode) has been performed in a draining dune river valley. Different factors are at the basis of the variability of the apparent ground conductivity: groundwater flow, lithology, topography and sea spray.
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Spatializing Water Tension in Heterogeneous Sandy Soils with Surface ERT During Rain-Evaporation Cycles
Authors M. Descloitres, O. Ribolzi, Y. Le Troquer and J. P. ThiebauxDunes are almost exclusively the support of herbaceous vegetation in the Sahel. There is a need to better understand their hydrological functioning during the rain cycles. This paper evaluates the interest of using surface Electrical Resistivity Tomography (ERT) for spatializing water tension. A representative micro-dune has been studied in the field using 3 artificial rain cycles. ERT inversion was corrected from temperature and optimized using a comparison with in-situ resistivity measurements, giving an estimate of ERT accuracy. A porosity-independent relation between resistivity ratio (final / initial state) and water tension differences (final – initial) is found for the first 2 rains with demineralised water. When using salted tracer (third rain), the slope of this relationship changes. We found that the ERT image smoothing doesn’t allow us to track accurately some localized phenomena. Moreover, the interest of spatializating water tension using non-destructive surface ERT is limited because inversion has to be constrained using a priori information given by destructive measurements at depth. We conclude that surface ERT imaging reconstruction needs improvements to reliably spatialize resistivity variations linked with processes in heterogeneous sandy soils.
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Non Destructive ERT Survey at the South Wall of Akropolis of Athens, Greece
Authors P. Tsourlos, G. N. Tsokas, N. Papadopoulos, V. Manidaki, M. Ioannidou and A. SarrisResistivity tomogaphies were carried out on the south wall of the Akropolis in Athens. The electrode lines were deployed vertically and horizontally on the wall plus on the top of the hill. Further, some tomogarphies were conducted encompassing the volume of the material between the wall and open pits (baulks). Bentonite mud was employed instead of metal stakes in order to comply to the non destructive character of the whole operation. An existing algorithm was modified to cope with the particular requirements of this unconventional survey. Areas of increased moisture were revealed in the prospected volumes. Further, the thickness of the wall was assessed in particular locations. It is showed that this type of surveys are perfectly feasible and can produce results informative and elucidating images to archaeologists and restoration scientists.
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Estimation of Subsurface Pipe Radius Using Ground Penetrating Radar Data
Authors A. Dolgiy, A. A. Dolgiy and V. ZolotarevIn this paper, we consider three techniques for radius estimation of subsurface metal pipes using Ground Penetrating Radar (GPR) data. The GPR acquisition system was Zond-12c with shielded antennae of frequency 900 MHz. Seven buried pipes from 0.01 m up to 0.36 m in radius were studied. For GPR data processing were applied the methods of weighted least squares, the recursive Kalman filter and the maximum likelihood. The recursive Kalman filter method shows the minimum value for average error of 3.08% for radius estimation. The maximum value for average error of 13.3% for all considered methods corresponds to pipe with the minimum radius of 0.01 m. For pipes with radius from 0.024 m up to 0.36 m the average errors are varied in the range of 0.74 - 5.17%. We studied the influences of random errors of measurements on the accuracy of radius estimations for the metal pipe of radius 0.36 m. We also inspected the influence of the angle between the antennae scan direction vector and the pipe axis on the accuracy of radius estimations by recursive Kalman filter technique for all studied pipes.
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The Effect of Surface Polarization in Galvanic Modelling
More LessSurface integral equation solution, where surface polarization is included to model galvanic measurements. The model and the corresponding software are best suited in forward modelling. The problem is formulated for finite conductivity contrast.
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Applying Syscal-Pro Resistivity-Meter in Fast Measurement Mode to Soil Investigation
Authors A. A. Bobachev, I. Cousin, H. Robain and A. A. GorbunovThe paper deals with geophysical aspects of studying soils by resistivity method.
Electrical resistivity measurements have recently come to use in field experiments to explore soil structure. Rapid resistivity-meters like 10-chanels Syscal-Pro (Iris Instruments) are able to record up to six hundreds of measurements per minute. Such units open new possibilities for soil investigation.
The authors fitted Syscal-Pro for soil exploration. All 96 electrodes were fixed in 8 plastic slabs with 10 cm inter-electrode spacing. Fast and exact setup of array is thus provided. Three different types of measurements were implemented: 2D and 3D resistivity imaging and the infiltration monitoring. Some valuable geophysical results are discussed.
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Surface-to-Hole FEM Modeling in Conductive Half Space
More LessThis work presents several modeling examples for frequency-domain electromagnetic (FEM) system. Measuring system consists of a large surface transmitter loop and a borehole receiver probe. In EM methods a typical target is a conductive body in a resistive host. The primary magnetic field at the borehole receiver stations is first calculated theoretically and then reduced from the measured response. This residual field is used for the interpretation. This method provides also visual interpretation, but if the surrounding rock is not very resistive, the method leads to difficulties and may cause false results. In conductive subsurface, currents inside the conductive body are induced, instead of a magnetic dipole, by more complicated electromagnetic field, known as the "smoken ring" in the time-domain EM method. On the contrary, when a conductive body is lying in a resistive environment, the induced currents are flowing and decaying inside the body. That gives information about a shape of the body, but in the conductive environment the current flows freer trough the interface of the body. Interaction between the body and the host increases and the shape of the body becomes more difficult to interpret from the data.
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DC, EM and Direct Push Resistivity Surveys for Pollution Pathways Delineation – A Case Study from Poland
By M. RudzkiThe paper presents the results of the geophysical multi-method survey that was carried out in order to map the contamination migration pathways in the vicinity of waste disposal ponds of one of the biggest Polish chemical plant, located in central Poland. Three geophysical methods were adopted for this survey: EM profiling, ERT resistivity surveys and direct push resistivity logging. While EM profiling was chosen for surface area mapping, the application of ERT method was aimed at vertical characterization of the contamination distribution. Finally, the direct push resistivity logging was performed at two locations, with the aim of verifying the ERT interpretation. As a result, several extremely high conductivity/low resistivity anomalies has been identified, that can be attributed to the migration pathways from waste ponds. The survey results for each method applied here are shown, and emphasis is placed on comparison between the results obtained with each individual method. Differences in the resistivity/conductivity image of the area can be explained as arisen from different spatial characteristics of each method applied.
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DERT and Magnetotelluric Investigation of Bradano Foredeep Deposits
Authors E. Rizzo, M. Balasco, P. Harabaglia, V. Lapenna, A. Siniscalchi and G. TamburielloGeoelectrical and magnetotelluric surveys have been carried out to better understand the deep water circulation system in the Venosa area (Southern Italy) located in the frontal portion of the southern Apenninic Subduction. In this area, measurements undertaken in some deep wells give a water conductivity of 3mS/cm and a temperature of about 35°C. A deep dipole-dipole geoelectrical tomography has been carried out along a profile of 10.000m and an investigation depth of 900m. Furthermore a broad band magnetotelluric profile consisting of six station was performed in order to infer the resistivity distribution up to some kilometerers of depth. The MT profile was almost coincident to the geoelectrical outline. The applied methods allow us to obtain a mutual control and an integrated interpretation of the data. The high resolution of the data was the key to reconstruct the structural asset of buried carbonatic horst which top is located at about 600m deep. On the basis of data wells, geothermal analysis and geophysical data, it results that the horst is saturated with salted water and an anomalous local gradient of 60°C/km is present. The proposed mechanism is that of a mixing of fossil and fresh water.
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A General Approach for Introducing Information into Inversion and Examples from DC Resistivity Inversion
Authors T. Guenther and C. RückerWe present a more general inversion and regularization method based on constrained minimization on arbitrary grids.
Weighting matrices are used to control both model cells and their boundaries.
This allows for involving additional information as bore hole data, known boundaries or other geophysical methods, e.g., in form of a joint inversion.
We show different examples, each with additional information that is incorporated into the inversion and, thus, enhances the interpretation significantly.
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Comparison of the Spatial Resolution of Standard and Optimised Electrical Resistivity Tomography Arrays
Authors O. Kuras, P. B. Wilkinson, P. I. Meldrum, J. E. Chambers and R. D. OgilvyTwo new ERT array optimisation strategies are presented that attempt to maximise the spatial resolution of the image for a given number of measurements. Synthetic modelling studies and inversions are used to assess the performance of the optimised arrays against arrays of standard Wenner-Schlumberger and Dipole-Dipole configurations. For the same number of configurations, the optimised arrays are found to perform at least as well as, and mostly better than, the standard arrays.
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Kelvin Inversion of PEC and PMC Spheres for Geoelectric Imaging
Authors E. H. Eloranta and J. J. HänninenThe classical Kelvin inversions in spherical geometry are revisited in the context of geoelectric imaging. A point source of current is outside a perfect electrically conducting (PEC) sphere or outside perfect magnetically conducting (PMC) sphere. Perfect electric conductor is very good conductor with zero electric resistivity. Perfect magnetic conductor corresponds to very good insulator with infinite resistivity. The sphere is embedded in a homogeneous whole space which has constant electric conductivity. Analytical solutions can be obtained by the method of images. The methods are described and numerical demonstrations based on them are presented.
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3D Resistivity Monitoring - An Effective Electrode Array to Follow Preferential Flow during Rainfalls
Authors L. Marescot, N. Denchik and D. ChapellierThe aim of this study is to select an appropriated array type to figure localized horizontal movements of fluid in the subsurface for 3D resistivity monitoring using a permanent electrode device. Due to technical limitations, the number of current and potential electrodes is limited in the field. Therefore, the choice of the appropriated array for the survey is crucial. Using numerical modelling, the most appropriated array (pole-dipole) has been selected to detect lateral variations of the electrical resistivity in the framework of a permanent surveying installation system. A good correlation between results of numerical modelling and a field data example is shown and preferential infiltration can be observed in the field.
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Target Detectability Depths of DC Arrays for Various Models
More LessWe have compared target detectability depths for six different DC geoelectric arrays. Five various 2D inhomogeneity models and two noise levels (5 pc and 10 pc) were assumed. The maximum detectability depths were determined by using the RES2DMOD software. Although the results are model-dependent (they depend both on geometry and resistivity contrast), the best results (namely: the maximum detectability depths) were obtained usually with the pole-dipole (P-DP) and the dipole axial (DP-ax) arrays. The worst results (namely: the smallest detectability depths) were obtained (with one exception) in case of the pole-pole (P-P) and the Wenner-alfa (W-alfa) arrays. The results by using the Wenner-beta (W-beta) and dipole equatorial (DP-eq) array groups are slightly below or above the average. Detectability depth values are comparable exclusively for the same model (partly due to the variable resistivity contrast), but in case of a certain model it can be unambiguously declared, which array is the most effective one.
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Optimizing Resistivity Array Configurations by Using a Non-Homogeneous Background Model
Authors E. Athanasiou, P. Tsourlos, C. B. Papazachos and G. N. TsokasIn this work an algorithm designed to find optimum resistivity arrays is examined and tested for various synthetic data. Further, tests with real data are presented as well. The algorithm is based on existing model resolution matrix approaches for finding optimum arrays, but it differentiates since it studies the effect of using an initial model to calculate an optimum resistivity array. Results obtained with this approach are compared to those obtained from existing algorithms which operate for homogeneous ground and to the most commonly used traditional arrays. The optimum array type acquired by using this approach seems to represent the subsurface structure in the best manner, since it takes into account all existing information. The described approach has practical application mainly in time-lapse ERT surveys.
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Resistivity Imaging Using Flexible Electrode Combination Data Collected with a Multi-Channel, Full Waveform System
Authors L. Marescot, J. Zhe and S. GreenhalghIn this paper we suggest a major break from the usual practice of resistivity imaging using standard electrode arrays, such as Schlumberger, Wenner or dipole-dipole. Rather, one should take as many measurements as practical from a fixed set of electrodes and record from numerous combinations of current and potential electrodes. With this intention, a new prototype multi-channel and multi-electrode, full waveform resistivity acquisition system which permits such recording strategy has been developed. It allows for significant data collection capacity and flexibility and is therefore suitable for 2D or 3D surface or cross-hole resistivity tomography and for general electric monitoring.
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Geophysical Investigations for Hydrogeological Characterization at the Kappelen Test Site (Switzerland)
Authors Y. Hacini, L. Marescot and D. ChapellierVertical electric soundings, 2D resistivity imaging and several logging measurements were performed at Kappelen test site to identify the various geolelectric facies that allowed determining the tabular and horizontal structure of the aquifer. The surface-based geoelectric methods allowed for a reliable characterization of the overall structure and the geometry of the aquifer, while geophysical logging methods allowed for inferring detailed hydrogeophysical characteristics, such as the electrical resistivity, total porosity, global and matrix density and hydraulic conductivity. The synoptic interpretation and integration of this broad and diverse database allows for constraining the key hydrological characteristics and hence forms the basis for the detailed hydraulic modelling of flow and transport process.
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Improving the Signal-to-Noise Ratio of Surface NMR Data Due to the Remote Reference Technique
By T. RadicA new instrument was developed for Surface NMR measurements in noise-contaminated areas. Heart of the new measuring concept is the Remote Reference Technique (RRT), which helps to minimize cultural and natural noise in the SNMR data. The RRT is a very efficient tool because it needs no additional measuring time.
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Feasibility Study of AMT Application to Sedimentary Structure of Tidal Flat
Authors G. Park, B. D. Kwon, C. K. Lee, S. Y. Choi and S. HuhWe have applied the audio-frequency magnetotelluric (AMT) method to the intertidal deposits of Gunhung Bay, west coast of Korea, and analyzed the field data both qualitatively and quantitatively to investigate the morphology and sedimentary stratigraphy of the tidal flat. In order to test the sensitivity of AMT responses, the AMT inversion results were compared with the seismic section. The AMT survey and seismic survey both show the three-layered structure comprising unconsolidated deposits, semi-consolidated sedimentary rocks, and metasedimentary basement. The inversion of AMT data well reveals the upper sedimentary layer of Holocene intertidal sediments. However, it is thought that the AMT inversion tends to overestimate the depth of basement when compared with the seismic section. Since MT responses are not significantly sensitive to the resistivity of middle layer or the depth of basement, the AMT inversion result for the basement may have to be adjusted using other geophysical information like a seismic section or logging data if possible.
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GPR Structure of the Parana Delta (Argentina) - Preliminary Results
Authors A. Casas, F. Colombo and L. RiveroAn important research object when studying river deltas is predicting their internal structure, stratigraphy, lithofacies, depositional history and volume of sand or gravel accumulated. Until recently, prediction was based on morphologic-depositional models developed by comparing surface sediment texture and morphology with subsurface structure in carved exposures and/or drill cores. While this approach is commonly used to make a first approximation about delta structure and processes, ground-penetrating radar now provides a more accurate alternative, at low cost and without disturbance to the ground.
Ground-penetrating radar (GPR) was used to image the internal structure of the Paraná River Delta (Argentina). The topographically low parts of some deltas are unsuitable for ground penetrating radar method because the existence of marine conditions and fine-grained sediments deposited in restricted environments, creating an electrically conductive medium unfavorable for the penetration of radar waves. Then, the second aim was test the ability of GPR to acquire enough subsurface information for radar-facies imaging of the complete structure.
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First Results from Spaceborne Radar Interferometry for the Study of Ground Displacements in Urban Areas
Authors L. F. Loddo, C. M. Crosetto, C. A. Casas, R. G. Ranieri, A. I. Atzori and M. M. ManuntaThe Differential Synthetic Aperture Radar Interferometry (DInSAR) is a remote sensing technique that allows us to produce spatially dense deformation maps with centimetre to millimetre accuracy and to detect and follow the temporal evolution of deformations via the generation of time – series. To do this, the information available from each interferometric data pair must be properly related to those included in the other acquisitions, via the generation of an appropriate sequence of DInSAR Interferograms. In this work we process a large set of data acquired by the European Remote Sensing (ERS) satellites on the Palau y Solitari de Plegamans area located near Barcelona (Spain) and on the Cagliari area (Italy). In the second work we apply an algorithm referred to as Small Baseline Subset (SBAS) proposed by I.R.E.A. - C.N.R. The results demonstrate that the DInSAR technique is a fast and precise method to detect and follow the temporal evolution of deformations, so it is a powerful technique for monitoring the territory and to reduce its vulnerability.
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Information Enhancement for Peatland Investigations - Case Study Torronsuo
Authors G. van den Dool and J. LerssiGeological Survey of Finland (GSF) has contributed considerably to the rapid expansion of the Finnish peat industry and simultaneous helped to implement the National Mire Conservation Programme. In order to process this growing demand of information GSF has enhanced their procedures with modern data / information processing techniques.
To support the peatland investigations the GIS-orientated Data Base Management System (DBMS) was enlarged. This database (Geotietoydin / GTY) holds geological theme-based layers and includes now also information about all peatlands (over 20 hectares) in Finland.
The Peatland Information Layer contains field-, depth- and laboratory-measurement data. In the planning process GTY is used to select the most urgent peatlands (either by industrial or conservational interest), using all available spatial information. The fieldwork procedures are modified to enable the collection of borehole information through hand-held GPS devices. In the field bases this information is automatically validated and transferred into the structure of GTY. While reporting the findings of the fieldwork, with the use of computational data manipulations, the form and structure of the investigated peatlands are more accurate estimated.
This information utilisation has benefited to a more efficient planning, direct information use and knowledge integration in GSF.
Additional information about the methods and techniques used in this presentation can be found at http://www.eracnet.fi/publications/
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An Algorithm for Fast 3D Inversion of Parallel 2D ERT Data-Sets
Authors N. G. Papadopoulos, P. Tsourlos, C. Papazachos, G. N. Tsokas and A. SarrisIn this work, a new automated 3d resistivity inversion algorithm, for parallel 2D ERT lines which is at least two times faster than a standard inversion algorithm, is presented. The basic idea of this algorithm is based on the examination of the Jacobian matrix entries and the fact that due to the 2D arrangement of measuring electrodes there are many 3D parameters for which the calculated Jacobian matrix values are practically zero. These values are automatically identified and excluded from the Jacobian calculation, which is the most time-consuming part of forward modeling. Further, to take full advantage of the sparseness of the resulting Jacobian matrix, inversion is preformed using the LSQR (Least Squares Regression) method which results in faster inversion and has less memory requirements than the conventional approach. The algorithm is compared to a standard 3D inversion routine and proved its efficiency in both synthetic and field data.
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Multi-Salinity Measurements of Spectral Induced Polarisation on Sandstones
More LessThe polarisation effects are studied while saturating selected sandstone samples with brines of different cations. The real part of conductivity shows - as expected - a linear relation to the brine conductivity. The imaginary part of conductivity depends on the kind of cations in the brine with the strongest effect for sodium chloride. At higher concentration, a saturation is observed.
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Inversion of SIP Data for Cole-Cole and Constant-Phase-Angle Relaxation Model Parameters
Authors R. Blaschek, A. Hördt and A. KemnaWe present a tomographic inversion procedure for the computation of intrinsic Cole-Cole or Constant-Phase-Angle (CPA) model parameters from spectral induced polarization (SIP) data. The idea is to invert data at all frequencies simultaneously while using a subsurface model which consists only of the parameters of the relaxation model for each cell, instead of inverting each frequency separately for complex resistivities and fitting a relaxation model to each cell afterwards. Connecting the information from all frequencies during the inversion is expected to yield more consistent models. Both approaches are compared and also applied to a set of surface field data.
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2D and 3D Resistivity Imaging in an Investigation of Boulder Occurrence and Soil Depth in Glacial Till
Authors R. Wisen, F. Linders and T. DahlinThe uncertainty regarding depths to bedrock and occurrence of boulders in glacial till can make the planning of a major road construction more difficult. Despite the fact that geophysics is frequently used in many applications and also occurs as a natural method to solve problems in geotechnical engineering, it has often a subordinate role as a site investigation method prior to road construction in Sweden. This project aims at evaluating 2D and 3D resistivity imaging to determine soil depths and track boulders in glacial till. Hopefully, the extent of the excavations preceding a road construction will thus be better estimated in the future, as the geotechnical investigations are suitably complemented. When compared with existing geotechnical data and brought together, both the 2D and 3D resistivity methods are regarded as good producers of reliable soil depth models. However reference data from alternative methods must always be used for validation and calibration of results. It is shown that a 3D resistivity dataset, consisting of a number of parallel CVES profiles, in some cases can give significantly improved resistivity models.
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Least-Squares Inversion of DC Resistivity Data Acquired over Dynamically Changing Earth Model
By J. H. KimIn the geophysical monitoring to understand the change of subsurface material properties with time, the time-invariant static subsurface model is commonly adopted to reconstruct a time-lapse image. This assumption of static model, however, can be invalid particularly when fluid migrates very quickly in highly permeable medium in the brine injection experiment. In such case, the resultant subsurface images may be severely distorted. In order to alleviate this problem, we develop a new least-squares inversion algorithm under the assumption that the subsurface model will change continuously in time. Instead of sampling a time-space model into numerous space models with a regular time interval, a few reference models in space domain at different times pre-selected are used to describe the subsurface structure continuously changing in time; the material property at a certain space coordinate are assumed to change linearly in time. Consequently, finding a space-time model can be simplified into obtaining several reference space models. The regularization along time axis is introduced assuming that the subsurface model will not change significantly during the data acquisition. The performance of the proposed algorithm is demonstrated by the numerical experiments using the synthetic data of crosshole dc resistivity tomography.
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Resistivity Monitoring of an Irrigation Experiment at Högbytorp, Sweden
Authors T. Dahlin, P. Aronsson, A. Hagevi, E. Wall and M. ThörnelöfAn irrigation experiment was carried out close to the Högbytorp waste site outside Stockholm, Sweden, in which leachate water from the waste site was used for irrigation of a Salix plantation. Three different irrigation levels were used, plus no irrigation as control. Resistivity imaging was used at a number of time steps with the objective to try to monitor changes in water saturation and salinity in the ground. The monitoring results show strong potential for the method. Variation in soils properties are indicated by the variation in resistivity. Changes in resistivity that correlate well with differences in irrigation quantities and plant growth are evident. The experiment is planned to continue, and more in-depth analysis of all available data will be carried out. It is beyond doubt, however, that a lot of information related to variation in water quantity and quality is contained in the resistivity imaging results.
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The Use of the CVES Method in a Basaltic Environment - A Case Study from the Faroe Islands
Authors T. Varming and F. P. DalsgarðA CVES survey has been conducted in order to test the method and locating a coal-bearing sedimentary layer in an otherwise dominated basaltic environment showing considerable topography. The data has been initially interpreted using the smooth 2D minimum structure inversion. The results showed that data did not contain much information in the deeper section and the sedimentary section could therefore not be confidentially mapped. However good estimates of the thickness of the overburden and the presence of a shallow underground down-hill water flow can be mapped with confidence.
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1D Inversion of Resistivity and Water Content of Magnetic Resonance Sounding
Authors U. Yaramanci and M. BraunMagnetic Resonance Sounding (MRS, or Surface Nuclear Magnetic Resonance, SNMR) is used for groundwater exploration and aquifer characterization. It provides valuable information about geometry of the aquifer, water content and hydraulic conductivity.
In MRS, since the excitation magnetic field depends on the resistivity of the subsurface, it has to be taken into account in the inversion. Currently, resistivity is provided by a priori information from supplementary geoelectrical measurements or by assumptions. We developed an inversion scheme for MRS to invert also for the resistivity directly from the MRS data.
Results for synthetic data show that the water content, layer boundaries and resistivity can be derived using only the amplitude of the MRS data. However, using amplitude and phase of the MRS signal improves the inversion result. The feasibility of the new inversion scheme has been demonstrated by the successful implementation of field data. The resistivities derived from SNMR are comparable with those derived from conventional electromagnetic methods.
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Large Scale Geophysical Landslide-Genetic Models for the Adriatic Apennine-Ridge Slopes
More LessApennine geodynamic and geophysical evidences suggest models generating landslides affecting numerous Italian Regions, namely Emilia-Romagna, Marche, Umbria, Abruzzi, Molise and also Campania and Basilicata. These Adriatic Foredeep landslide models use concepts of Visco-Elasticity and Catastrophe Theory, recently including original contributions, i.e. Quasi-Sufficient Catastrophe Condition and Hopeless System. These models should be up-to-dated by processing field survey data and realtime Remote Sensing imageries, so using them like expert instruments helping civil protection organisations to alert people and protect natural and built environment.
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Integrated Approach for Exploration of Geothermal Energy in Sri Lanka
Authors H. M. R. Premasiri, U. G. A. Puswewala and S. WeerawarnakulaOver 70 percent of the island's total energy consumption was satisfied by firewood, agricultural residues, and animal waste, mostly for household use. Coal or petroleum deposits have not been discovered in the country yet, and the only other indigenous energy source was hydropower. Though Sri Lanka does not fall within very active tectonic zone in the earth , as nature’s wonderful gift, several hot water springs occur in the island (Map 2). However, no adequate steps have been taken to develop these thermal resources for use as energy resources.
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Some Geophysical and Geomechanical Remarks on Recognition Sinkhole Processes in Post-Mining Areas
Authors Z. Pilecki and H. MarcakA geomechanical process leading to sinkhole appearance in post-mining areas has been analysed. Changes of rock mass physical properties can be expected in the vicinity of inhomogeneities moving up towards the surface. Some geophysical measurements can be used for locating the risk site. The examples presented prove the effectiveness of the geophysical methods used to identify a sinkhole formation process in geological conditions of post-mining areas in Poland. Finally, some basic remarks on limitations of the geophysical methods have been underlined as well as the direction for further research work.
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Influence of Black Schists to Metal and Sulphur Content of Peat
Authors J. M. Lerssi and K. VirtanenThe purpose of this study was to find out the influence of black schists to metal and sulphur content of peat. Geophysical ground measurements, logging and peat sampling were done over the chosen profiles in selected peatlands. Element concentrations of peat samples were analysed in laboratory.
Exact positions of black schists were mapped using airborne data and ground EM-multi-frequency measurements. Resistivity logging probe was used to map the variation of resistivity by depth in peat. Different species and the stage of decomposition of peat and elevated metal concentrations can be observed from some logging results.
The influence of black schist is clearly observed in some high element concentrations compared to normal concentrations, in spite of quaternary layer between the peat and black schist. Some element concentrations are roughly twice as big as the normal ones. In some single samples very high concentrations were observed. Element concentrations clearly above normal level were: Barium, cobalt, chromium, iron, manganese, nickel, lead, sulphur, titanium, vanadium and zinc.
Most of the elements are concentrated near the black schist formations. Some of the elements have drifted several hundred meters and concentrated evenly in high contents over the whole bottom layer of the peat basin.
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Application of Shallow Seismic during Groundwater Filtration Investigations in the Vicinity of an Electric Power Station
Authors V. P. Lisin, I. A. Sanfirov, B. A. Bourdjukov, K. Celminsh and V. A. GrinjokDifferent aspects of shallow seismic methodology features in order to evaluate the condition of a protective dam and for an upgrade of the drainage system of the Plavinas HEPS on the Daugava River are considered. Within the framework of unified field technique, data was obtained for investigating soils, morainic deposits and buried karst manifestations on the bedrock surface in the foundations of hydraulic engineering structures using different kinds of seismic waves. Based on the obtained data, optimal locations of hydrogeological and testing boreholes were determined at the probable sites of increased filtration zones within the regional confined groundwater horizon.
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Diagnostic of Reinforced Concrete Constructions on Base GPR Technology
Authors V. V. Glazounov, N. N. Efimova, A. I. Kulikov and C. M. DanilievPractical capabilities of Ground Penetrating Radar (GPR) method in engineering diagnostics of defects arising and developing in time under operating loads in bulky reinforced concrete constructions are considered. GPR method makes it possible to detect and locate defects in concrete and metal structural components. As an illustration of vast GPR method capabilities, the results of many years’ investigation of underwater Kanonersky road tunnel in St-Petersburg are adduced. Beginning from 1998, GPR method was successfully used for qualitative estimation of the state of pavement at the tunnel foundation both before and after repair work, the latter was the cementation. Merits of GPR method and its development prospects are described.
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Studies for the Dependence of the Geotechnical Ground Parameters on the Hydrogeological Conditions
Authors V. Ciszkowski, T. Moldoveanu and G. TicuThe modifications of the geotechnical parameters lead to the decrease of the bear capacity of the foundation ground and to the yield of the ground under the pressure of the foundation causing the degradation of the building.
For establishing the supplying sources and the flowing directions of the ground water it is required to examine the ground through geophysical studies.
The image of the map of the electric field indicates the zone where the excess of water that came from the loss out of the underground pipelines network is situated in the loess dust but it cannot show the source of these infiltrations.
The map with the isobaths at the surface of the phreatic water allows placing the sources that generate water infiltrations.
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High Resolution 3D Geoelectrical Imaging of Quaternary River Valley Deposits at a Brownfield Site, Midlands, UK
A case study is described in which 3D electrical resistivity tomography (ERT) is used to investigate a chlorinated hydrocarbon contaminated site. The geophysical survey formed a component of a wider site investigation programme that included intrusive geological, hydrogeological and geochemical sampling. The primary goal of the 3D ERT survey was to characterise the thickness and lithological variations within the superficial deposits covering the site, and to assist in identifying suitable targets for the intrusive sampling programme. Interpretation of the 3D resistivity model has been aided by data collected from borehole sampling and cone penetration tests (CPT).
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Determination of Water Wells Location Based on Geophysical Investigations - An Example in an Alluvial Plain
Authors P. S. Sénéchal, G. Sénéchal, D. Rousset and J. M. MartinIn order to determine the best location of well for pumping drinking water in an alluvial plain, different non destructive geophysical methods (Ground Penetrating Radar (GPR), electrical prospecting, seismic refraction) are tested on the studied site. The goal of this study is to determine non destructive geophysical protocol for the lithological and geometrical characterization of the alluvial deposits (6 to 10 meters thick) and the bedrock constituted of clays. We have evaluated the efficiency of geophysical methods on the same site. The preliminary results are presented here. The results obtained with each method depend essentially of the lithology inside alluvial deposits and thick of vegetal soil. The combination of GPR and electrical measurements appears to be the best solution to have geometrical informations with good resolution for the top of the bedrock and informations about the lithological variations inside the alluvial deposits.
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Aquifer Modeling Using Geostatistics to Include Regional, Qualitative Heterogeneity Information
Authors W. J. Trainor and J. K. CaersClimatic changes and extremities make Aquifer Storage and Recovery (ASR) an increasingly attractive and practical water management tool. Homogeneous models will be insufficient for predicting the recovery potential of a particular aquifer or for predicting problems with flow barriers. This study generates many heterogeneous models using geostatistics without traditional reservoir data (wells/seismic). These models are used in a sensitivity analysis to determine the most influential flow structures; this analysis can then be used to design an informative geophysical survey. Lastly, ASR activities are simulated to evaluate the local aquifer's potential for groundwater storage and recovery.
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Geophysical Studies to Detect a Favourable Location for a Thermal Water Well
Authors J. P. Carvalho, E. C. Ramalho, R. Gonçalves, F. Santos and J. Martins de CarvalhoSeveral geophysical surveys were carried out in order to determine the best location for a new production well for the spa of Caldas de Monção, in order to increase production of thermal waters due to an increasing demand of the facilities. After a dipole-dipole survey conducted in 2001 that reached a depth of investigation of about 40 m and 2 shallow wells, three seismic reflection profiles were acquired to locate deep rooted faults capable of groundwater transport to the surface. Vertical electrical soundings and the refraction interpretation of the first arrivals of the reflection data were also undertaken to complement the information. This paper presents the integrated interpretation of geophysical and geological data in order to achieve the above mentioned goal. After the interpretation and lateral correlation of the structures found in the geophysical profiles, two locations appear as the strongest candidates for a successful thermal water production well. This example shows the utility of geophysical methods in detecting favorable areas for water borehole drilling, reducing economical costs considerably.
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Groundwater Flow Model to Simulate Drawdown Due to Leakage into Planned Rock Caverns at the Centre of Helsinki
Authors T. Vaittinen, P. Raudasmaa, J. Korpi and K. MelanderA groundwater flow model was compiled to simulate drawdown due to groundwater leakage into planned rock caverns under the city area of Helsinki. Numerical model was applied to complete standard groundwater management plan.
The main features to be described in the model were hydrostratigraphic units forming a system of an unconfined and a confined aquifer, leakage of the groundwater into existing underground facilities, e.g. metro tunnels at the northern boundary, and the sea water intrusion depending on the sea level. Both aquifers carry out a risk from the point of view decreasing groundwater head, condition of wooden piles in the unconfined aquifer and subsidence of the clay layer above the confined aquifer.
Groundwater leakage to planned maintenance tunnel at two possible locations was simulated. As a steady-state result, leakage at Kluuvi crushed zone is not as critical as leakage at the western boundary of the model. Compensatory sea water can intrude along the crushed zone, but based on the present concept, there is no such hydraulic connection from the sea to the western boundary. Therefore at the western area recharge wells may be needed to maintain groundwater table and hydraulic head.
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Groundwater Modelling - A Geophysical Approach
Authors J. Vendelbo, J. S. Damgaard and N. A. EdsenA regional groundwater model covering a 375 km2 large Danish island is established on the basis of 1D interpretation of geophysical datasets including almost 3.000 electromagnetic TDEM soundings. The model is based on 13 layers with downward increasing thickness and initial model parameters were determined on a presumed statistical dependence between electric resistivity and hydraulic conductivity.
During the last decade, geophysical methods for groundwater purposes have been intensively developed and applied in Denmark as electric and electromagnetic methods measuring the subsurface resistivity distribution have shown most suitable for Danish conditions.
The modelling strategy is based on a relationship between geophysical models and the hydraulic properties of the geological sediments. The actual case has proven a relationship between the electrical resistivity and hydraulic conductivity, though for some sediment-/rock-types with some uncertainty. As expected, interpretation difficulties are connected with transferring the geophysical information (resistivity from 1D interpretation of TDEM-soundings) to hydrogeological information. However, by combining electrical resistivity with borehole information, a model that yields a good representation of the aquifer properties can be defined.
Conclusively, a significant advantage can be achieved by including geophysical surveys when setting up regional groundwater models.
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Geophysical and Hydraulic Properties in Rock
Authors B. E. Danielsen and T. DahlinAn extensive database with data from southern Sweden invites for a thorough investigation of the geophysical and hydraulic properties. In the first attempt to find a relation between geophysical and hydraulic properties the information from core drillings and CVES are used. The records from the drillings include lithology, weathering and hydraulic conductivity. From the inverted CVES profiles separate soundings are extracted at positions close to the core drillings.
The results from the investigation are not easy to interpret. Some drillings and resistivity soundings shows good correlation and some do not. The problem might be that the resistivity measurements have a too low resolution compared to the very detailed observations from the core sample. Another problem could be that the core drilling and resistivity sounding most likely are made at positions close to each other but not the exact same place.
As expected this type of investigation is too simple for a complex relationship as the one that might exist between geophysical and hydraulic properties. It shows the importance of further investigations of existing and new data.
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First Steps in Hydrogeophysical Characterisation of Buried Valley Aquifers
Authors H. M. Rumpel and H. WiederholdIn the scope of the BurVal project buried valleys are explored in the North Sea region. They developed as subglacial tunnel valleys during Quaternary times and are refilled with sediments afterwards. Depending on the distribution of permeable to non-permeable layers (sand and clay) they are important for future water supply. In order to address the intrinsic complexity of the buried valleys, high-resolution reflection seismic, vertical seismic profiles (VSP) and electromagnetic (EM) surveys are used amongst other geoscientific measurements for investigation. In the two German pilot areas, the Cuxhavener Rinne and the Ellerbeker Rinne, we can map the extension of a covering clay layer in the buried valley. It is visible in the EM surveys as good conductor. In combination with the seismic sections the relative position of the clay layer in the valley is further specified. Side deposits on the valley flanks influence the protective function of the clay along the valley rim. VSP measurements show the wide range of the seismic velocities in an unconsolidated and water-saturated environment. The collected data is integrated in a 3D structural model which enables the visualisation of the buried valley and will serve as basic model for future groundwater modelling.
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Mapping Limestone Aquifer by Electrical and Seismic Methods in the Zmajevac Area (Croatia)
Authors F. Šumanovac, Z. Pekaš and H. Majtanić;Exploratory area is located in the Pannonian basin, 95 km east of Zagreb, with a geological model atypical for basin conditions. The aquifer is a “package“ of limestones, clayey limestones and marls (for geological and petroleum exploration purposes named “Croatica layers”), characterized by secondary porosity and very slight increase in relation to the impermeable “package“ of the Sarmatian deposits, which are similar to turbid sediments. Due to a complex geological model, electrical and seizmic methods were applied, with 2-D electrical tomography being the main exploration method. The Croatica layers were successfully isolated and mapped, located at depths ranging from several to approximately 40 metres, as well as the fault zones with increased rock fragmentation. The forecasting model was confirmed by the exploratory borehole, set in one of the identified fault zones in the Croatica layers, and satisfactory quantities of ground water (about 5 l/s) were obtained by exploratory pumping.
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Spatially Constrained Inversion of Area Covering Datasets
Authors A. V. Christiansen, E. Auken, N. Foged and K. SørensenWe present a new inversion methodology for large area-covering datasets. Many new data acquisition systems are capable of covering large areas with densely sampled data sets. However, it is not possible to apply 2D or 3D inversion algorithms on a routine basis because of computational problems.
We propose a Spatially Constrained Inversion (SCI) inversion scheme using a local 1D model description ensuring fast computation times. Information on the geological variability is included as constraints between model parameters applied on an all-to-all basis. The 1D formulation means that the SCI works best on a sub-horizontal layered geology.
To ensure flexibility even for very large surveys we sub-divide the area in hexagons, which are independently inverted and afterwards stitched together. Continuity over hexagon edges is ensured by applying an overlap between neighbouring hexagons.
A field example demonstrates that the SCI enables better resolution of otherwise poorly resolved model parameters. Lineations observed in line-inversion are removed.
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Geophysical Investigation of a Mud-Volcano in the Northern Apennines
Authors A. Bratus, F. Accaino, U. Tinivella, S. Conti and D. FontanaIn the frame of a co-operation between the Earth Science Department of the University of Modena and Reggio Emilia and the OGS, a geophysical investigation, including geo-electrical profiles and a 3D seismic acquisition, was detected in Nirano (Italy, Northern Apennine) where mud volcanoes are present.
The aim of the investigation was to determine the geometries of the shallower structures, across a mud volcano (Fig. 1), using information obtained by tomographic inversion of first arrivals of 3D seismic data and resistivity models obtained by 2D ERT (Earth resistivity Tomography) data.
The joint information from different geophysical methods furnished a detailed map of the structures of the first 30 - 50 m in depth, and the final results are in agreement.
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Seismic Interferometry in Near-Surface Seismics - An Experiment
Authors D. S. Draganov and R. GhoseSeismic interferometry (SI) is the process of generating new seismic records from the cross-correlation of existing records. The interest to SI is growing, but mainly in hydrocarbon exploration and global seismology. The application of SI to shallow seismics could help to extract more information form the data and at the same time save efforts and time in the field.
We tested the SI method on a dataset from physical model with a VSP geometry. We could reconstruct direct arrival events and some reflections.
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Stress Dependence of Shear-Wave Velocity in Unconsolidated Sand: Implication for New Monitoring Possibilities
By R. GhoseWe have performed an experiment on an unconsolidated sand sample in a biaxial pressure chamber to observe the effect of horizontal stress on seismic shear-wave velocity. Multi-offset seismic reception was successfully implemented in the laboratory scale. From changes in shear-wave moveout velocity observed in a receiver array we could distinguish the effect of horizontal stress change. The approach appears to be realisable in field. This may lead to a new methodology for in-situ monitoring of horizontal stress change in soil. Further, we found a good match between the observed and the modelled shear-wave velocity change as a function of horizontal stress. This has potential to offer the magnitude of in-situ stress in soil, which has so far remained difficult to estimate in the field. Our observation also reinforces the possibility of implementing the model-based integration of seismic and CPT data, proposed earlier, for obtaining unique estimates of in-situ porosity and horizontal stress.
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