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Near Surface 2011 - 17th EAGE European Meeting of Environmental and Engineering Geophysics
- Conference date: 12 Sep 2011 - 14 Sep 2011
- Location: Leicester, UK
- ISBN: 978-90-73834-15-6
- Published: 12 September 2011
21 - 40 of 127 results
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Geophysical Techniques Improving Geological Mapping in Urban Areas at Two Different Scales
Authors B. Benjumea, A. Gabàs, F. Bellmunt, A. Macau, S. Figueras and M. VilàWe present a geophysical study where different geophysical techniques are combined to support an urban geological mapping project in NE of Spain. The study has been carried out at two different scales. Seismic and electrical resistivity tomography methods have provided a subsoil image up to 30/40 m depth supported by surface-wave and H/V microtremor analysis. The Quaternary/Neogene contact is related to a sharp velocity contrast in one of the profiles producing a significant secondary peak in the H/V microtremor ratio. Therefore, the identification of this second peak could be used for Quaternary thickness estimation in urban areas. Furthermore, large scale study has been carried out combining CSAMT/MT and H/V microtremor methods. The resistivity model obtained from electromagnetic data allows imaging Neogene/Paleogene contact and Paleozoic bedrock. Combination of this model and bedrock depth estimation obtained from H/V technique has led to identify the geological contact linked to the soil fundamental frequency. Therefore, H/V measurements extended to the whole urban areas can be used to assess bedrock depth. The methodology introduced in this study will improve geological mapping at 1:5000 scale in other urban areas of Catalonia region.
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3D Electrical Resistivity Tomography to Locate DNAPL Contamination in an Urban Environment
Authors V. Naudet, J.C. Gourry, F. Mathieu, J.F. Girard, A. Blondel and A. SaadaThis study presents results from electrical resistivity campaigns performed around a housing estate located downstream to an old coke tar site in France. This coke tar has been previously studied with geochemical analyses and geophysical prospections. The previous results have shown a possible migration of the contaminant plume toward the housing estate. As geophysical measurements are difficult to perform in such an urban environment, the electrical array has been deployed all around the housing estate with an innovative arrangement of surface electrodes in C-shape and acquisition geometries in order to achieve a real 3D imaging of the subsoil and locate the contaminant plume above the housing estate. The electrical resistivity data were inverted with the ERTLabTM 3D inversion software developed by Multi-Phase Technologies and Geostudi Astier. Results show, at the depth of the aquifer, a very conductive plume emanating from old tar ponds and a slag heap and spreading through the housing estate.
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Suppression of Traffic Noise on Vibroseismic Data
By E. NørmarkIn Denmark, vibroseismic data acquisition is commonly used for mapping groundwater reservoirs. A significant part of the reflection seismic profiles are acquired on asphalt roads, which makes the data very exposed to traffic noise. Normally, several sweeps are carried out at each source location. This gives an option for suppressing traffic noise on uncorrelated data. The method assumes that the vibroseismic source signal shows high repeatability in terms of both amplitude and phase. In this context, noise problems from cars and pedestrians will be addressed, but the present procedure is capable of suppressing any kind of noise that is non-repeating and reasonably localized in space and time. The method is simple and robust. It improves signal-to-noise ratio and reduces the need for manual trace editing when processing the data. Thus, by addressing the problem before correlation, significant improvements in the noise suppression can be achieved.
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Geophysical Monitoring of Simulated Clandestine Graves Using Electrical and GPR Methods - 0-3 Years after Burial
Authors J.K. Pringle, J.R. Jervis, J.D. Hansen, N.J. Cassidy, G.M. Jones and G.T. TuckwellThis study provides forensic search teams with systematic geophysical monitoring data over simulated clandestine graves for comparison to active cases. Simulated ‘wrapped’ and ‘naked’ burials were created. Multi-geophysical surveys were collected over a three-year monitoring period. Bulk ground resistivity, Electrical Resistivity Imaging, multi-frequency Ground Penetrating Radar and grave ‘soil water’ conductivity data were collected. Resistivity surveys revealed the naked burial had consistently low-resistivity anomalies, whereas the wrapped burial which had small, varying high-resistivity anomalies. GPR 110-900 MHz frequency surveys showed the wrapped burial could be detected throughout, with the ‘naked’ burial difficult to resolve after 18 months. 225 MHz frequency data was optimal. ‘Soil water’ analyses showed rapidly increasing (year one), slowly increasing (year two) and decreasing (year three) conductivity values. Results suggest resistivity and GPR surveys should be collected if target ‘wrapping’ is unknown, with winter to spring surveys optimal. Resistivity surveys should be collected in clay-rich soils.
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MASW Characterization of Ground Subsidence Induced by an Outburst of Water From a Sump Gallery at St-Vaast, Belgium
Authors J. Deceuster, P. Lizin, T. Martin and O. KaufmannIn February 2009, several damages were recorded on 9 houses and a road at St-Vaast (Belgium) a few days after a sudden outburst of water coming from a former coal mining sump gallery. To identify the origin of these damages and understand the mechanism of the ground subsidence, geological, hydrogeological geotechnical and geophysical investigations were conducted. Five MASW (Multichannel Analysis of Surface Waves) profiles were carried out inside and around the impacted zone to delimit its extent. S-velocity anomalies were pointed out near the surface and at depth just beneath the centre of the impacted area.
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Reliability of Surface Wave Inversion
Authors J. Hausmann, H. Steinnel, S. Popp-Hoffmann, U. Werban and P. DietrichSurface wave seismic can be used to evaluate near surface geological information out. In order to get a reliable bedding model of the near surface loose soil layers the quality of the data and the inversion constraints are the most important influencing factors. This study discusses the influence of the thickness model as a boundary constraint doing data inversion according to MASW processed data (Park et al. 1999, Park et al. 2007). It can be shown that there is a relevant influence of the used thickness model of the uses SurfSeis sofware on the inversion results. A fixed depth model leads to unwanted stretching effects. The assessment of a free depth model using an equal or a variable thickness model gives good and comparable results. But there are high differences between the results if the profile was shot in opposite direction. The geological interpretation may vary according to the direction of the profiling. Forward and reverse measured profiles show significant differences in the inversion results, even there is only one geological setting. Therefore it is recommended to have a critical view of one dimensioned profile section.
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GPR for the Inspection of Industrial Railway Tracks
Authors J. Hugenschmidt, C. Kasa and H. KatoIndustrial railway tracks are important for the loading and unloading of goods and therefore for the railway system in general. Often, industrial tracks are embedded in concrete and/or asphalt to enable truck and other traffic across rails. Thus, most of the construction is hidden from visual inspection. If repair work is planned or if damage occurs, details of the construction have to be known for the planning of repair work or for the evaluation of damages. This paper describes the non-destructive testing of industrial tracks using GPR. Typical testing problems are described and the application of GPR for these problems is demonstrated using data from field measurements. Data from different types of equipment are compared and benefits and limits of the method are discussed. It is shown that GPR is a powerful tool for the inspection of industrial railway tracks.
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Noise Cancellation for Surface NMR - Application of Time and Frequency Domain Approaches
Authors M. Mueller-Petke and U. YaramanciEven though surface NMR is the only geophysical technique that provides hydrogeophysical rock properties on the base of direct sensitivity to subsurface water, in many cases surface NMR measurements suffers bad signal-to-noise ratio, and measurements can be carried out only far from sources of electromagnetic noise. To overcome these restriction some approaches using a reference loop based system were developed during the last years (Radic, 2006; Walsh, 2008; Müller-Petke and Yaramanci, 2010; Neyer, 2010). All approaches have demonstrated useful capability to improve S/N. But a comparison that allows for determining properties, i.e., pro and contra of each approach is missing. Thus, to compare time domain with the frequency approach, we developed an own frequency domain code beside the existing time domain code (Müller-Petke and Yaramanci, 2011). The algorithms were tested using synthetic examples and a field examples. It shows that with increasing complexity of the noise the frequency domain approach provides better results compared to the time domain for the synthetic examples. Concerning the field data both approaches are equivalent. We found that using the multi-channel reference the signal enhancement is improved.
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Noise Cancellation of Multichannel Magnetic Resonance Sounding Measurements with Wiener and Adaptive Filters
Authors E. Dalgaard, E. Auken and J. J. LarsenMagnetic resonance sounding (MRS) is a unique and promising hydrogeophysical technique. Due to its ability of providing a direct estimate of the water content and estimates of the porosity in the subsurface, MRS has the potential of being a great team player in the field of hydrological prospecting. However, the MRS measurements suffer from a low signal to noise ratio due to the signal being inherently weak and the susceptibility to electromagnetic interference. The low signal to noise ratio currently impedes the applicability of the technique, particularly in urban areas. In recent years the development of multichannel MRS has opened up new possibilities for advanced noise cancellation methods. Before multichannel MRS can realize its full potential robust and reliable methods for noise cancellation must be developed. In this work we compare two noise cancellation methods: The multichannel Wiener filter and a multichannel adaptive noise cancellation filter. The comparison is performed on noise records from a multichannel MRS instrument with or without synthetic signal added. Our results show that adaptive noise cancellation performs better than the Wiener filter for both pure noise removal and for recovery of the parameters of the synthetic signal.
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Full FID Inversion of MRS Data
Authors A. Behroozmand, E. Auken, G. Fiandaca, E. Dalgaard and J. LarsenThe Magnetic Resonance Sounding (MRS) method, also known as surface NMR, is a non-invasive geophysical method which directly studies groundwater reservoirs from surface measurements. It is capable of direct estimating of water content distribution (from initial amplitudes) and indirect information on pore sizes (from relaxation time). The purpose of this paper is to introduce a new inversion scheme for the full MRS data set. The multi-exponential behaviour of the NMR signal is taken into account and estimated by the Stretched-Exponential model (Hilfer, 2002), in which the time constant and stretching exponent determine the centre and the width of the distribution. The model space is then described with much less parameters, compared with the multi-exponential description. The FID time series are logarithmically gated and the discretization in the z-direction in the forward algorithm is decoupled from discretization of the inversion model. The inversion routine supports smooth and block inversion of the data and a full linear model parameter sensitivity analysis can be calculated (Tarantola and Valette, 1982), an essential criteria for evaluation of the inversion results.
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Block Joint Inversion (BJI) of MRS and DC Resistivity Soundings for Aquifer Imaging at the North Sea Island Borkum
Authors T. Gunther, J. Liebau, I. Akca and M. Müller-PetkeWe present a joint block inversion of DC resistivity and spectral inversion of magnetic resonance sounding (MRS) data. Coupling is achieved by using identical layer thicknesses. We apply the method to data from the north sea island Borkum and demonstrate that the joint inversion is superior to single inversions. Spectral inversion is necessary and finally allows for distinguishing lithology and salinity. Both deterministic, Gauss-Newton (GN), and stochastic, Genetic Algorithm (GA), approaches yield identical results within the resolution limits.
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3D Monte Carlo Inversion of Surface Magnetic Resonance Measurements
Authors AC Chevalier, A. Legchenko, M. Descloitres, H. Guyard, C. Vincent and S. GaramboisMagnetic Resonance Tomography (MRT) is the only geophysical method where the measured signal is directly related to water distribution in the ground. This property allows three-dimensional imagery of water content by signal inversion routines. Because of its non linearity, the inverse problem has a quasi-infinite number of solutions implying as many possible spatial distributions of water content. A good answer to this problem, relevant for ice cavity detection and karstic structures mapping, is to provide a set of solutions consistent with the measured data. Markov Chain Monte Carlo (MCMC) algorithm applied to the MRT inverse problem provides a random exploration of the solutions giving the ability to compute probabilistic answer to a particular data set. For saturated structure detection, first results on synthetic cases demonstrate the routine ability to show an important anisotropy in the MRT resolution. Finally, a real case study, where the MCMC and linear inversion are compared, also shows the benefit of the method for a French Alp glacier cavity detection.
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Defining Formation Boundaries in Multi-layer Models Through Statistical Analysis
More LessEstimating formation boundaries from 1D inversion of electric and electromagnetic data with multi-layer models can be challenging because of the inevitable regularization of this type of inversion resulting in more or less smeared transition zones between formations. Conventional wisdom has it that inversion with few-layer models will solve the problem by providing models with well defined layer boundaries. However, in modern profile-oriented, laterally correlated inversion, the number of layers is the same for all models along the profile. This may cause lateral formation boundaries to be poorly indicated, and sometimes a specific formation will "change layers" along the profile meaning that layer boundaries are no longer formation boundaries. I suggest a new approach to the definition of formation boundaries. It is based on multi-layer inversion models and finds formation boundaries through a statistical analysis of the set of equivalent models obtained in a stochastic process with a correlation function defined by the posterior covariance matrix of the inversion. The method surmounts several of the difficulties mentioned above. A field example will show a successful application of the method.
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How to Incorporate Prior Information in Geophysical Inverse Problems - Deterministic and Geostatistical Approaches
Authors T. Hermans, D. Caterina, R. Martin, A. Kemna, T. Robert and F. NguyenMany geophysical inverse problems are ill-posed leading to non-uniqueness of the solution. It is thus important to reduce the amount of mathematical solutions to more geologically plausible models by regularizing the inverse problem and incorporating all available prior information in the inversion process. We compare three different ways to go beyond standard Occam’s inversion for electrical resistivity tomography (ERT) using electromagnetic logging data in the context of salt water infiltration: a simple reference model, a structural constraint and a geostatistical constraint based on a vertical correlation length. Results with the traditional smoothness constraint yield small contrasts of resistivity, far from the reality revealed by borehole measurements. Incorporating prior information from boreholes clearly improves the misfit with logging data. If a good reference model can always be used, it can lead to misinterpretation if its weight is too strong. When the computation of the correlation length is possible, the geostatistical inversion gives satisfactory results everywhere in the section. In this specific case, the geostatistical approach seems to be a more robust way to incorporate prior information. The structural constraint seems to be more indicated when integrating information from other geophysical methods such as GPR or seismic.
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A Complicate Response of Compact EMI Sensors over Shallow Local Conductive Targets
By E.V. BalkovThe paper presents a modeling of different type of compact EMI sensors response over a local conductive shallow depth targets. The target as well as Tx and Rx coils are approximated by the vertical magnetic dipoles. All the calculations are made under the model of uniform conductive space. Three types of coil configuration are considered. First is semi two coil device with fixed transmitter that carries out the sounding by the changing both the intercoil separation and frequency. The second type is three coil multi frequency sensor with fixed geometry. All the coils in previous configuration are arranged at the same plane. The last new type is multi frequency sensor with the receiver coils that placed at the line inclined to the horizon. A field example is considered as well. The theoretical and practical investigations show that shallow local conductive anomalies produce sophisticated response for compact EMI sensors with spaced coils. It yields from one to four anomalies over one target. The new proposed coil configuration produces the single anomaly
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3D Quantitative Interpretation of Archaeo-magnetic Datasets
Authors S. Cheyney, I. Hill, N. Linford, S. Fishwick and C. LeechRecent advances in magnetic surveying have meant high-resolution data over archaeological sites can now be quickly obtained. However, post-survey processing still generally comprises a sequence of data correction and filtering prior to a 2D visual interpretation based on pattern recognition. Developments in the processing and modelling of aero-magnetic datasets have led to techniques that can identify the location and shape of anomalous sources, including providing depth information. This paper explores the possibility that several of these techniques can be adapted for use on archao-magnetic datasets. 3D models of subsurface magnetic susceptibility can be generated using inverse methods. Prior to inversion it is important that processing and filtering techniques do not alter the character of the measured signal. Inverse models suffer from the “inverse problem” where many models fit the data equally well. In order to help overcome this Euler deconvolution has be used to constrain the inversion routine, and provide confidence in the final result. Results from a case study over a Romano-Celtic temple, Silchester, UK are shown, and in order to test the robustness of the technique, the model has been compared to a GPR survey collected concurrently. Both models show equivalent features for each depth slice.
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Some Geophysical Survey Methodologies for Archaeological Research in the Brú na Bóinne World Heritage Site, County Meath, Ireland
More LessThe presented paper is a project update of integrated research to evaluate the potential for developing engineered geothermal systems (EGS) in Alberta, Canada. This process balances both locating the highest concentrations of the resource and assessing where there is a demand for those resources. In the context of the geothermal exploration being undertaken by Helmholtz Alberta Initiative (HAI), this requires that heat sources for heavy oil and bitumen production are found close to the deposits. In the passed year, 25 2D seismic lines in different locations of Northern Alberta were reprocessed and interpreted. Reference and Horizon maps were produced in Athabasca area and it would be developed by adding and processing more data sets. This work is innovative as we are developing the first geological model of the area that integrates seismic data, all data prior to this has been developed on the basis of well logs.
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Geophysical Investigations in UK Graveyards - Re-use of Existing Burial Grounds
Authors J.D. Hansen and J.K. PringleThis series of linked studies tests the capabilities of ground penetrating radar (GPR) and fixed-offset bulk resistivity surveys in the search for unmarked burials in UK graveyards. 225 MHz GPR antennae was deemed the optimum dominant frequency due to success in delineating the positions of known burials and also the relatively rapid data acquisition rate compared to higher frequencies. Resistivity data were collected using two mobile probe spacings (0.5m and 1m) simultaneously, 1-m probe spaced data was judged to be less affected by near-surface heterogeneities. Comparisons of both resistivity datasets were deemed worthwhile to detect unmarked burials. Soil type has a major influence on the effectiveness of each geophysical technique. It is recommended that both GPR and resistivity surveys be conducted to optimise the detection of unmarked burials.
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Bedrock Detection and Mineral Thickness Assessment Using 3D Electrical Resistivity Tomography (ERT)
Authors J.E. Chambers, P.B. Wilkinson, A. Hameed, I.A. Hill, C.A. Jeffrey, D. Wardrop, P.I. Meldrum, O. Kuras, R.D. Ogilvy, D. Gunn, M. Cave and J. AumonierIn this study, we demonstrate the use of 3D ERT for detecting depth to bedrock below river terrace sand and gravel, and quantify its performance against borehole control data. The approach to ERT bedrock detection considered here assumes that the interface is located at the maximum slope of the resistivity-depth curve, and is therefore referred to as the ‘steepest gradient method’ (SGM). The study site was located within a geological setting that has proven to be impossible to adequately characterise using conventional discrete sampling approaches (i.e. boreholes and trial pits) due to the considerable heterogeneity of the deposit. Comparisons between borehole and the SGM derived bedrock surface elevations indicated a reasonable agreement between the two, thereby establishing a basis, in this case, on which to estimate mineral volumes using 3D ERT and the SGM. The bedrock surface calculated from the resistivity data provided a means of estimating mineral volumes at a site for which a meaningful reserve calculation could not be made using conventional approaches to site investigation.
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Geophysical Quality Assessment of Sand and Gravel Deposits
Authors I. Hill, C.A. Jeffrey and A. HameedQuality assessment of onshoresand and gravel deposits must be low-cost and efficient to be economically attractive. This paper reports initial results of a thorough comparison of assessment methods for such deposits, by comparing results from geophysical surveys and exploration drilling with a variety of drilling methods, with ground truth obtained by direct sampling of the deposits as the working face of the quarry moved through the areas of study. Here, the results of the geophysical assessment of the deposits uses resistivity and EM methods. While immedaite results are practically useful to the industry, further development of 3D inversion, and petrophysical models are needed to fully exploit the data quality from current survey instruments.
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