- Home
- Conferences
- Conference Proceedings
- Conferences
Near Surface 2010 - 16th EAGE European Meeting of Environmental and Engineering Geophysics
- Conference date: 06 Sep 2010 - 08 Sep 2010
- Location: Zurich, Switzerland
- ISBN: 978-90-73781-88-7
- Published: 06 September 2010
121 - 140 of 153 results
-
-
Selecting Electrical Resistivity Tomography Arrays to Define Geological Structures Aimed by Relative-sensitivity Matrix
More LessResistivity tomography is a widely used geophysical technique. One of the most relevant points while performing ERT surveys is to choose a suitable electrode- array configuration. The best definition of the structure which is under the target depends on it. The proper interpretation of the electrical imaging is essential. On this purpose, criteria for checking the reliability of the model have to be found in order to detect possible artefacts. Structurally well known areas have been studied with ERT using Dipole-Dipole and Wenner-Schlumberger arrays in the NE Spain. The previous knowledge of the structures and the blocks sensitivity imaging has been used as reliability criterions. Also direct model calculations have been performed with a theoretical terrain in order to compare them with the field-results. The results show that the presence of conductive materials at deep position can improve the data acquisition, especially in the case of Dipole-Dipole array. Although this array tends to be noisy and uncertain, it could be the best option in certain structures. Additionally, the imaging of block sensitivity can be useful as criterion to check the coherence between the calculated resistivity and the maximum and minimum sensitivity values.
-
-
-
Accurate and Robust Evaluation of Singular Boundary Element Integrals for Computing 3D Radiomagnetotelluric Fields
Authors Z. Ren, T. Kalscheuer and H. MaurerWe present a new surface integral formulation designed to efficiently simulate effects of 3D topography and displacement currents on radiomagnetotelluric data. Surface charges, magnetic surface currents and electrical surface currents are introduced to ensure the stability over a wide rage of frequencies. Using a triangular parameterization and the Galerkin approach, the surface integral formulation is transformed into a linear system of equations, in which several double surface integrals must be evaluated. If the triangles in the outer integrals are close to, adjoin or overlap with the triangles in the inner integrals, then these double surface integrals will be weakly or strongly singular. An accurate and robust singularity extraction technique is applied to transform these singular integrals, such that they can be evaluated by analytical expressions. The remaining logarithmic singularities in the outer integrals are removed with a smart strategy of interchanging the order of integration. If the triangles fully overlap, strongly singular integrals can be transformed into weakly singular ones using Gauss' divergence theorem. The efficacy of our singularity extraction technique is validated by means of numerical examples.
-
-
-
Enhancing the Resolution Power of Least Squares Inversion Results of 2-D Magnetotelluric Data
More LessThe paper presents the results of applying a new approach on 2D inversion of Magnetotelluric (MT) data in order to enhance the resolution and stability of the inversion results. Due to non-linearity and limited coverage of the data acquisition in an MT field campaign, minimizing the error by linearization of the problem in the least squares inversion usually leads to an ill-posed problem. Determination of a suitable Lagrangian multiplier in order to balance minimization of error and model roughness could be a useful approach to achieve both resolution and stability in inversion. We tested the capability of Active Constraint Balancing (ACB) approach in enhancing the resolution power of least-squares inversion results by applying it to 2-D synthetic MT data generated from forward modeling code of Geotools-MT for a model with a conductive anomalous body of 5 Ohm-m beneath the earth. The result of joint inversion of TM and TE mod for the conventional approach with fixed Lagrangian multiplier of 0.06 leads to the rms misfit of 0.036 and reduces to 0.008 when ACB is applied. By comparing the resistivity sections, the anomalous object can be seen much clearer and distinct in the case of ACB approach.
-
-
-
Incorporation of a 3-D lossy (conductive) Earth into Surface NMR Tomography
Authors J. Lehmann-Horn and M. HertrichThe surface nuclear magnetic resonance (NMR) tomography technique uses electromagnetic (EM) fields to locate and quantify groundwater occurrences. The magnetic fields interact with the spin magnetic moments of water protons situated within the static Earth’s magnetic field. In conventional surface-NMR forward and inverse modeling, semi-analytical solutions for the electromagnetic fields on a layered earth are employed. This simplification restricts the model to regular loop shapes on a flat terrain and 1-D conductivity distributions. To remove such limitations and to extend the analysis to rough terrain (e.g. mountainous regions) and 3-D heterogeneous media incorporating conductive (lossy) zones (e.g. clay lenses), we have developed a hybrid electromagnetic modeling scheme that combines integral equations with the finite-element method. Local mesh refinement enables the sensitivity function to be sampled in an appropriate manner near the principal singularity (i.e. the transmitter). Here, we apply the novel numerical forward and inverse modeling scheme to synthetic data to illustrate its effectiveness.
-
-
-
2-D Resistivity Imaging for the Crooked Survey Line on the Tough 3-D Topography
Authors M.J. Yi, J.H. Kim, C. Kim, J.S. Son and P. TsourlosIn the 2-D resistivity survey, survey line is sometimes detoured because of geographic or man-made structures. Although the effect of the crooked line is believed to be negligible in most of cases, it should be considered to obtain accurate image of the subsurface. In this study, we examine the effect of crooked survey line on the 2-D resistivity data. A strategy of 2-D resistivity inversion based on the 3-D inversion code with 2-D model structure was suggested to incorporate the crooked line into the inversion, which provided accurate image of the subsurface compared to a conventional 2-D inversion result. As another important practical issue of 2-D resistivity survey, tough topographic variation in the strike direction is also implemented using this strategy to obtain enhanced imaging capability. Field data example of 2-D resistivity survey for mineral exploration at the mountainous area shows that our strategy is very useful in the combined case of crooked survey line on the tough topographic area.
-
-
-
Frequency Soundings at Low Induction Numbers: Transformation and 1D Inversion Comparing to 2D ERT Inversion
Authors E.V. Balkov and Y.G. KarinThe paper is devoted to an approach for calculating an apparent resistivity and a layered earth parameters from transformation and inversion of FEM soundings data. First part of study presents theoretical estimation of the sounding capability of the multifrequency sensor at simple layered earth model. Second part includes processing of the both FEM and DC ERT data sets collected at the same profiles. The results of transformation and 1D layered inversion of frequency soundings comparing to 2D ERT inversion are considered.
-
-
-
Related Interrelations Between Filtration and Electric Parameters of Rocks
By S.Z. KozakOn the base of the model consisting of a batch of capillaries part of which is sealed with clayly plugs, the analytical dependencies of electrical parameters (specific electric resistance and polarizability) on filtration coefficient of water-bearing rocks is acquired. It is shown that one of the main factors influencing on the value of electrical and filtration parameters is distribution of clayly particles in the rock. It is determined for water prospecting that polarizability is the most challenging parameter for quantitative evaluation of filtration parameters by geophysical data. Based on the obtained results, an inference is deduced that empirical relations between electrical and hydraulic parameters are to be composed and analised for uniform lithology and genetic aquifers.
-
-
-
Sensitivity of Experimental Dynamic Stiffness of the Vibrator-Earth System
Authors F. Georgiades and P. ScholtzThe dynamics of the vibrator-earth system is driven by the properties of both the vibrator and the earth. Using a model for the dynamics of this system, we determine theoretically the dynamic stiffness curves that describe the system. In order to define them we need to specify the ground force and the acceleration at the surface of the earth. In our case we use a simplified model of the vibrator to determine the ground force and then assuming that the vibrator baseplate is properly coupled to the surface of the earth (that is very close to the reality due to the hold down force) we use the acceleration of the baseplate to extract these characteristic dynamic stiffness curves of the system. Then we examine, using experimental data, the effect of location, and vibration polarity in dynamic stiffness curves of the system. The sensitivity analysis showed that at low frequencies there is no effect but at medium and high frequencies these curves are very sensitive to the variations. This work is a starting point for the determination of effective earth properties through dynamic stiffness.
-
-
-
Investigations on Kinematic Refraction Inversion at Different Geological Models
Authors A.N. Paripas and T. OrmosIn this paper a refraction inversion technique developed in the Department of Geophysics, University of Miskolc is investigated for different laterally changing geological models. It is important because the solution of the forward problem uses several approximations for significant reduction of the calculation time and these can affect the inversion results. The effects of these approximations depend on the investigated geological structure. According to our investigations it can be stated that the applied approximation gives very good results in case of slowly changing structures, while even in case of a highly changing model the inversion technique gives a result where the target model is recognizable.
-
-
-
2D Quantitative Imaging by Elastic Full Waveform Inversion: Application to a Physical Scale Model
Authors F. Bretaudeau, D. Leparoux, R. Brossier, S. Operto and O. AbrahamQuantitative imaging of the first meters of the underground with seismic methods is an important challenge for many applications. This task is difficult because of heterogeneity, strong attenuation, low frequency content and large dominance of surface waves in the data. Tomography, migration, and dispersion analysis of surface wave are not efficient in all contexts. Elastic full waveform inversion was developed for crustal imaging but has a great potential for near surface applications as it could help to image very heterogeneous media by exploiting the whole complexity of the seismograms. We try here to (1) understand how behaves this method on near surface context, and (2) anticipate on the field application. We use for that a simultaneous approach by applying the waveform inversion on a simple three layer medium from synthetic data obtained by numerical modeling, and from experimental data obtained by small scale modeling. We present the small scale model, the acquisition of synthetic and experimental data, and we present an inversion result. Images of the Vp and Vs parameters are reconstructed. Both lateral and vertical variations are well determined. Images from synthetic and experimental data can be compared. The results are similar but we identify differences related to noise.
-
-
-
Estimation of the Depth and Shape of Intrusive Bodies by 3D Inversion of Ground Magnetic Data
Authors B. Yousefi and B. OskooiThis paper evaluates application of 3D inversion of geomagnetic data for estimating depth and shape of an intrusive body in northeast of Iran. For this purpose, we used an algorithm that has presented by Yaoguo Li and Oldenburge (1996). They have obtained the inverse solution by minimizing an objective of the weighted model subject to fit the observations to the degree determined by the errors. The algorithm discretizes the surveyed area into a large number of vertical rectangular cells and starting from an initial depth, ends with the best fit depths to all the blocks. At last, it determines the best model to show susceptibilities of bodies with an iterative approach. For validating the result, a comparison between the borehole data and predicted model was performed. After comparing the borehole data with the predicted model in well location, a good correlation obtained between thickness of high magnetic susceptibility minerals in borehole data and high magnetic susceptibility parts of predicted model. Finally after 57 iterations, we obtained a model with variable top depths between -20m to -60m and concentration of high magnetic susceptibility increases with depth in northern direction.
-
-
-
Some Applicability Problems of Euler Deconvolution to the Interpretation of the Results of Microgravity Survey
By S. PorzucekThe author presents some problems arising when Euler deconvolution is applied to microgravity survey. The application of Euler deconvolution requires the values of three orthogonal gravity gradients. These values rather cannot be measured ant the values is calculated from measured gravity values using the Fourier or the Hilbert transformations. In microgravity survey the essential problem for correct calculation of the transformation is the proper selection of sampling step. This paper presents the empirical dependences between characteristic properties of microanomalies and measuring (or calculating) steps, which can be useful for microgravity quantitative interpretation.
-
-
-
Linearized Electrical Impedance Tomography
More LessWe address the problem of Electrical Impedance Tomography (EIT) as this is encountered in various applications of near surface geophysics. Based on a closed form integral model that relates the boundary observations to the electrical properties of the domain we suggest some extensions to the conventional `linearization--regularization' modeling approaches. Instead of truncating the Taylor series expansion of the forward operator to first-order accuracy, our methodology accounts for the linearization error as a multivariate random variable and uses stochastic simulation to obtain its statistical information.
-
-
-
Petrophysical and Electrical Study of Soil Properties in Sinaloa, Mexico
Authors V.A. Shevnin, H. Peinado, O. Delgado and A.A. RyjovIn area of study near the channel Valle del Fuerte in Sinaloa state, Mexico, geological and geoelectrical characteristics of soil were measured in seven key areas. In each of seven key areas an electrical resistivity tomography (ERT) profile was fulfilled, three boreholes were made with soil samples collection in each borehole, filtration coefficients of soil, cation exchange capacity, porosity and grain size distribution, groundwater salinity and ionic content in water were determined. ERT data were interpreted and soil resistivity curves versus pore water salinity were measured. Joint analysis of all data obtained was performed to find correlations between them to create petrophysical model of soil. Verification of this model was performed by correlation between the same petrophysical parameters measured in laboratory and obtained from geophysical data.
-
-
-
Seismic Survey for Characterization of Shallow Depth Materials of Bam City, S.W. Iran
Authors M.A. Riahi and S. Hashemi Tabatabaeisecond and third layers decreases from south west towards the north east of the study area. Attentive to the velocity distribution of the P and S wave velocity, the Poisson’s ratio distribution as well as attenuation coefficient obtained for the identified layers of the area under investigation in the Bam city, it is concluded that the low velocity layer with high thickness and attenuation coefficient of the first layer can be attributed to be the cause of the strong motion vibration of the soil due to Bam earthquake on 26th December 2003 in the city.
-
-
-
Sensitivity of Electrical Properties to Soil Compaction - Case Study
Authors H. Petersen, W. Rabbel, R. Horn and L. VolkSoil compaction on cropland caused by human activity is realized to be a serious problem. The development of prevention and remediation strategies requires an accurate knowledge of the real areal distribution of soil compaction. Within the project “Interdiscplinary investigations of soil compaction” we performed in-situ compaction experiments on different field sites with Luvisols from loess and glacial till for different tillage systems (with and without use of plough) to investigate the sensitivity of electrical conductivity measured by EMI method to compaction. Results show a clear influence of the compaction on the apparent electrical conductivity, where the conductivity increases with increasing the compaction.
-
-
-
Velocity and Attenuation Dispersion Relations for the Effective Biot Double Porosity Model: Total Field Formulation
Authors S.A. Greenhalgh, X. Liu and B. ZhouTwo approaches – the host phase fields and the total fields - have been applied to formulate the effective Biot governing equations from the double-porosity, dual permeability model. The host phase formulation, given previously by Pride and Berryman (2003) is made under the assumption that the volume fraction of phase 2 (the inclusions) is very small. The total-field formulation is new and has no such limitation and gives rise to more general equations which cover the host phase field approach as a special case. We have computed the phase velocity and attenuation dispersion curves for the fast and slow Biot compressional waves using both approaches for a sample rock of water-saturated sandstone having sand as the inclusions. The curves are consistent across a wide frequency range for the fast P wave but deviate at higher frequencies and for large volume fractions of the inclusions .For the slow P wave there is a significantly increasing discrepancy in wavespeed as frequency increases, even for low inclusion concentrations
-
-
-
Seismic Monitoring of a Simulated Radioactive Waste Repository During Water Saturation
Authors E. Manukyan, S. Marelli, H.R. Maurer, A.G. Green and S.A. GreenhalghIn the framework of a radioactive waste disposal and monitoring program as part of the EC Integrated Project ESDRED, we have conducted for the NDA (UK) in co-operation with Nagra (Switzerland) high resolution (0.2 - 4 kHz frequencies) seismic experiments in an anisotropic clay unit at the Mont Terri underground rock laboratory in Switzerland. The objective was to explore the possibilities and limitations of seismic measurements for remotely monitoring, at distances of tens of metres, the changing properties of material filling a small scaled–down version of a repository (1 m diameter tunnel) embedded in a clay formation. Recordings using vertical-component geophones attached to the inside wall of the microtunnel revealed significant waveform variations for different fill materials (empty and sand-filled) and experimental conditions (dry, wet, and pressurised). Initially, the presence of water weakens the clay, but at a later stage the seismic measurements indicate swelling of the clay material, which likely causes healing in the excavation damage zone. On the basis of our results, it is judged worthwhile to develop wireless seismic sensors that could be employed for non-intrusive monitoring during the water saturation of a repository.
-
-
-
Vertical Shaft Pilot Hole Investigations in Pre-assessment of Orientation, Transmissivity and Continuity of Fractures
Authors E.J. Heikkinen, T. Ahokas, P. Rouhiainen and T. PereSpent nuclear fuel disposal in crystalline bedrock requires knowledge on long and hydraulically conductive fractures near disposal holes and tunnels. It is possible to investigate fractures with different techniques before drilling of pilot holes, before excavation from pilot holes, between tunnels as crosshole survey, or from the surfaces of excavated tunnel. This work included review of materials obtained in a 105 m long section of vertical shaft, investigated before raise-boring, and using single-hole and detailed crosshole investigations performed in and between three drillholes located at 3.2 m distances from each other, within shaft perimeter. The geological mapping of shaft took place after the raise-boring. From the results it was possible to draw conclusions on applicability of these methods in detailed model compilation. Combined use of geological, imaging and geophysical logging will assist detecting the fracture responses and assigning the orientation. The hydraulic difference flow logging will assign the transmissivity. Crosshole hydraulic measurement will detect the hydraulic connections between drillholes. Crosshole mise-à-la-masse survey grounded on fractures and other conductive zones will effectively reveal electrical connections. Single-hole radar survey indicates continuity of conductive features. Explanation of the features will require geological correlation.
-
-
-
Airborne Electromagnetic Survey of the Groundwater Mineralisation in the Potash Mining District of The Werra River, GER
Authors B. Siemon, A. Ullmann, M. Vasterling, U. Meyer, W.W. Beer and J. PlümacherDissolution of Zechstein salt and discharge of saltwater from potash fertilizer production into the river Werra has led to a considerable mineralisation of ground and surface waters. Injection of waste water into the subsurface was introduced to reduce the discharge directly into the river. The waste water injected displaced the formation water upwards and, in areas where fault zones exist, saltwater rising occurred. In order to map and monitor the near-surface groundwater mineralisation BGR conducted regional airborne geophysical surveys in 1996/97 and 2008. Of particular interest was an area where a test disposal of saline waste water (9.5 million m³) in the Gerstungen syncline took place between 1999 and 2007. Although complicated by the use of different helicopter-borne electromagnetic systems both surveys could be successfully compared with respect to map the near-surface groundwater mineralisation and to outline possible changes due to the injection of saline waste water caused by the potash mining industry. Based on apparent resistivity maps no significant increase in groundwater mineralisation could be substantiated within the achievable exploration depths of about 20 m (saltwater ponds) and 200 m (hard rock)
-