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8th EEGS-ES Meeting
- Conference date: 08 Sep 2002 - 12 Sep 2002
- Location: Aveiro, Portugal
- ISBN: 972-789-071-7
- Published: 08 September 2002
121 - 131 of 131 results
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Realization and assessment of T1 measurements with surface nuclear magnetic resonance
Authors O. Mohnke, U. Yaramanci and G. LangeThe method of Surface Nuclear Magnetic Resonance (SNMR) is based on the excitation of hydrogen protons in the mobile pore water by a magnetic field oscillating with the local Larmor frequency. In practice the excitation is realized by an alternating current flowing through a transmitter loop at the surface. The excitation intensity defines the investigation depth of the method and is characterized by the pulse moment q (the product of the current I and the pulse duration τ). Increasing the pulse moment q, the NMR excitation focuses on greater depths. After the termination of the stimulating pulse the magnetic resonance field caused by the precession of the hydrogen protons (acting as small dipoles) around the axis of the geomagnetic field, is measured using the same loop as receiver [1, 2]. The signal amplitude is directly proportional to the amount of free water in the pore space. The relaxation behavior of the NMR signal (decay time constant) is dependent on the effective pore size of the material (Fig. 1). SNMR is the only geophysical method, that allows a direct characterization of aquifers by surface measurements. In SNMR usually the free induction decay (FID) of the excited hydrogen protons is measured (Fig. 2, A). This decay is described by the relaxation time T2* and is strongly affected by diffusion processes caused by static geomagnetic field inhomogeneities (Fig. 1). In well logging NMR and laboratory NMR such field dependent diffusion processes can be eliminated by the application of suitable pulse sequences (e.g. Inversion Recovery, CPMG). Thus, the measured relaxation times are defined only by the pore sizes of the material and are not affected by diffusion processes [3]. With the upgraded NUMIS Plus system it is possible to determine the longitudinal relaxation time T1 of SNMR signals that is independent of diffusion processes. Analogous to a 90° – 90° pulse sequence in the laboratory NMR, two successive NMR pulses of the same intensity q are emitted (Fig. 2, B). However, in SNMR there is no constant excitation over the full range of the investigated volume (i.e. constant tilt angle distribution of the magnetic moments), but rather an area of maximum excitation (tilt angle) within the focused depth. Therefore, for T1 measurements with SNMR only a quasi 90° - 90° pulse sequence can be realized. From the NMR response of the ground water a corresponding time constant T1 can be derived [4]. For sake of measurement progress default T1 measurements are conducted using only a single pulse sequence, i.e. two reading points. With the recent upgrade of the NUMIS Plus system of the Federal Institute for Geosciences and Resources (BGR) this technique is now also available in Germany.
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Geoelectrical null-arrays
More LessThe term “geoelectric null-array” is introduced for those D.C. electrode configurations, where the measured or the interpreted potential difference is zero above a homogeneous half space. A classification of all known null-arrays is shown in Figure 1. While the arrays belonging to the second column (where not the measured, rather the derived quantities are zero) and to the third one (that is the focussed arrays) have already been used earlier, the arrays in the first column (where directly the measured value is zero) – in spite of their simplicity – were used at first by the authors. These latter arrays represent current fields at various typical distances from the current electrodes, and at the same time, they can be easily constructed from traditional arrays (Szalai et al., 2002). The pairs of traditional and non-traditional arrays, presented in Figure 2, are as follows. I) MN (M0N0) is close to one of the current electrodes. In this case the current lines are nearly radial around the current electrode. According to the analogy of the classical three-electrode configuration AMN, the AM0N0 configuration is called as the three-electrode null-array; II) MN (M0N0) is between the two current electrodes. Here the current lines are nearly parallel. A special situation in this group is the Schlumberger (or AM0N0B) null-array, when the center of M0N0 is in the connecting line between A and B, at an equal distance from both current electrodes; III) MN (M0N0) is far from the current electrodes. In this case the current source may be considered as a dipole. Two null-arrays can be easily constructed from well-known traditional configurations: IIIA) the dipole axial null-array; IIIB) the dipole equatorial null-array.
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Shallow, S-wave reflections over lagoon deposits
Authors R. Ghose, F. Almeida, H. Hermosilha, F. Bonito and C. CardosoThe use of seismic shear (S) wave reflections to geotechnical characterization of the shallow subsoil has got a new meaning in the recent years with the availability of specialized hardware like the high-frequency shear-wave vibrator (Ghose et al., 1996), and new approaches for integration of seismic and geotechnical parameters (Ghose and Drijkoningen, 2000; Ghose and Goudswaard, 2001). The importance of S waves in geotechnical engineering is well-known. However, systematic correlation of S-wave reflections, indicating change in small-strain, elastic properties, to changes in large-strain, failure or strength properties of the soil is a recent observation. The correlation of S-wave reflections to changes in CPT (Cone Penetration Test) cone resistance has been found at common soil sites composed primarily of sand, clay and peat layers (Ghose et al., 1996; Ghose and Goudswaard, 2000; Brouwer et al., 1996, 1997; Goudswaard and Ghose, 2001). Recently, in relation to planned engineering constructions on landfills overlying non-tropical lagoon deposits, we have carried out shallow seismic reflection experiments, and investigated the correlation between S-wave reflections and CPT data. S-wave reflectivity information for the mud-silt and mud-sand interfaces, typical for lagoon deposits, was unknown. The objective of our research, directly related to practical engineering needs, was two-fold: (1) to check the potential of shear wave reflections for laterally continuous delineation of the subsoil boundaries in a lagoon deposit, and (2) to verify the correlation of S-wave reflections with CPT cone resistance.
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Integrated use of geo-electrical methods for subsurface salinity distribution mapping
Authors E. Slob, E. Bloem. V. Post and K. GroenGeophysical methods are established in groundwater investigations (Meju et al., 1999, Meju, 2000); their importance is underpinned by the fact that (1) groundwater distribution is controlled by mapable geological factors, (2) groundwater quality is controlled by geochemical factors, and (3) rock resistivity is inherently related to porosity, fluid content and chemistry. The electrical conductivity of the subsurface is highly influenced by dissolved solids in groundwater making electrical and electromagnetic (including GPR) methods indispensable in groundwater quality studies. Because of this, the use of electric and electromagnetic methods in hydrological research in coastal areas is well established for a long time (van Dam, 1976; Boekelman, 1991). These methods were used to obtain information on the vertical changes in the subsurface resistivity at a limited number of points on the surface (vertical soundings), or on the lateral changes at a fixed apparent depth (profiling). Both are in fact one-dimensional methods. Modern electric earth resistivity meters allow us to make true two- and three-dimensional images of the subsurface resistivity distribution. Modern computers and software allow the construction of these 2D and 3D models within a reasonable amount of computation time compared with acquisition time. Depending on the size of the research area, the use of these modern methods is feasible if used in an integrated way. We have investigated this approach to map the subsurface salinity distribution in de Nieuwe Keverdijkse Polder, NKP, in the Netherlands. As the area of investigation is approximately 15 km2, a first quick survey is carried out with a fixed frequency, fixed coil-spacing induction method (EM34), then at several locations in-situ water conductivities are measured, based on these combined results a number of detailed multi-electrode electric resistivity line surveys. To calibrate the results of these surveys, the in-situ water conductivities are translated to bulk resistivity values using the formation factor of the soil. The main purpose in this study was to investigate the added value of this integrated approach in the detailed mapping, at a scale of several meters, of the subsurface fresh and brackish water distribution. These recent developments enable the hydrologists to reconstruction of the fresh and brackish water distributions to a level of detail that is unprecedented (Post et al., 2002).
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Two and three-dimensional resistivity surveys in a landfill leachate investigation
Authors E. Slob, E. Bloem, B. van Breukelen and K. GroenGeophysical methods are established in contaminated land and groundwater investigations (Meju, 2000); their importance is underpinned by the fact that (1) groundwater distribution is controlled by geological factors that can be mapped, (2) groundwater quality is controlled by geochemical factors, and (3) rock and soil resistivity is inherently related to the porosity, fluid content and chemistry. The electrical conductivity of the subsurface is highly influenced by dissolved solids in groundwater making electrical and electromagnetic (including GPR) methods indispensable in groundwater quality studies. There is an enormous variety of contamination sources, one of which is our past waste disposal practise. It has resulted in increased salinity of groundwater and a large variation in different kinds of non-aqueous phase liquids (NAPL), both dense and light (compared to water). For electric methods, the increased salinity of the groundwater due to the spills is the key factor. For low concentrations of NAPL’s, borehole samples must be analysed, yet correlations with increased salinity may be found. For heterogeneous subsurface lithology, the salinity distribution is also heterogeneous and hence so is the NAPL distribution. From line data in an area with cross-line heterogeneities, at the scale of a survey, inaccurate model reconstructions will be obtained. Therefore, we have investigated the difference in model reconstruction results near a former landfill in the Netherlands, called Banisveld. It is shown that well positioned 3D surveys at small scales do provide better model reconstructions than those based on line data as it eliminates side effects. The flow direction of the leachate plume can be obtained from 3D surveys, or from a suitable combination of different 2D surveys.
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Improving separation of shallow S-wave reflections by source-response deconvolution of raw vibrograms
By R. GhoseSeismic shear (S) waves are important in geotechnical engineering, as shear waves address directly the small-strain rigidity of the soil. Moreover, in recent years it has been found that the amplitude of shear-wave reflections provides information of the soil strength distribution (Ghose and Goudswaard, 2000). In soft, water-saturated soils shear waves offer smaller wavelength and hence much higher resolution than the compressional (P) waves. Recently the strength of shear-wave reflection seismic has been enhanced by the introduction of a small, electromagnetic vibratory source that can generate relatively high frequency shear waves (Ghose et al., 1996). One common problem in shallow, high-resolution reflection seismic is the reverberating nature of the reflection events. This severely limits the separation between two successive reflections, posing serious problem to interpretation. Such reverberations are commonly caused by ringing nature of the source wavelet. Because of the high resolution offered by the high-frequency shear waves generated by the vibrator, the problem of lacking separation is more critical in shear-wave vibrator data than in P-wave data. For an impulsive source data, reverberation can be removed by spiking deconvolution. Spiking deconvolution requires that the seismic wavelet is minimum phase, but on a cross-correlated vibroseis trace, the seismic wavelet does not meet the minimum-phase requirement necessary for spiking deconvolution (because on a cross-correlated vibroseis trace, the seismic wavelet is the convolution of the zero-phase Klauder wavelet with the component minimum-phase wavelets due to the effects of recording instruments, coupling, attenuation, ghosts, reverberations, and other multiple reflections), and hence the final result of spiking deconvolution is less than optimal. In this paper, we have evaluated on a shear-wave reflection dataset obtained on soft soil, the effect of source response deconvolution on the separation of the very shallow reflection events.
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Role of enormous earth's magnetic fields (EMF) in etiopathogenesis of cardiovascular diseases (CVD)
Authors N. Trifunovic and S. KomatinaIn the paper, CVD etiopathogenesis occurred under the influence of enormous EMF intensities is discussed. It has been studied for more than ten years how changed values of EMF cause diseases. Effect of enormous EMF zones has been revealed, and the diseased was moved away from the influence of these enormous zones. Synthesis of two data groups has been made. Processing the first data group correlation between present enormous EMF zones and body region overtaken by the disease was determined. The second data group are theoretical explanations of unclearness found in literature concerning CVD from the EMF knowledge aspect. Coincidence between practical measuring results and theoretical explanations of many unclearness has been determined. It was concluded that enormous EMF are the main cause of many CVD and that moving the diseased away from the enormous EMF is great help to the patient and his doctor in the treatment.
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Evaluating various electrode configurations for the detection of DNAPL's with ERT
Authors B. J. M. Goes and J. A. C. MeekesIn the Netherlands there are many polluted sites that contain the highly toxic Dense Non-Aqueous Phase Liquids (DNAPL). Pure DNAPL's penetrate through sand and often accumulate on less permeable layers in the subsurface. Locating pure DNAPL product is often a bottle-neck for the successful remediation of these sites Various methods for the localisation of DNAPL's have been tried in recent years. All these methods have, up to now, hardly been applied after their trials due to high costs and/or the long duration of these methods. For this reason a project started (January 2002, see acknowledgements) to demonstrate the applicability of Electrical Resistivity Tomography (ERT) to locate pure DNAPL's in the Dutch situation. ERT is a geophysical technique that has been applied to detect or monitor electrical resistivity contrasts between boreholes (e.g. Newmark et.al. 1998). DNAPL's have a very high electrical resistivity (e.g. Schneider and Greenhouse 1992). A problem with applying ERT is that the definition of the 'best' measurement schedule (a list of four-electrode configurations to be addressed by the resistivity meter) remains a poorly resolved problem (Labreque quoted in Slater et.al. 2000). The project consists of three phases: 1) selecting promising electrode configurations on the basis of a literature study and modelling, 2) demonstrating ERT at a site with a shallow (~5 m below the surface) pollution, 3) demonstrating ERT at a site with a deep (~20 m below the surface) pollution. This paper presents some of the results from the modelling and the field measurements at the first site.
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A coaxial transmission line for accurate permittivity measurements from 300 MHz to 3 GHz
Authors A. G. Gorriti, E. C. Slob and J. BruiningThe permittivity of porous media is usually measured from very low frequencies up to the giga- Hertz region, where it becomes constant for most natural materials and which is also the range of interest for field applications using geo-radar techniques. On the low frequency regime, up to hundreds of mega-Hertz, the so-called direct methods are being used. Generally, the material is placed between two parallel plates and the impedance or admittance of this capacitor is measured so that the permittivity is calculated directly from these measured quantities, see for i.e. [Bona et al. 1998] and [Shen et al. 1987]. On the high frequency regime, the relation between the permittivity and the measured quantities is no longer linear for reasonably sized sample holders and more complex set-ups have to be implemented. It is common to place the material in a coaxial transmission line [Shen 1985] and [Nguyen 1999] or coaxial-circular wave guide [Taherian et al. 1991] and measure the S-parameters of the set-up with a Network Analyser. The S-parameters are then modelled using transmission line theory. By means of an optimisation procedure the permittivity is calculated, or when possible, the S-parameters of the section of the line filled with the material are extracted [Shen 1985, Compensation Parameters method] and then the permittivity is calculated directly from an explicit analytic expression [Weir 1974]. We have designed our tool such that the same material can be characterised over the whole frequency range, so that it can work as a coaxial capacitor in the low frequency regime and as a coaxial transmission line in the high frequency regime. Moreover, we want to test which of the two existing methods to calculate permittivity from the S-parameters lead to the best results. The method known as the Compensation Parameters was not satisfactory because it suffers from many resonant frequencies at which the calculated parameters are unstable. In this paper we present the tool and its forward model, in a companion paper [Gorriti et al., 2002] we present the results obtained for several materials and the limitations of the technique.
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Accurate reconstruction of permittivity from coaxial transmission line measurements
Authors A. G. Gorriti, E. C. Slob and J. BruiningIn the high frequency regime (mega-Hertz to giga-Hertz) accurate permittivity measurements become complicated, as the relation between the measured quantities from which it can be derived and the permittivity itself, becomes highly non-linear. It is common to place the material in a coaxial transmission line [Nguyen 1999] or coaxial-circular wave-guide [Taherian et al. 1991] and [Belhadj-Tahar et al. 1990] and measure the S-parameters of the set-up with a Network Analyser. The S-parameters are then modelled using transmission line, or full-wave theory depending on the type of the sample holder. By means of an optimisation procedure the permittivity is then calculated. However, the coaxial-circular wave-guide technique requires relatively small samples, and so far, no fluid flow through the sample has been achieved. To our knowledge, this is the first time that the full scattering (S-parameter) matrix is modelled with transmission line theory, improving the accuracy of the results. We have designed a coaxial transmission line to perform accurate permittivity measurements in the high frequency regime for relatively large sandy samples. It also allows for simultaneous measurements of capillary pressure, so that electrical properties can be related to the saturationdesaturation history of the sample. In a companion paper [Gorriti et al., 2002] we present the tool and the accuracy of its corresponding forward model. In this paper we present the results obtained for several calibration materials and the limitations of the technique so far.
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Multilayered model for the electro-kinetic effect
Authors A. Ranada and C. P. A. WapenaarThe electro-kinetic effect represents a class of processes in which there is a conversion from electromagnetic to kinetic energy and vice versa. In the case of this transfer taking place in a saturated porous medium we name the effect electro-seismic for the transfer from electromagnetic to kinetic energy, and seismo-electric for the transfer from kinetic to electromagnetic energy. There exist other effects sometimes called electro-kinetic like the piezo-electric effect or the modulation of rock resistivity by seismic waves, but we will not take those effects into account in this paper. This interaction between seismic and electromagnetic waves is due to the relative motion of the electrically charged ions in the pore fluid. When in equilibrium a porous medium saturated with an electrolyte is electrically neutral, but if a wave, seismic or electromagnetic, perturbs this equilibrium, the relative motion of the ions in the pore fluid will generate both seismic and electromagnetic waves. In the case of a passing seismic wave the flow of ions and the consequent electric imbalance generates electromagnetic waves. If the passing wave is electromagnetic there will be an induced flow of fluid in the pores that will be transmitted as a seismic wave. The existence of the seismo-electric conversion is known since the early 1930s, however in all the published papers the main topic is the generation of an electromagnetic wave as a fast P wave hits an interface (usually the water table), showing this method to be a useful tool to characterize parameters like fluid content and fluid geochemistry in the Earth's subsurface. Although the seismo-electric effect is the most known, in our model we deal with it as a particular case of the electro-kinetic effect, which contains all the interactions between seismic and electromagnetic waves in a layered porous medium. It is then very interesting to look at the possibilities offered by the use of shear waves as well as the not so well known electro-seismic effect, which could be a very promising prospecting tool. Possible applications include groundwater detection and monitoring of pollutant migration. This effect can possibly be also employed in borehole measurements as a way to determine permeable formations or monitoring multiphase flow through porous areas.
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