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4th EEGS Meeting
- Conference date: 14 Sep 1998 - 17 Sep 1998
- Location: Barcelona , Spain
- ISBN: 978-94-6282-127-9
- Published: 14 September 1998
61 - 80 of 236 results
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Gravity and seismic reflection applied to the hydrogeological research in a large Alpine valley
Authors A. Rosselli, R. Olivier and G. VeroneseThe determination of the bedrock morphology is an important parameter for the purpose of the hydrogeological research. In the case of large alpine valleys the bedrock depth may vary from few hundreds to thousand meters. Furthermore, structural differences in the sedimentary body, generally linked to lateral valley contribution, are another important parameter to take into account. In the outline of a research project of the Geological Survey of the Province of Trento (ltaly), several gravity profiles was measured in the Adige valley, neighboring the urban area of Trento. A 2.5 D model was elaborated on the base of the gravity data. Geology, borehole information and seismie reflection was used to constrain the 2.5D gravity modeling.
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Hydrogeological and geophysical investigation for evaluating the salt/fresh water distribution in Saidia plain (North-Eastern Morocco)
Authors K. Walraevens, N. El Ualimi, A. Chaouni Alia and E. BeeuwsaertNorth-eastern Morocco has a Mediterranean c1imate, with low rainfall, which is irregularly distributed both within the year and over succeeding years. The c1imate is very dry in summer, and moderately to weakly humid in winter. The coastal area shows groundwater salinization, which is due to the presence of connate saltwater or to sea water intrusion. Low salinity water resources are scarcc, and should be carefully managed. The present study focuses on an aquifer system in the Mediterranean coastal area of northeastern Morocco. It is situated in the most easterly coastal plain, up to the Aigerian border: the eastern part of Saidia plain. This plain is mostly dispersely populated, with agricultural activity, while the city of Saidia is a summer resort. In the western part, outside the studied area, a large sea fish farm is causing extensive groundwater salinization by leakage from sea water filled basins. Saidia plain is an elongated area, squeezed in between the Ouled Mansour hills and the sea. To the east, it is bounded by the Aigerian border, following the Kiss River. From north to south, several parts, occurring as longitudinal, parallel strips, can be distinguished (fig. I): the shore, the northern dunes, the Sareg mud flat, the southern dunes, and finally the Merzakan mud flat, up the Ouled Mansour hills.
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Optimization of water wells through conventional logs
Authors J. M. Dîaz Curiel, D. Martin, N. Caparrini and A. MaldonadoThe need of increasing the quality of water wells is contributing to extend the use of geophysical well logging, as an essential technique to optimize the exploitations and to control of the ground-water resources also. Besides the development of new technologies and interpretation methods, the popularisation of this technique requires the optimization of the results obtained from the different welllogs (even maintaining the employment of a relative low cost devices), arousing the interest of the technicians involved with the different fields related with boreholes. In this communication, we present a summary of the conventional weIl logs with a higher hydrogeological application, The possibilities of each of them are pointed out in a schematic way. Overall, the use of geophysical logs in the hydrogeology's field can be divided in two main groups: the conventional logs used during the drilling phase, which will determine the constructive characteristics of the borehole, and the specific logs used in casing(piped) boreholes, which will define the optimum design of the exploitation.
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Control network (/T for the investigation of the marine intrusion in Castell de Ferro (Granada, Spain)
Authors D. A. Martin Sánchez, J. Diaz Curiel, A. Maldonado and N. CaparriniThe detection in every moment of the interphase (fresh/salt water) implies a great advantage controlling the advance of the marine intrusion. In this respect, the simultaneous and continuous measurement of the conductivity and the temperature in a sufficient number of borings, looks like the very best solution for this control. This publication shows the experience achieved by the Instituto Tecnológico Geominero de Espafia (I.T.G.E) and the Engineering Geology Department of the Madrid School of Mines in the coastal aquifer of the Gualchos in Caltell de Ferro, Granada province, where a control network was designed, optimised and installed. Through the analytic investigation of the results, a series of mathematic approximations have been obtained in a fast and simple way to show the possible processes of advance or regression of the sea interphase.
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H-Sense': harbour sediment mapping using Chirp reflection surveys in Norway and Sweden
Authors P. M. Maurenbrecher and T. WeverThe EdgeTech (4-24 kHz -Fish) is used in the range of 4 to 16 kHz to map the most recent, usually contaminated, top layer in the harbour and estuaries of the low tidal harbours of Bergen, Norway and Gothenburg, Sweden. The principal objective is to relate distribution patterns of the sediments with natural and man-induced processes such as sediment supply, currents, waves. storms, and vessel movements. In February and March surveys were carried out which covered the harbour areas of Bergen and Gothenburg. Both surveys succeeded in not only mapping the top sediments but also in obtaining profiles at Bergen in water depths exceeding 300 m despite the signal return period exceeding the transmission periods. In the Gothenburg estuary reflectors were obtained from 40 m below seabed despite the high frequency range of the Chirp signal showing profiles infilled channels (see Figure 2). The initial results for the H-SENSE study (Harbours: Silting and Environmental Sedimentology European) are presented for two harbour basins in Bergen and Gothenburg respectively. The study is being carried out jointly between universities and institutes in Sweden. Norway, the UK. Netherlands and Latvia within the EC-Directorate General 6 (Transport) research programme.
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Seismic amplitude versus offset analysis of gas hydrates
Authors A. Mazzotti and S. GrionWe describe the application of AVO (Amplitude Versus Offset). analysis and high resolution velocity analysis to evaluate the elastic properties of Bottom Simulating Reflectors located in the South Shetland margin and in the South Chile offshore. Two marine seismic data sets are available for the study and were subjected to a full amplitude-preserved processing. This presented some difficulties due to the presence of noise related to the bad weather conditions during the acquisition. For this reason, after array directivity compensation and geometrical spreading correction, surface consistent amplitude corrections were applied and particular attention was given to the picking of the AVO trend of the target reflectors. This AVO has been used for a single interface elastic inversion of the PP reflection coefficient. The results given by high resolution velocity analysis, based on the a priori estimation of the wave let, and by AVO inversion indicate that both the South Chile and the South Shetland Bottom Simulating Reflectors are associated with interfaces showing high P-wave velocity (Vp) in the upper medium, followed by a consistent drop in Vp and in VpNs ratio in the lower medium. These results are consistent with the presence of a layer fully saturated and consolidated by gas hydrates overlying unconsolidated sediments with free gas (partial) saturation.
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The SEISCAN image archive: an exercise in environmental geophysics
Authors P. R. Miles, M. Schaming, A. Casas, M. Sachpazi and A. MarchetriSeveral hundred thousand kilometres of marine seismic reflection records collected up to 30 years ago remain only as paper records; and this applies to European Exclusive Economie Zone (EEZ) surveys alone. These data records were collected during scientific programmes by academie institutions since the 1960s. Owing to deteriorating recording media (magnetic tape) and the absence of appropriate instrumentation these data cannot be re-played. Often the records remain boxed and un-referenced while gradually deteriorating; those of greater interest having suffered more by handling. SEISCAN is a project to rescue these early seismic records which, with few exceptions, have not been archived in any photographic or other reproducible form. The 3-year project is funded by the European Commission MAST-III Supporting Initiative Programme and involves 5 European partner institutions. These partners will oversee an initial phase compiling seismic record metadata prior to digital scanning and image processing. The image database created will not involve any cost to the contributing organisations.
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The Ricker pulse and sediment density measurement by acoustics
By N. RoosnekMeasuring the density of sediment in delta harbours is a major issue in proper harbor management. These measurements are carried out in the port of Rotterdam by nuclear radiation absorption. Carrying out sediment measurements by acoustics might be possible. The gradient of the sediment density causes sound echoes. Assuming that there is a one-to-one relation between impedance and density, a measure for the gradient is obtained by deconvolution of the echo. Integration of the gradient after correcting for the loss of the probing acoustic pulse gives a measure for density. To obtain the sign information a wide frequency-banded signal has to be used. Such a signal can be generated by nonlinear acoustics.
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Beach dewatering system, results from a pilot plant installation in the Ebro Delta
Authors J. Serra, R. Bautista, C. Montori and L. Parente MaiaBeach erosion problem has been considered and combated for a long time by different solutions since the loss of beaches has a direct impact on local economy, mainly the touristic one. Hard structures defences are in comparison with soft engineering solutions less effective and environmentally acceptables, and between the last ones the Beach Dewatering System, or BMS, has been tested in the regressive deltaic coast of the Ebro delta. The system is based in the lowering of the water table and the creation of an unsatured zone in the beach face that facilitates the percolation of the water from the up and backwash. With the less water in the backwash and reduced surge height, less sand will be brought back to sea and then trapped in the beach face (Vesterby 1997; Oversen 1992). Because of the complexity of the beach structure and controlling parameters, a geophysical support by seismic or georadar of that sedimentary bodies can help the knowledge and the response of the system.
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High resolution marine seismic surveying as quality control tool for the construction industry
Authors G. U. J. Swoboda and D. OrlowskyMulti Channel Marine Seismics is well known for Oil and Gas surveying. The application of this technique at problems of the construction industry or at engineering problems needs some modifications of the seismic equipment and high accuracy in evaluating geometric parameters. The result of these modifications is the hydro-acoustic measurement system. It examines with sound waves in the range from 50 Hz to 25 KHz the underground of shallow water. Targets are reservoirs, rivers and objects in waterfilled construction areas The penetration into the bottom is down to 30m under the sea floor, depending on the seismic impedance between the layers. The use of high frequencies allows measurements of small layers in the range of one decimeter. Inevitable for good results and high resolution ist the digital seismic postprocessing of the multichannel dataset.
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Flow paths in a full scale model
Authors S. A. al Hagrey, T. Schubert-Klempnauer, O. Wachsrnuth, J. Michaelserr and R. MeissnerFor the restoration of groundwater and for an assessment of its pollution by agrochemical, e.g. pesticides, and industrial contaminants, an intensive study of flow processes through the unsaturated near surface vadose zone is necessary. Such a study is being presently carried out by a joint effort of the Institute of Geophysics and the Institute of Water Management and Landscape Ecology of Kiel University. It was known from many field experiments with tracers that rain water does not penetrate the upper vadose zone as a uniform water "front", but prefers "preferentiaI" flow paths (ScanIon et al., 1997: Hagrey & MichaeIsen 1996). This is due to organic heterogeneities caused by wormholes and roots and by inhomogeneities like a variability of soil textures, background humidity or water repellence (Ritsema et al., 1993; Hagrey & MichaeIsen, 1998). A preferential flow leads to an increase of flow velocity and diminishes the filter effect of the subsurface (Kung, 1993). A full-scale model of homogeneous sand regarding the vadose zone, was needed to develop a suitable high resolution mapping technique in space and time, using different geophysical and hydrological sensors to be distributed in the sand.
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Nuclear well logging in soils calibration of slim hole nuclear tools gamma-gamma and neutron-thermal neutron
Authors L. Baron and D. ChapellierThe calculation of elasticity parameters by sonic and ultra sonic wave propagation in saturated soils using Biot's theory needs the following variables : forpiation density and porosity (p, ø), compressional and shear wave velocities (Vp, Vs), fluid density, viscosity and compressibility (Pfi Ilfi Ki), matrix density and compressibility (p" K), The first four parameters can be determined in situ using logging probes. Because fluid and matrix characteristics are not modified during core extraction, they can be obtained through laboratory measurements. All parameters necessitate precise calibrations in various environments and for specific range of values encountered in soils. The slim diameter of boreholes in shallow geophysics and the high cost of petroleum equipment demand the use of specific probes, which usually only give qualitative results. The measurement 'of density is done with a gamma-gamma probe and the measurement of hydrogen index, in relation to porosity, by a neutron probe. The first step of this work has been carried out in synthetic formations in the laboratory using homogeneous media of known density and porosity. To establish borehole corrections different casings have been used. Finally a comparison between laboratory and in situ data in cored holes of known geometry and casing has been performed.
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Tomographic resistivity 3D mapping: filter coefficients and depth correction
Authors P. Cosentino, D. Luzio and R. MartoranaIn this paper further adjustments are presented regarding the method given by Cosentino et al. (1995, 1997) to construct rapidly 3D tomographic pictures from resistivity data. The purpose of the procedure (a filtered back-projection of the experimental data) is to obtain 2D or 3D pictures which match as closely as possible the geometry of the structures which are being investigated. In order to optimize the target some problems should be solved. Firstly, one should try to detect correctly the depth of electrical discontinuities (or anomalous bodies); secondly and not less important, one should try to determine the size as weIl the shape of the structures which are buried. In this paper we study the conneetion between the resistivity of the anomalous bodies and their effect on the apparent resistivity distribution. The results suggest that is best to optimize the values of the filters using also the negative values of the influence coefficients; they give also some information which appear useful to improve the estimate of both the depth and geometry of the bodies.
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Comparative study relating the specific water content deduced from the neutron-neutron and electromagnetic conductivity logs
Authors I. M. Fernández, J. Estaire and A. CarbóThis paper presents the transformation of the neutron-neutron and conductivity logs to specific water content. The transformation of the neutron-neutron log is made using calculated curves deduced from calibration bank, and the transformation of the conductivity log is made using laboratory data obtained in borehole samples. Out of the logs of specific water content obtained by these two procedures of calculation it is posible to draw conclusions concerning the quality of the methods and the correct application of these logs in the determination of the specific water content.
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Relations between natural gamma radiation, mineralogical and chemical properties of clays and muds from the Aveiro region - Portugal
Authors A. C. Galhano, J. Saraiva, L. Torres. F. Rocha and C. GomesThe Aveiro region (Fig. I) corresponds to the northern sector of the Portuguese Occidental Meso-Cenozoic sedimeritary basin. At the Aveiro region the upper Cretaceous is represented by the "Argilas de Aveiro" formation (Campanian-Maastrichtian), which are composed of greenish clays and marls intercalated with thin dolomitic layers. In the region, the outcrops of this formation are relatively extensive, with a maximum length (oriented N-S) of about 20 Km and a maximum width of about 5 Km, partially covered by quaternary old beach and alluvial deposits.
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Subsurface resistivity imaging using two single-step algorithms
Authors M. Gasulla, J. Jordana and R. Pallás-ArenyThis paper compares two single-step algorithms to obtain 2D subsurface resistivity images based in the sensitivity theorem The first algorithm is based in the Marquardt-Levenburg method whereas the second algorithm uses a weighted backprojection technique. An approximate' analytical solution yields the data (surface potential) for a spherical anomaly immersed in a homogeneous medium when using dipole-dipole and Schlumberger-based electrode arrays A fidelity measure quantifies the reconstruction error of each algorithm, which is defined as squared inner product of the normalized difference between the true resistivity profile and the estimate. The Marquardt-Levenburg method yields smaller errors but requires a damping factor that must be obtained experimentally. The reconstruction algorithms have been validated experimentally by placing a spherical object in a plastic water tank. Data for the dipole-dipole array are less accurate at higher depths because of the small voltages detected. Errors in the reconstructed image because of the uncertainty in electrode positioning and the finite dimensions of the phantom are partially canceled by taking a reference measurement.
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Tomographic and spectrometric sonic measurements at historical buildings
Authors H. Lindner and C. PretzschnerThe estimation of seismic veloeities in rocks with ultrasonic transmission methods is well known. Our idea is both to apply the tomographic method and to prove the usefulness of spectrometric analysis of sonic signals transmitted through materials of old buildings. The advantage of tomography is that it produces a two-dimensional picture of physical properties inside the rock. Fig. I shows this principle of transmission measurements around the surface.
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Application of electrical tomography in environmental study of oil pipeline zone in Luthuanian karst region
Authors D. Michelevicius, V. Narbutas and R. SeckusThe geological and hydrogeological and geomorphological conditions have been investigated along the oil pipeline dislocated in the Northern Lithuanian karst region. The pipeline crosses a region of geological and natural preservation - a zone of intense karstification. A particularly distinct karst structure is being seen between Roveja and Apascia rivers. Karstic subsurface represênts a vulnerable environment, where potable groundwater aquifer can be easily polluted. The objectives were to evaluate already caused and potential damage to nature as weIl as ecological hazards factors in various places along the 18 km long pipeline profile. Electrical tomography measurements have been performed along more than 50 profiles of this region. Also borehole drilling was involved for further interpretation of geoelectrical modeis. 32 control wells up to 20 m deep and 6 wells up to 35 m deep have been drilled.
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Resistivity and lP probability tomography in near-surface geophysics
Authors D. Patella and P. MaurielloWe describe a new tomographic method for the interpretation of resistivity and induced polarization (lP) field data. In recent years, tomographic imaging of geophysical data has become an important topic, due to the need of a high-resolution approach to the delineation ofburied structures, mainly in near-surface exploration (Noel and Xu, 1991; Loke and Barker, 1996). Our approach to tomographic interpretation of resistivity and lP data differs from standard inversion schemes, since it is essentially based on a probabilistic point of view.
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Drilling and borehole geophysics in alpine permafrost
Authors D. S. Vonder Muehll and C. HauckDrilling in mountain permafrost is quite different from drilling at other places: the inhomogeneous formation consisting of 'hard' rocks and 'soft' ice, and the temperature close to the melting point of ice are the most difficult of several factors. A number of parameters can be measured along the borehole facilitating the understanding of surface geophysical investigations and of ongoing processes in the permafrost Here, we present all overview of experiences from several permafrost drill sites and list some possibilities for drilling and borehole geophysical rrieasurements in mountain permafrost. This includes the saving and analysis of cores, borehole television camera, borehole logging and long-term monitoring installations (temperature, deformation). The aim of the borehole logging (Gamma-ray, Gamma- Gamma, Neutron-Neutron, Resistivity, Sonic) is to determine the thickness of the perennial frozen debris layer above the bedrock and to investigate P-wave velocity, resistivity and ice content of the whole permafrost body. In addition, main results from several permafrost boreholes in the Swiss Alps and Svalbard (Spitsbergen) are presented.
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