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  • 1
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 119 (1994), S. 0 
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The problem of 3-D inverse modelling in Direct Current (DC) surveys is addressed in this paper. First, forward modelling of the response of 3-D bodies in DC surveys is carried out by the moment method. It consists of dividing a volume into N small cells, equivalent to 3N dipoles. the numerical code is checked against published results obtained through algorithms that use either equivalent surface charge densities or a finite-difference approach. Good agreement is found between these methods and a maximum discrepancy of 3 per cent is computed on a widely published test model.Secondly, inverse modelling is carried out by a classical least-squares (LS) scheme that includes the Levenberg-Marquardt constraints formalism. We have tested two approximations: Born, and localized non-linear (LN). the difference between resistivities calculated with and without these approximations is found to be too large for inverse modelling, especially in the case of conductive bodies. We use this inversion scheme for different theoretical 3-D models that consist of two layers (34 cells) under an overburden. It is found, in the case of a vertical contrast, that, when a resistive feature overlays a conductive one, resistivities are resolved very accurately, with a low number of iterations and with a better accuracy than in the case where the conductive feature overlays the resistive one. Despite a slower convergence rate, in the case of both vertical and lateral constrasts, the shape of the body is well resolved, even if a slight discrepancy in the absolute values is noticed, especially for conductive cells. Finally, the stability of the inversion is tested with noisy data.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The aim of the Couy experiment in the ‘Géologie Profonde de la France’ programme was to reach the source body of the magnetic anomaly of the ‘Bassin de Paris'. The drilling stopped at 3500 m. Magnetic field logging and susceptibility logging were undertaken to obtain as complete a description as possible of the magnetic parameters of rocks crossed by the borehole. Remanent magnetization and susceptibility measurements on the core were also carried out. The entire set of results is presented in this paper for both sedimentary layers and the basement. The values obtained for magnetic field variations and for susceptibility lead to the conclusion that the magnetic source body of the AMBP was not reached.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 44 (1996), S. 0 
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: Electrical conductivity mapping is a prerequisite tool for hydrogeological or environmental studies. Its interpretation still remains qualitative but advantages can be expected from a quantitative approach. However a full 3D interpretation is too laborious a task in comparison with the limited cost and time which are involved in the majority of such field studies. It is then of value to define the situations where lateral variations are sufficiently smooth for a 1D model to describe correctly the underlying features.For slingram conductivity measurements, criteria allowing an approximate 1D inversion are defined: these mainly consist of a limited rate of variation over three times the intercoil spacing.In geological contexts where the weathering has generated a conductive intermediate layer between the underlying sound rock and the soil, this processing can be applied to determine the thickness of the conductive layer from the apparent resistivity map when the other geoelectrical parameters are known. The examples presented illustrate this application.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Science Ltd
    European journal of soil science 54 (2003), S. 0 
    ISSN: 1365-2389
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: There is no simple and general relationship between the thermal conductivity of a soil, λ, and its volumetric water content, θ, because the porosity, n, and the thermal conductivity of the solid fraction, λs, play a major part. Experimental data including measurements of all the variables are scarce. Using a numerical modelling approach, we have shown that the microscopic arrangement of water influences the relation between λ and θ. Simulated values for n ranging from 0.4 to 0.6, λs ranging from 2 to 5 W m−1 K−1 and θ from 0.1 to 0.4 can be fitted by a simple linear formula that takes into account n, λs and θ. The results given by this formula and by the quadratic parallel (QP) model widely used in physical property studies are in satisfactory agreement with published data both for saturated rocks and for unsaturated soils. Consequently, the linear formula and the QP model can be used as practical and efficient tools to investigate the effects of water content and porosity on the thermal conductivity of the soil and hence to optimize the design of thermal in situ techniques for monitoring water content.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    Oxford, UK; Malden, USA : Blackwell Science Ltd
    European journal of soil science 55 (2004), S. 0 
    ISSN: 1365-2389
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Soil cracks formed by natural processes play a key role in water and gas transfer. Patterns of soil cracks are, however, difficult to characterize. Our aim here is to assess the effectiveness of three-dimensional electrical resistivity surveys in detecting soil crack networks. A three-dimensional electrical survey was carried out by a square array quadripole with Cu–CuSO4 electrodes (electrode spacing of 3 cm). The measurements were made with two orientations (0° and 90°) on a block (26 cm × 30 cm × 40 cm) of soil while it dried for 18 days under controlled conditions. Two indexes, calculated from the apparent resistivity values, were evaluated to detect the degree of soil heterogeneity: (i) an anisotropy index based on the ratio between the apparent resistivity at 0° and that at 90°; and (ii) the angle-array orientation corresponding to the preferential anisotropic orientation (maximum resistivity). The anisotropy index provided information on the presence of cracks and the orientation for crack width 〉 1 mm in the first pseudo-depth (i.e. depth of investigation), while the angle-array orientation provided information on crack extension for the whole pseudo-depth. Information about the presence, position, orientation and extension of cracks can be obtained from an analysis of apparent resistivity obtained by a three-dimensional electrical survey. Such direct analysis will help the resistivity inversion to detect the crack network.
    Type of Medium: Electronic Resource
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  • 6
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: The aim of this paper is to point out the advantages of multipoles for the exploration of the very near subsurface (0–3 m) by continuous profiling. We propose a new geometry with eight poles for a MUltipole Continuous Electrical Profiling (MUCEP) measuring system, where the array has a V-shape and is thus called ‘Vol-de-canards’. A series of criteria including 3D numerical simulations are performed (direct and inverse modelling) to determine the optimal geometry and to compare its performance (in terms of depth of investigation and resolution of the geometry of the targets) with the other arrays (quadrupoles or rectangular-type multipoles). This multipole was built together with a real-time acquisition system. The multidepth maps obtained confirm the characteristics predicted by numerical simulations.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Science Ltd
    Geophysical prospecting 49 (2001), S. 0 
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: Electrical resistivity tomography was used in Beauce (France) to assess the water extraction by corn plants (evapotranspiration). The acquired pseudosections show conductive anomalies under the plants. A 2D inversion of measurements led us to identify clear resistive features associated with the water losses under the corn-plant rows. New models have been calculated with two different 3D algorithms (finite-difference and moment-method) to take into account 3D structure of the ground and to confirm that periodic resistive features may generate shifted apparent-resistivity anomalies.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    Amsterdam : Elsevier
    Advances in Space Research 12 (1992), S. 3 
    ISSN: 0273-1177
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Type of Medium: Electronic Resource
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