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- Volume 7, Issue 1, 2009
Near Surface Geophysics - Volume 7, Issue 1, 2009
Volume 7, Issue 1, 2009
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Correlation of electrical resistivity, electrical conductivity and soil parameters at a long‐term fertilization experiment
Authors Ulrike Werban, Katrin Kuka and Ines MerbachGeophysical methods are progressive, non‐destructive but indirect techniques for characterization of soil properties and mapping of soil heterogeneities. Geophysical surveys for soil mapping lead generally to ambiguous results since geophysical parameters are influenced by several soil properties, e.g., organic content, clay content and bulk density.
The investigations presented here focus on the effect of different stages of organic content on DC‐geoelectrics and electromagnetic induction (EMI) at a long‐term fertilization experiment. This experiment gives, after 105 years running, an excellent opportunity to study the correlations between electrical resistivity, apparent electrical conductivity and soil parameters. Results from DC‐geoelectrical measurements (profile length 80–160 m, electrode distance 0.5 m) twice, in August after harvest and in January during black fallow period, are presented. Additionally electromagnetical investigations were conducted in January.
Correlations of resistivity and carbon input into the soil are significant and very strong; especially in January with but contradictive in summer and winter. The analysis of resistivity and apparent electrical conductivity is critical since bulk density and water storage capacity is influenced by fertilization and plant growth. Interpretation of a combination of DC‐geoelectrics and electromagnetical techniques on agricultural areas has to be done with respect to management aspects.
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Electrical conductivity mapping for precision farming
Authors E. Lück, R. Gebbers, J. Ruehlmann and U. SpangenbergPrecision farming overcomes the paradigm of uniform field treatment by site‐specific data acquisition and treatment to cope with within‐field variability. Precision farming heavily relies on spatially dense information about soil and crop status. While it is often difficult and expensive to obtain precise soil information by traditional soil sampling and laboratory analysis some geophysical methods offer means to obtain subsidiary data in an efficient way. In particular, geoelectrical soil mapping has become widely accepted in precision farming. At present it is the most successful geophysical method providing the spatial distribution of relevant agronomic information that enables us to determine management zones for precision farming.
Much work has been done to test the applicability of existing geoelectrical methods and to develop measurement systems applicable in the context of precision farming. Therefore, the aim of this paper was to introduce the basic ideas of precision farming, to discuss current precision farming applied geoelectrical methods and instruments and to give an overview about our corresponding activities during recent years. Different experiments were performed both in the laboratory and in the field to estimate first, electrical conductivity affecting factors, second, relationships between direct push and surface measurements, third, the seasonal stability of electrical conductivity patterns and fourth, the relationship between plant yield and electrical conductivity. From the results of these experiments, we concluded that soil texture is a very dominant factor in electrical conductivity mapping. Soil moisture affects both the level and the dynamic range of electrical conductivity readings.
Nevertheless, electrical conductivity measurements can be principally performed independent of season. However, electrical conductivity field mapping does not produce reliable maps of spatial particle size distribution of soils, e.g., necessary to generate input parameters for water and nutrient transport models. The missing step to achieve this aim may be to develop multi‐sensor systems that allow adjusting the electrical conductivity measurement from the influence of different soil water contents.
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Influence of tillage methods on soil water content and geophysical properties
Authors Martin Müller, Gerhard Kurz and Ugur YaramanciThis paper focuses on the development of a geophysical methodology as a tool for agricultural soil research particularly concerning the impact of different tillage. Geophysical interest laid on the suitability (practically and technically) resolution limits of the applied methods; on the reliability of the data on small horizontal and vertical scales (cm–dm) in volumetric water content (VWC) determination as well as consequences for the interpretation in terms of water retention capacity.
In parts of the investigated area a large field was divided into sub‐fields, each of them subjected to different tillage since about 15 years: normal tillage (field I, with a 0.3 m deep impact plough), conservative tillage (field II, zero tillage, just seeding) and mulch tillage (field III, just the uppermost 0.1 m ploughed). Measurements were performed on two parallel profiles with geoelectrics, radar, induced polarization and nuclear magnetic resonance.
The contrast of VWC and electrical resistivity was always larger between fields I and II on profile 1 as well as on profile 2 but for one exception: for the ground‐penetrating radar (GPR) data of August 2004 on profile 2 the largest contrast was between fields I and III, nevertheless the contrast between fields I and II was nearly as large as between fields I and III. How could the type of tillage of fields I and II be distinguished by the contrast of their physical parameters? Whereas on profile 1 field I was dryer and field II the wetter, on profile 2 field I was the wetter and field II the dryer one. On the other hand the laboratory samples from profile 2 showed a similar behaviour as the field data from profile 1. This (apparent) contradiction may have come from the respective precipitation history. Whereas some of the measurements have been performed during and shortly after a rain event, other measurements were performed after a longer dry period. During and directly after precipitation the soil on the unploughed field II could take up more, whereas the soil on the ploughed field could not hold as much water.
The first layer of field I was more inhomogeneous than the first layer of field II and the second layer of field I was more homogeneous than the second layer of field II. The differences in homogeneity were caused by destructive process of the plough, which loosens the soil and exposes the soil to the weathering.
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A least‐squares standard deviation method to interpret magnetic anomalies due to thin dikes
Authors E.M. Abdelrahman, K.S. Soliman, E.R. Abo‐Ezz, T.M. El‐Araby and K.S. EssaWe have developed a least‐squares method to determine simultaneously the depth and the horizontal position (origin) of a buried thin dike that extends in both strike direction and down dip (2D) and in which the depth is much greater than the thickness from horizontal gradients obtained numerically from magnetic data using filters of successive window lengths. The method involves using a relationship between the depth and the horizontal position of the source and a combination of windowed observations. The method is based on computing the standard deviation of the depths determined from all horizontal gradient anomalies for each horizontal position. The standard deviation may generally be considered as a criterion for determining the correct depth and the horizontal position of the buried dike. When the correct horizontal position value is used, the standard deviation of the depths is less than the standard deviation using incorrect horizontal position values. This method can be applied to residuals as well as to the observed magnetic data. The method is applied to synthetic examples with and without random errors. The present method was able to provide both the depth and horizontal position of the source accurately. The practical utility of the method is tested on an outcropping dike in Canada.
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The use of 2D electrical tomography to assess pollution in slurry ponds of the Murcia region, SE Spain
Authors P. Martínez‐Pagín, A. Faz and E. AracilA large quantity of pig slurry is produced in the Murcia Province, raising serious environmental concern. Monitoring the evolution of the slurry ponds with time in the subsoil is a prime requirement for environmental safety. Our aim was to determine the temporary effects of pig slurry ponds on the subsoil in the Murcia Province to establish the degree of slurry pond infiltration in semi‐arid climates. A non‐destructive, geophysical, 2D electrical tomography technique was used to: 1) monitor the vertical movement of pig slurry into the subsoil and 2) determine the possible depth of this movement. The results of our studies showed that the method works well. Areas affected by pig slurries have, indeed, developed environmental problems over time. The method identified the lithological layers with sufficient resolution in order to study the possible pollution of the soils by the slurry, including salts. The method is also capable of monitoring seasonal changes and the time‐dependent behaviour of the polluting plume within different subsoils in the ponds.
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Inversion of airborne time‐domain electromagnetic data to a 1D structure using lateral constraints
Authors Marc A. Vallée and Richard S. SmithWe invert time‐domain airborne electromagnetic data in such a way as to obtain a model that varies slowly along the profile. This is achieved by modifying a typical one‐dimensional inversion algorithm to include lateral constraints. The lateral constraints are included as a roughness matrix that is solved simultaneously with the Jacobian matrix in an iterative eigenparameter inversion. In this case, multiple soundings along a line are all solved simultaneously. The lateral constraints can be applied to the resistivities and the thicknesses, both separately and together. We apply these techniques in two situations where airborne geophysical data are applied to near‐surface exploration. The first case is in a resistive environment where we are interested in quantifying a superficial conductive overburden. In this case, lateral constraints improve the geological image compared with those images obtained using unconstrained layered‐earth inversion. In the second case, we want to map the thickness of a resistive aquifer covering a saline layer. In this case, we show how varying the weights on the lateral constraints can change the image of the thickness of the aquifer. The presence of numerous cultural artefacts makes the inversion problematic. Application of a first‐difference constraint did the best job at removing culture but resulted in oversmoothing of the hydrogeology. The use of a second‐difference lateral constraint gave a good rendition of the hydrogeology but did not suppress the culture as well as the first‐difference constraint.
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Volumes & issues
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Volume 22 (2024)
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Volume 21 (2023)
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Volume 20 (2022)
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Volume 19 (2021)
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Volume 18 (2020)
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Volume 17 (2019)
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Volume 16 (2018)
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Volume 15 (2017)
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Volume 14 (2015 - 2016)
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Volume 13 (2015)
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Volume 12 (2013 - 2014)
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Volume 11 (2013)
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Volume 10 (2012)
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Volume 9 (2011)
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Volume 8 (2010)
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Volume 7 (2009)
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Volume 6 (2008)
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Volume 5 (2007)
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Volume 4 (2006)
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Volume 3 (2005)
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Volume 2 (2004)
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Volume 1 (2003)