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- Volume 6, Issue 5, 2008
Near Surface Geophysics - Volume 6, Issue 5, 2008
Volume 6, Issue 5, 2008
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Multispectral processing of high‐resolution radiometric data for soil mapping
More LessABSTRACTSoil radioactivity is dominantly a product of the local geology and mineralization, physical and chemical weathering processes within the profile, and soil organic content. Recent studies have shown that local soil patterns can be confidently mapped at scales of 1:10 000 using new high spatial resolution airborne and ground radiometric data acquired with standard 256‐channel gamma‐ray spectrometry systems and sodium‐iodide (NaI) crystal packs. In this study, a new multispectral processing methodology was developed to assess the 256‐channel spectral soil response. The study identified seven distinct gamma‐ray energy peaks, all products of the potassium‐40, uranium‐238 or thorium‐232 decay series, whose spatial and spectral relationships could be used to interpret soil properties and/or soil conditions.
In the case studies presented, interpretation of complementary thorium and uranium channels (isolated using the new multispectral processing methodology) reveal additional spatial patterns and spectral information that were unattainable using standard processing methods. In one example, soil conditions attributed to a local, developing clay‐soil unit were resolved by the increased 228Ac (actinium) to 208Tl (thorium) channel response. In a separate example, a change in ratio between 214Bi (uranium) energies 1120 keV and 1764 keV, produced during the same decay emission, was used to differentiate soil conditions in the top 40 cm of an expansive sandy loam on clay duplex soil unit. A decrease in the 1120 keV to 1764 keV ratio was interpreted as an increase in soil density as the lower gamma‐ray energy emissions are preferentially attenuated with increased density. Relative increases in the 1120 keV and 1764 keV channel responses were interpreted as an increase in uranium concentration in the soil profile. With the clay as the dominant uranium contributor, integrated interpretation of the 214Bi channels suggested that in these areas there was an increase in clay percentage within the surface sandy loam, which may result in local increased water‐logging susceptibility.
This paper also theorizes that interpretation of the spatial displacement of thorium‐232 decay series 228Ac and 208Tl peaks, produced from decay events approximately 2 years (half‐life) apart, can be used to identify decay series disequilibrium conditions resulting from local soil–water processes or other recent soil activity. This information contributes to an understanding of the soil processes active in the area. In addition, identification of disequilibrium decay conditions may highlight locations where standard radiometric (radioelement) processing, which relies on equilibrium conditions, may be erroneous.
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Agronomic consequences of potential management zones delineated on the basis of EM38DD measurements
Authors U.W.A. Vitharana, M. Van Meirvenne, D. Simpson, L. Cockx and G. HofmanABSTRACTThis study was conducted to investigate the feasibility of dual dipole apparent electrical conductivity (ECa) measurements obtained with an EM38DD sensor as an explicit information source to delineate potential management classes in an agricultural field in the Polder area of Belgium. The success of class delineation was evaluated in relation to soil physical‐chemical properties and sugar beet yield. The average apparent electrical conductivity (ECa‐A) derived from vertical and horizontal dipole measurements was capable of delineating 3 relatively large management classes. The analysis of variance of soil properties indicated that topsoil sand and lime and subsoil clay, sand and lime were largely different across these classes (all having a proportion of the variance accounted for by a classification of >0.5). During the growing season of 2005, we monitored topsoil and moisture content and found strong differences among classes. As a result, the crop biomass at harvest (roots plus leaves) was strongly variable between classes (ranging from 105 Mg ha−1 to 153 g ha−1), as well as the sugar content (ranging from 15.4% to 17.2%). However, due to a compensation effect between the crop biomass and sugar accumulation, differences in sugar yield and financial income between classes were relatively small (the income ranged from 3950 € ha−1 to 4230 € ha−1). However, these income values resulted from strongly different growing conditions, calling for a class‐specific management. The image of the average ECa was found to be a reliable basis for delineating agronomically relevant management zones.
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Quality control of compacted grounds using seismic velocities
Authors Osman Uyanik and Emin U. UlugergerliABSTRACTThe speed‐up plaque load experiment and a seismic refraction survey were used in quality‐control tests of compaction in the runway grounds of the Süleyman Demirel Airport, Isparta, located to the north of Burdur Lake, Turkey. The straight‐line correlation between plaque load experiments and seismic velocities ( and ) has shown that compaction is sufficient when both the and values exceed 590m/s and 379m/s, respectively. In addition, two‐dimensional velocity models for both P‐ and S‐waves map out the level of compaction along the profiles.
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Underwater self‐potential measurements in the safety assessment of dams
Authors A. Rozycki, C. Olalla, J.M. Ruiz Fonticiella and A. CuadraABSTRACTA small earth‐cored dam, the water balance of which indicated water losses, was chosen to test the applicability of the underwater self‐potential (SP) method. The data were recorded using a measurement system composed of an electrode dragged over the upstream face of the dam and attached to a well‐logging instrument. The measured electrical potential was referred to a fixed point inside the reservoir and near to the pool water level.
The obtained SP data were split into regional and local components. The regional component was generated using an existing correlation between a vertical SP water gradient and the water depth of the upstream slope shape of the dam. The residual SP field was obtained by subtraction of the regional component from the recorded data.
The residual SP field showed several anomalies, one of which coincided with the axis of the outlet conduit. Depth estimation gave a value equal to the position of the tube. Some of the anomalies suggested the presence of suffusion processes that had developed along the upstream slope, similar to sink‐hole dissolution. The SP anomalies and the conductive bodies interpreted from the direct current measurements which were carried out on the crest of the dam, showed good spatial correlation; this indicated the possibility of hydraulic connections through preferential flow paths between the upstream face and the body of the dam.
In quantitatively interpreting the SP anomalies, their relative positions on the upstream face must be taken into account, due to possible electrical‐potential distortions at the rock‐fill/water and water/ air limits.
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An application of CSAMT to a geological investigation for railway route selection
Authors Da Lei, Guoze Zhao, Zhihong Yang, Fugang Zhao and Xiaochang LiABSTRACTThe controlled‐source audio‐frequency magnetotelluric method is a frequency domain electromagnetic sounding technique using a controlled source. This method, though characterized by efficiency, high lateral resolution, weak shielding effect from resistive beds and moderate detection depth has been rarely used for near‐surface geological investigation before in China. However, in the upper 200 m geological investigation for the pre‐route selection of the Lan‐Xin railway, the method has been successfully adopted for quickly mapping hidden fracture zones and structural framework, as well as assessing geological stability, so providing a basis for the railway route selection.
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Karst and saltwater intrusion mapping using electric resistivity tomography: case study at the site of Cap Rhir, Southern Morocco
ABSTRACTA 2D electrical resistivity method was used to survey the site of Cap Rhir, which is located on the Atlantic Coast, about 30 km north of Agadir, Southern Morocco. This site was proposed by the National Office of Electricity (ONE) for the construction of a large thermal plant for the provision of electrical power to the region. Nineteen 2D electrical resistivity tomography profiles with electrode spacings of 3 m and 5 m were acquired across the study area using the dipole‐dipole electrode configuration. The apparent resistivity measurements were taken by the Sting R8 system (AGI) and the inversion of the data was accomplished by the EarthImager 2D software.
The integration of the apparent resistivity inversion results, the surface geology and the borehole cores data indicate, in addition to the near surface topsoil cover, two prominent high and low resistivity areas on all the resistivity profiles. The high resistivity readings of up to 2000 Ohm.m correlate with the very compact reef limestone that crops out in the southern portion of the study area and with the more or less resistive block‐faulted limestone that occurs in the northern portion of the study area.
Nevertheless, the most important findings that might have a negative impact on the thermal plant project construction, local ecosystems and groundwater resources farther inland, are the low resistivity anomalies that dominate all the 2D resistivity profiles at relatively shallow depth. They show low resistivity values between 10 Ohm.m and 20 Ohm.m and have geometrical forms of alternating nearly spherical and elongate bodies. In accordance with the outcrops and the borehole cores, these features are likely to represent saltwater‐filled buried karstic cavities connected by channels, caused by limestone dissolution. An alternative interpretation could be that excessive carbonate dissolution has resulted only in a nested network of large communicating pores filled with marine waters. Major faults, spanning the entire study area, were imaged on the resistivity profiles; sharp cliffs were also identified and were interpreted as marks of ancient shorelines.
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The use of GPR to detect active layers in young periglacial terrain of Livingston Island, Maritime Antarctica
Authors Georg Schwamborn, Dirk Wagner and Hans‐W. HubbertenABSTRACTThe South Shetland Islands offer ice‐free margins with periglacial surfaces that are only a few decades old after the recent glacier retreat. Ground‐penetrating radar (GPR) profiling was used to acquire information on young permafrost occurrence as deduced from active layer advance. Local GPR measurements included grids of parallel single‐offset 2D reflection profiles and multi‐offset measurements to determine wave velocity in the ground. Excavations served to determine the sedimentary composition and to backup GPR profile interpretation. GPR results show that the active layer could easily be traced at a site 140 m above sea level (asl), which is placed in volcanic soil. In contrast, GPR data were ambiguous at a site low in altitude (35 m asl), where frozen and unfrozen ground was imaged next to each other and GPR interpretation relied on ground verification.
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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)