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- Volume 5, Issue 4, 2007
Near Surface Geophysics - Volume 5, Issue 4, 2007
Volume 5, Issue 4, 2007
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Three‐dimensional probability tomography of self‐potential anomalies of graphite and sulphide mineralization in Orissa and Rajasthan, India
Authors Bimalendu B. Bhattacharya, Shalivahan, A. Jardani and Aparna BeraABSTRACTCharge occurrence probability (COP) and dipole occurrence probability (DOP) tomographic imaging of graphite and sulphide orebodies, which were earlier detected by a self‐potential (SP) survey, were carried out. Three typical examples, two shallower graphite lenses in Bender and Berni, Orissa State, and one relatively deeper sulphide deposit in Kayar, Rajasthan State, are presented. An exploratory test hole, drilled on the basis of the SP results in Bender and Berni, struck graphite. A mise‐à‐la‐masse survey was also carried out on these two deposits to delineate the orebody. The graphite deposits have subsequently been mined. The shapes of the graphite bodies, especially the upper parts, are now known. The Bender deposit is a single graphite lens, whereas the graphite occurrences in Berni are complex multilens‐type deposits. Both the COP and DOP techniques clearly imaged the single lens at Bender, as well as the multilens deposits at Berni. The Kayar sulphide deposit, unlike the graphite deposits, has not yet been mined, and therefore it is not delineated as precisely as the graphite orebodies. However, its outline is known from the drilling data. The imaging results of COP and DOP show good agreement with the drilling results of the sulphide deposits.
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HELP ACEH – A post‐tsunami helicopter‐borne groundwater project along the coasts of Aceh, northern Sumatra
Authors Bernhard Siemon, Annika Steuer, Uwe Meyer and Hans‐Joachim RehliABSTRACTAfter the earthquake and the tsunami event on December 26, 2004, the Indonesian and German governments set up a project dedicated to re‐installing the public life of the people in the coastal region of Aceh Province in northern Sumatra, Indonesia. As the tsunami waves caused large‐scale coastal salt‐water intrusions and destroyed thousands of shallow drinking‐waterwells, the focal point was water assessment along the shorelines of Aceh about nine months after the tsunami. The target areas were the city of Banda Aceh and the district of Aceh Besar on the north coast and the area on the west coast between the towns of Calang (district of Aceh Jaya) and Meulaboh (district of Aceh Barat). A helicopter‐borne survey including electromagnetics (HEM), magnetics and gamma‐ray spectrometry was conducted by the airborne group of the German Federal Institute for Geosciences and Natural Resources from August to October 2005. The HEM surveys revealed shallow coastal salt‐water occurrences caused by the tsunami, deep salt‐water occurrences up to several kilometres inland, and a number of potential freshwater resources. Numerous requests for information on the geological and hydrogeological situation at sites for planned water‐wells were successfully evaluated.
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High‐resolution seismic investigation of Lake Tana, northern Ethiopia
Authors C. Richard Bates, Henry F. Lamb and Mohammed UmerABSTRACTHigh‐resolution single‐channel seismic reflection data and sediment core samples from Lake Tana, Ethiopia, reveal a dynamic Holocene and late Pleistocene history of the lake. The seismic data show a sequence of sediments in excess of 45 m thick with four strong reflection horizons. The horizons are interpreted to be a consequence of climate conditions resulting in complete lake desiccation. The youngest of these surfaces, at a depth of 8–12 m beneath the present lake floor, is dated at about 17 k cal BP and is correlated with desiccation of Lake Victoria during Heinrich event 1. Similar surfaces were mapped at depths of approximately 25, 30 and 45 m below the lake floor, suggesting numerous late‐Pleistocene lowstands. No features within the seismic sections suggest deep‐water depositional environments. Deeply incised channel sequences cut through the near‐surface sediments above the strong lake‐wide reflection horizons and represent periods of significant erosion during the early Holocene. These events are tied to core intervals dated to 8.4 and 12.1 k cal BP, which also correspond to climate change during regional arid periods. On the northern lake margin, the geometry of sediment accumulation suggests recent faulting with associated continuing basin subsidence in this active tectonic region. The seismic and core data from Lake Tana can be summarized as showing a sedimentation history through the Holocene and late‐Pleistocene that reflects the interplay between rapid climate change and long‐term basin subsidence.
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Characterization of small local noise sources with array seismology
Authors Joachim R.R. Ritter and Henriette SudhausABSTRACTSeismic noise is often regarded as a stochastic, incoherent and unwanted signal. However, at a closer look, noise also contains coherent signals which can be used to characterize its sources and/ or the subsurface. Here, we propose the use of array seismological methods for engineering purposes to locate and identify small, local noise sources. A detailed knowledge of noise sources enables appropriate measures to be taken to mitigate unwanted noise effects. As a case study, we report results from a temporary array of ten seismic broadband stations which were installed at the building site of a high‐precision physics laboratory. Standard analyses of power spectra and amplitude measurements indicate that the highest noise amplitudes are caused by heavy trucks passing by. Burst‐like coherent waves, excited by vehicles, appear to have a recurring pattern. Using array techniques, we were able to measure the slowness and backazimuth of the coherent arrivals. These could be identified as Rayleigh waves generated at small pot‐holes and bumps of a near‐by road and were found to propagate with a speed of 130–200 m/s.
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Non‐destructive DC resistivity surveying using flat‐base electrodes
Authors E.N. Athanasiou, P.I. Tsourlos, G.N. Vargemezis, C.B. Papazachos and G.N. TsokasABSTRACTThe application of flat‐base electrodes to geoelectrical measurements is examined. This study is motivated by the inability to apply geoelectrical techniques in environments where conventional spike electrodes cannot be inserted into the ground. The performance of flat‐base electrodes is examined in various environments, using different measuring modes. It is shown that flat‐base electrodes can be satisfactorily used in most cases, producing data that are almost identical to the measurements obtained using standard electrodes. Several case studies in various urban locations in Greece, where flat‐base electrodes have been successfully employed, are also presented. The results indicate that flat‐base electrodes provide the advantage of a fully non‐destructive application and, therefore, the extension of geoelectrical methods to environments that, otherwise, would not have been considered suitable.
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GPR and microwave tomography for detecting shallow cavities in the historical area of “Sassi of Matera” (southern Italy)
ABSTRACTThe results of a geophysical survey carried out in the historical area of “Sassi of Matera” (Basilicata region, southern Italy) are presented and discussed. Matera is one of the most fascinating and ancient towns of the Mediterranean area and is listed in the UNESCO World Heritage List. It represents an example of a town where the communities developed a cave‐life culture in the cavities excavated through the substratum of the hill of Matera. The urban area of Matera, therefore, is characterized by a complex system of interconnected and overlapping ancient shallow cavities: residences, galleries, cisterns, crypts, graves, etc. Consequently, local surface deformation phenomena are often observed.
We describe the results from a ground‐penetrating radar (GPR) survey carried out in St. Rocco Square, one of the most interesting areas of the historical centre of Matera, that was affected by both spread out and local subsidence phenomena. Great attention was paid to GPR data processing and inversion. In particular, a novel microwave tomography approach was applied to obtain high‐resolution images of shallow ancient cavities that caused the local surface deformations. Moreover, the GPR survey was integrated with an electrical resistivity tomography survey in order to prospect those subsurface zones with a high electrical conductivity, where GPR investigation was not possible. The geophysical investigation enabled us to identify the cause of the subsidence phenomena and to reconstruct the geometry of the complex system of shallow cavities partially filled with unconsolidated material.
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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)