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- Volume 3, Issue 4, 2005
Near Surface Geophysics - Volume 3, Issue 4, 2005
Volume 3, Issue 4, 2005
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3D inversion of VES data from the Saqqara archaeological area, Egypt: a case study
Authors G.M. El‐Qady, F.A. Monteiro Santos, A. Gh. Hassaneen and L. TrindadeABSTRACTSaqqara is one of the most important archaeological sites in Egypt. The monuments in this area are suffering from the effects of urbanization and temporal and spatial variations in the groundwater level. In an attempt to understand better the problems associated with the groundwater, a geoelectrical DC resistivity survey, comprising 47 vertical electrical soundings using a Schlumberger array, has been conducted. These data have been inverted using a 3D code based on a smoothness‐constrained least‐squares method. To determine the reliability of the inversion results, a range of tests has been performed. The final 3D resistivity model obtained for the study area provides useful new subsurface geological and hydrogeological information. The new model allows a more integrated image and interpretation of the complex hydrogeological conditions. It also depicts the mutual relationship between the two water‐bearing units in the area that was not clear in the previous 1D and 2D interpretations.
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Seismic land‐marine acquisition survey on the Great Ancona Landslide
Authors E. Stucchi, F. Zgur and L. BaradelloABSTRACTThe paper deals with a reflection seismic acquisition survey carried out on the Great Ancona Landslide, a huge landslide body on the Italian Adriatic coast involving part of the city of Ancona. To investigate the area we carried out a joint land‐marine seismic acquisition that gave us a continuous image of the subsurface in the land‐sea transition zone where the landslide foot is supposed to be located. A specifically designed interface unit connected the analog ocean‐bottom cable (OBC) lying along the sea‐floor to the telemetry system used on land. The effectiveness of different source types for both marine and land shooting was tested in the field, by computing and analysing the frequency‐time (FT) response of the recorded seismograms. The various sub‐datasets obtained (land‐land, land‐marine, marine‐land and marine‐marine) were kept separate because of the different signal properties and the kinds of noise present; a short description of such noise is given. Finally, the acquisition statistics are discussed, and a preliminary image of the subsurface in time has been achieved from the marine‐marine data.
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Seismic prospecting in archaeology: a 3D shear‐wave study of the ancient harbour of Miletus (Turkey)
Authors Susanne Woelz and Wolfgang RabbelABSTRACTA 3D multicomponent shear‐wave survey was performed in the area of the ancient city of Miletus (western Turkey). To achieve high spatial resolution of near‐surface targets, data were acquired with a geophone array of 1 m grid spacing. The survey served: (1) to investigate archaeologically important details of the basement of one of the silted harbours of Miletus, and (2) to evaluate the potential of close‐meshed shear‐wave seismics for shallow high‐resolution prospecting in a more general sense. Methodically, we could show that isotropic radiation patterns of compressional shear‐ and surface waves can be reconstructed with simple vector arithmetic even if hand‐driven hammer blows are applied as a horizontal seismic source. Presenting time‐slices of S‐wave propagation, we demonstrate that side‐swipe may significantly affect the propagation of refracted waves. It is shown that the wavefronts of surface waves may be far from circular in the case of typical near‐surface lateral heterogeneity. As a further phenomenon of wave propagation, we could visualize the coda of a ground‐coupled air wave probably caused by mud cracks. Regarding the archaeological background of our study, we could show that the basement of the Milesian Lions’ Harbour is shallow enough to provide appropriate ground for ancient construction work. However, by ‘true 3D’ refraction mapping, we could image small‐scale topographic details of the harbour basement, proving that no large ancient stone walls are hidden under the ground. Magnetic anomalies were found to coincide with a lateral increase in Rayleigh‐wave velocity. The rectangular shape of this velocity contrast may indicate the remnants of an artificially shaped rock platform or an embankment. Spectral analysis of the surface wavefield shows that this seismic anomaly will be found at a depth of 3–6 m.
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The links between MRS parameters and the hydrogeological parameters
More LessABSTRACTMagnetic resonance sounding (MRS) is a non‐invasive geophysical method that can be used to discriminate between the geophysical signals of pore water of the surrounding rock. This method is thus particularly attractive for hydrogeological applications.
In order to investigate MRS for the benefit of applied hydrogeology, a comprehensive interdisciplinary study was carried out. The aim of this project was to develop an optimal methodology which could be easily adopted by hydrogeologists. This paper summarizes the main results obtained, with the objective of translating the MRS parameters into parameters that can be directly used by hydrogeologists.
It was found that MRS is well adapted to the working scales of the hydrogeologist (i.e. field scale, well scale). It is able to provide significant information for the hydrogeologist, including:
- Reliable detection of the presence of water in the subsurface. This is the basic and most significant advantage of the method, which proved to be useful, particularly in arid to semi‐arid environments.
- Locating water‐saturated formations (top and bottom), situated at depths between 0 and 100 m approximately.
- Estimation of the hydrodynamic parameters of detected aquifers:
- – Through a rigorous inversion and calibration process, the MRS method enables the quantification of the specific yield (effective porosity) of aquifers and of the storage coefficient, the latter in unconfined aquifers only.
- – In the present state‐of‐the‐art, MRS is able to estimate the hydraulic conductivity (or trans‐missivity) of aquifers in localized favourable configurations, when calibration is available; otherwise, aquifers can be compared qualitatively.
Thus the MRS method provides data that cannot be obtained with other non‐invasive geophysical tools, and is already a valuable tool for applied hydrogeological projects.
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Magnetic resonance sounding (MRS) and resistivity characterisation of a mountain hard rock aquifer: the Ringelbach Catchment, Vosges Massif, France
Authors J.M. Baltassat, A. Legchenko, B. Ambroise, F. Mathieu, P. Lachassagne, R. Wyns, J.L. Mercier and J.J. SchottABSTRACTThe Ringelbach catchment, which has been studied since 1975, is highly representative of the crystalline Vosges massif, where the water supply is mainly derived from small aquifers in heterogeneous superficial formations and in the weathered and fissured bedrock. Vertical electrical soundings (VES), resistivity imaging profiles and magnetic resonance soundings (MRS) were applied in order to investigate the 3D structure and estimate the hydrodynamic characteristics of the subsurface, which is composed of granites partly covered by Triassic sandstone.
Despite poor MRS signal‐to‐noise conditions, the general structure of the catchment is defined using geophysics (electrical methods and MRS combined) and geology. MRS also makes it possible to map the water volume per unit surface within the different geological formations and structural blocks. The corresponding maps are proposed as a basis for evaluating the water storage within the catchment and improving the understanding of its hydrological functioning. The initially proposed geological model is refined and a four‐block structure intersected by faults is defined. The schema of a gradual stratified weathering of the granite from surface to depth is not clearly reflected in the resistivity images, partly because of insufficient investigation depth. It is completed with highly weathered, deeply rooted fractured zones associated with faults in order to explain deeply sited low‐resistivity zones.
The MRS water content in the weathered granite of Ringelbach appears abnormally low in comparison with the higher values observed in similar resistivity settings in other granite regions. Because of the rough catchment topography, these low values may be attributed to unsaturated weathered formations on hillsides or saturated, fine material (colluvial deposits, highly weathered granite) in the valley bottom. The control‐wells and laboratory measurements on core samples proposed in the next step of this research will attempt to calibrate the MRS results and validate the proposed structural and weathering schema. Re‐interpreted and validated geophysical results will then be used as effective input for improved hydrological modelling.
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Nuclear magnetic resonance (NMR) properties of unconsolidated sediments in field and laboratory
Authors Martin Müller, Stefan Kooman and Ugur YaramanciABSTRACTSurface nuclear magnetic resonance (SNMR) is a geophysical technique that has proved to be a useful tool for the investigation of hydrological properties of aquifers (porosity , saturated hydraulic conductivity ) in the past 10–15 years. Until recently, laboratory NMR has focused on consolidated sediments. In this research, an attempt is made to investigate unconsolidated sediments. To enable an enhanced understanding and interpretation of SNMR data, laboratory nuclear magnetic resonance (NMR) properties of synthetic and natural unconsolidated samples (glass beads, sand mixtures, bore‐core samples from a Quaternary environment west of Berlin (Germany) and coarse sand samples with a varying clay content) were analysed. To verify the NMR measurements, pore‐space properties (specific surface, porosity, pore‐size distribution) were analysed. Finally, hydraulic conductivity measurements were conducted to verify both the hydraulic conductivity derived from NMR relaxation times and the hydraulic conductivity estimates based on grain size. The laboratory data were compared to SNMR field data to assess scaling effects due to the dispersion of relaxation.
The results show that the relationship used to obtain hydraulic conductivity from NMR relaxation is suitable for predicting the saturated hydraulic conductivity of samples composed of clay, silt and coarse sand. Furthermore, it is found that the predictions of hydraulic conductivity of intermediate grain sizes with differing clay content vary over a wide range. A more individual approach with regard to the paramagnetic properties of the material might be needed to achieve successful estimations.
Clay (because of its high specific surface and large surface‐to‐pore‐volume (S/V) ratio) has a strong influence on NMR relaxation and the associated saturated hydraulic conductivity. The NMR relaxation of a series of coarse sand samples (d=1.0–0.5 mm) with a clay content in the range 3–20% shows an exponential decay for the hydraulic conductivity.
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MRS multi‐exponential decay analysis: aquifer pore‐size distribution and vadose zone characterization
Authors J. Roy and M.W. LubczynskiABSTRACT(the NMR signal decay‐time constant) spectra are analysed for two purposes: (1) to determine the pore‐size distribution below the water table for lithological and flow parameter purposes and (2) to determine the moisture content as a function of water‐film thickness/water‐drop size to characterize water storage and fluxes in the unsaturated zone. Unsaturated‐zone hydraulic conductivity versus moisture‐content relationships are non‐linear and normally have a memory effect (hysteresis). Such relationships are dependent on the partitioning of moisture into pores of different sizes, which are wetted and drained in an order that depends on the soil composition and texture. Magnetic resonance sounding (MRS) multi‐exponential decay analysis may supply a unique insight in this component of water‐resources quantification through the differentiation of the moisture content into different film/droplet sizes. Field examples from two sites, one in The Netherlands and the other in Botswana, are shown, in which the free induction decay NMR signal is analysed in terms of three spectral components.
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Improved karst exploration by VLF‐EM‐gradient survey: comparison with other geophysical methods
Authors F.P. Bosch and I. MüllerABSTRACTThe knowledge of size, density and both orientation and vertical distribution of fractures as well as their opening and filling material or overburden thickness is a valuable contribution to estimating hydraulic conductivity and to evaluating the vulnerability and protection strategy of karst aquifers. To obtain some of these parameters and to ascertain high‐permeability zones, the Very Low Frequency‐Electromagnetic Gradient (VLF‐EM GRAD) method was applied, together with Radio Frequency‐Electromagnetics (RF‐EM), Radiomagnetotellurics (RMT), Geoelectrical Tomography and refraction seismics, over a karstic terrain in the Swiss Jura Mountains. In this area, karst springs infiltrate a porous aquifer. This survey investigated the highly permeable karst structures, which provide fast water‐infiltration pathways into the karstic flow network. A dye tracer test validated the geophysical results. The results show the efficiency and reliability, particularly of the VLF‐EM GRAD method, for high‐resolution investigation at shallow depths and for its potential for fast data acquisition over large surfaces at catchment area scale without ground contact.
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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)