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- Volume 14, Issue 4, 2016
Near Surface Geophysics - Volume 14, Issue 4, 2016
Volume 14, Issue 4, 2016
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Multiwave Gaussian beam prestack depth migration of exploration‐scale seismic data with complex near‐surface effects
Authors Jianguang Han, Yun Wang and Changqing YuABSTRACTThe direct prestack depth migration methods of nonplanar data provide effective ways to address the complex near‐surface problem. Based on the study of local slant stacks considering the elevation and dip angle information of an irregular surface, an alternative method for multiwave Gaussian beam prestack depth migration of exploration‐scale seismic data with complex near‐surface effects is presented here. We first derive the downward‐continued wavefield for an irregular surface in terms of Gaussian beams, where we directly decompose the seismic records within a Gaussian window into a local plane‐wave component. The PP‐ and PS‐wave Gaussian beam prestack depth migration methods under complex near‐surface conditions are then presented, after the wavefield separation is performed with an affine coordinate‐system transform. Tests with numerical examples demonstrate the accuracy and effectiveness of the method.
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Detection of geological structures using impact‐driven piling as a seismic source
Authors Bagher Farmani, Nils‐Otto Kitterød and Elisabeth GundersenABSTRACTSeismic imaging during piling may reduce construction costs in sensitive areas, and combined with technology to look ahead of the pile tip, the risk of stopping the piling before reaching the bedrock may be reduced. Seismic images obtained during piling can give vital information about the subsurface structure in the vicinity of the pile. In this paper, we discuss how to produce seismic images as a part of the piling procedure without using any external seismic sources. Using surface and borehole synthetic data examples, we show that it is possible to use pressure (P) waves emitted from the pile tip for imaging purposes. A pilot field study was carried out to test the feasibility of this method. The field study revealed that high‐frequency pressure waves are emitted from the pile tip. However, they are masked by strong surface and shear waves, especially at the nearer offsets. The data processing approach allowed us to achieve reasonable signal‐to‐noise ratio for pressure waves at the further offsets, while the signal was not as well recovered at the nearer offsets. This study demonstrates that it is possible to acquire fair‐quality signals without using any seismic source other than the impact on the pile itself. More field data are needed and some acquisition procedures need to be optimized before the method can be applied for practical imaging.
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Locating mofettes using seismic noise records from small dense arrays and matched field processing analysis in the NW Bohemia/Vogtland Region, Czech Republic
Authors Hortencia Flores Estrella, Josefine Umlauft, Andreas Schmidt and Michael KornABSTRACTThe NW Bohemia/Vogtland region is characterized by currently ongoing geodynamic processes within the intracontinental lithospheric mantle. Among others, this activity results in the occurrence of mid‐crustal earthquake swarms and CO2 degassing zones called mofettes. These two natural phenomena are related to each other since it is considered that fluid flow and fluid‐induced effective stress can trigger earthquake swarms. At the Earth’s surface, they appear spatially separated, but their connection could be explained by the existence of pathways within the crust that allow efficient and permanent fluid transport. However, neither the structure nor the position of such pathways has been imaged yet.
With this background, we used a matched field processing analysis within the NW Bohemia/Vogtland region to locate mofettes and investigate their characteristics. Considering the CO2 degassing process as a high‐frequency noise source, we chose two different test sites: the Dolní Částkov Borehole, which is an artificial mofette that we used to validate the method, and the South Hartoušov mofette field, a natural CO2 degassing area. On both sites, we measured seismic noise in a continuous mode for several hours (7 hours to 9 hours), with a sampling frequency of 250 samples/second, in multiple campaigns using an array of about 60 × 60 m2 with approximately 30 randomly distributed stations.
For the matched field processing computation, the phase velocity of the study area is required, which we obtained from active seismic experiments with vertical hammer‐blow as the source. The phase velocity varies between 200 m/s and 420 m/s for Dolní Částkov and between 100 m/s and 280 m/s for South Hartoušov, both in a frequency range of 7 Hz–60 Hz.
With the matched field processing analysis at the artificial mofette in Dolní Částkov, we could relocate the noise source successfully. In the South Hartoušov mofette field, we detected one dominant vertically extended noise source, probably a fluid pathway, as well as a small matched field processing maxima at the surface that can be related to a dry mofette.
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Demonstrating the contribution of dielectric permittivity to the in‐phase EMI response of soils: example from an archaeological site in Bahrain
Authors Christophe Benech, Pierre Lombard, Fayçal Rejiba and Alain TabbaghABSTRACTElectromagnetic induction instruments (also called loop–loop, dipole–dipole, or Slingram) are now commonly used in archaeological prospecting. They are truly light instruments, which are able to measure both the apparent electrical conductivity and the apparent magnetic susceptibility of the ground. During a field test in Bahrain Island where the soil has a high clay content and a high salt content, surprisingly high values of in‐phase response were obtained at all inter‐coil spacings using CMD “Mini‐Explorer” (GF Instruments Ltd., Brno, Czech Republic) at 30 kHz, in both horizontal coplanar and vertical coplanar configurations, and the horizontal coplanar and vertical coplanar susceptibility variations were in total opposition. This apparent discrepancy is explained by considering the in‐phase responses to be dominated by the relative dielectric permittivity. Using the raw and in‐phase vertical coplanar and horizontal coplanar data, it is possible to determine and map the apparent permittivity and apparent magnetic susceptibility. For this case of slated soils with high clay content, the relative permittivity is strong but in agreement with both experimental data at lower frequencies and theoretical models reported in the literature.
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Comparison between thermal airborne remote sensing, multi‐depth electrical resistivity profiling, and soil mapping: an example from Beauce (Loiret, France)
ABSTRACTA wide variety of remote sensing and ground‐based (proximal sensing) methods have been developed to describe soil physical properties and their lateral variations. Remote sensing enables the estimation of soil properties over large areas, but the information is often limited to the soil surface. Ground‐based methods enable the derivation of soil properties for the whole soil column thickness, although these methods cannot be conducted over large areas. The aim of the present study is to contribute to the assessment of the efficacy of airborne thermal prospection over bare soils in soil mapping. This study focuses on a comparison between this technique, which can investigate over the whole soil column thickness after a sufficiently long transient heat exchange period, and pedo‐logical and electrical resistivity data that were recorded for three different depths of investigation.
The study area is located in the Beauce region, where the soils (haplic Calcisol or calcaric Cambisol) consist of a loamy–clay layer that is 0.3 m–1.4 m thick and overlies Tertiary Beauce limestone. Thermal measurements were recorded by ARIES radiometer in December, after six days of heat loss from the ground. The investigation depth could thus be considered to be larger than the thickness of the ploughed layer. Comparisons using statistical analyses between the thermal measurements, electrical resistivity, and pedological data demonstrated that: (i) the spatial organization of the thermal inertia map is similar to the spatial organization of the 0‐m to 1.7‐m resistivity map; and (ii) the thermal apparent inertia values are significantly different between the haplic Calcisols and the calcaric Cambisols and can thus be mapped with a high spatial resolution over large areas. The applicability of thermal prospecting in soil mapping opens up many possibilities considering the present advances in light‐infrared radiometers. Beside agronomical concerns, this methodology will also facilitate progress in engineering applications, including the cross‐estimation of electrical and thermal properties.
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Using electrical anisotropy for structural characterization of sediments: an experimental validation study
Authors Sadam Al‐Hazaimay, Johan A. Huisman, Egon Zimmermann and Harry VereeckenABSTRACTImproved characterization of subsurface heterogeneity is important to better understand a range of key processes in the hydrologic cycle, such as overland flow, infiltration into the soil, evaporation to the atmosphere, and transpiration by plants. Recently, synthetic modelling studies have shown that information on subsurface heterogeneity can be obtained from the anisotropy in electrical resistivity. The objective of this paper is to experimentally validate the findings of this synthetic modelling study. In order to do so, we developed a new measurement procedure to determine the effective complex electrical resistivity from a set of current injections and voltage measurements on a heterogeneous sample. A synthetic modelling study showed that this new measurement procedure was able to reproduce the results of the previous study that showed how the electrical properties and the correlation length ratio of bimodal distributions of two materials can be obtained from the effective complex electrical resistivity measured in two perpendicular directions. After validation of the new measurement approach, we constructed two bimodal distributions in a 2D measurement cell and applied the newly developed measurement strategy. We were able to estimate the electrical properties, the volume fraction, and the correlation length ratio with good accuracy from the complex resistivity measurements in two directions. The remaining differences were attributed to variations in sediment thickness that occurred during sample preparation. We conclude that anisotropy should not be ignored but embraced when dealing with subsurface heterogeneity and that proper interpretation of anisotropy may actually be used to characterize subsurface heterogeneity.
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Ambiguities in geophysical interpretation during fracture detection—case study from a limestone quarry (Lower Silesia Region, Poland)
Authors Tomislaw Golebiowski, Slawomir Porzucek and Bernadetta PasierbABSTRACTGeophysical studies were conducted over a shallow mining excavation near a sinkhole that was created by partial collapse of the excavation. The main aim of this study was to detect the fractured zones induced by mining activity and so threatening the stability of the surface. To this end, geophysical surveys using microgravimetry, georadar, and electrical resistivity tomography techniques were carried out. However, although the results of these measurements were clear, yet the distribution of the fractured zones in the rock mass remained unclear. In this paper, the results of each individual geophysical technique used to detect the fractured zones and the ambiguities in the interpretation are presented. In order to properly analyse the data, a more complex interpretation were carried out using numerical modelling. Georadar and electrical resistivity tomography data provided the basis for gravity modelling, the results of which enabled confirmation of the presence of fractured zones. This had not been directly possible from the measured microgravimetric curve.
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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)