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- Volume 10, Issue 3, 2012
Near Surface Geophysics - Volume 10, Issue 3, 2012
Volume 10, Issue 3, 2012
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High resolution 3D near surface imaging of fracture corridors and cavities by combining Plus‐Minus method and refraction tomography
Authors Jean‐Luc Mari1 and Manuela MendesABSTRACTTomographic inversion helps to generate detailed geophysical models of the subsurface.
This paper shows how the velocity model obtained by the conventional Plus‐Minus refraction travel‐time picking method can be used as initial input in a subsequent refraction tomography to build an even more accurate subsurface velocity model. This new fast procedure becomes an interesting alternative technique considering the important computational time effort required for the global search algorithms or the simple two‐layered model provided by the conventional refraction analysis, as the Plus‐Minus method.
The proposed methodology proved attractive when testing with synthetic and field data. The illustration with synthetic data shows the final result and reference model very well matched. It is shown that the ray coverage is a useful tool for assessing the quality of the reconstructed model. The application to field data enables us to map the irregular shape of the top of a karstic reservoir, to be sure that no cavities filled with clays are present near there (top of the reservoir) to obtain a 3D velocity‐depth model of a heterogeneous low velocity superficial zone revealing the main orientation of fracture corridors (N90°), and also to estimate appropriate static corrections for use in subsequent data processing.
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Velocity and attenuation dispersion relations for the effective Biot model: total‐field formulation
Authors Stewart Greenhalgh, Xu Liu and Bing ZhouABSTRACTIn this paper, two approaches ‐ the host phase fields and the total fields, were respectively applied to formulate the effective Biot governing equations from the original double‐porosity dual‐permeability (DPDP) model. The host‐phase formulation given previously in the literature is made under the assumption that the macroscopic fluid flux of the included phase is zero, so that this term can be ignored in the conservation of the momentum equation. The total‐field formulation developed here has no such limiting assumption and gives rise to new and general governing equations that cover the host‐field approach as a special case. By computing the phase velocity and attenuation dispersion curves of sample rocks, we show that the two sets of governing equations are consistent at a very low frequency but for larger volume fractions of phase 2, there is a significantly increasing discrepancy in the slow P‐wave as the frequency increases. The slow P‐wave, whilst difficult to observe, does exist and must be considered when computing the frequency‐dependent reflection coefficients at an interface with a porous medium.
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Multi‐method geophysical mapping of quick clay
ABSTRACTMarine clay deposits in coastal, post‐submarine areas of Scandinavia and North America may be subjected to quick clay landslides and hence significant efforts are being taken to map their occurrence and extent. The purpose of this paper is to assess the use of a number of geophysical techniques for identifying quick clay. The investigated area, Smørgrav, located in southern Norway has a history of quick clay sliding, the most recent event occurring in 1984. Geophysical techniques that are used include electromagnetic conductivity mapping, electrical resistivity tomography, seismic refraction and multichannel analysis of surface waves. These results are compared to geotechnical data from bore samples, rotary pressure soundings and cone penetration testing. A number of these approaches have proved promising for identifying quick clay, in particular electrical resistivity tomography and electromagnetics, which delineated a zone of quick clay that had previously been confirmed by rotary pressure soundings and sampling. Seismic refraction was useful for determining the sediment distribution as well as for indicating the presence of shallow bedrock whereas the multichannel analysis of surface‐waves approach suggested differences between the intact stiffness of quick and unleached clay. It is observed that quick clay investigations using discrete rotary pressure soundings can be significantly enhanced by using, in particular, electrical resistivity tomography profiles to link together the information between test locations, perhaps significantly reducing the need for large numbers of soundings.
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Depth estimation of cavities from microgravity data using a new approach: the local linear model tree (LOLIMOT)
Authors A. Hajian, H. Zomorrodian, P. Styles, F. Greco and C. LucasABSTRACTIn this paper an attempt is made to estimate depth and shape parameters of subsurface cavities from microgravity data through a new soft computing approach: the locally linear model tree, known as the LOLIMOT algorithm. This method is based on locally linear neuro‐fuzzy modelling, which has recently played a successful role in various applications over non‐linear system identification.
A multiple‐LOLIMOT neuro‐fuzzy model was trained separately for each of the three most common shapes of subsurface cavities: sphere, vertical cylinder and horizontal cylinder. The method was then tested for each of the cavity shapes with synthetic data. The model's suitability for application to real cases was analysed by adding random Gaussian noise to the data to simulate several levels of uncertainty and the results of LOLIMOT were compared to both multi‐layer perceptron neural network and least‐squares minimization methods. The results showed that the LOLIMOT algorithm is more robust to noise and is also more precise than either the multi‐layer perceptron or least‐squares minimization method.
Furthermore, the method was tested with microgravity data over a selected site located in a major container terminal at Freeport, Grand Bahamas, to estimate cavity depth and was compared to the results achieved by least‐squares minimization and multi‐layer perceptron methods. The proposed method can estimate cavity parameters more accurately than the least‐squares minimization and multi‐layer perceptron methods.
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Electromagnetic induction antenna modelling using a linear system of complex antenna transfer functions
ABSTRACTThe quantitative retrieval of soil apparent electrical conductivity using electromagnetic induction (EMI) has remained limited due to strong simplifications regarding EMI antenna modelling. In this paper, a new technique for EMI antenna modelling is applied for the common‐offset EMI systems. The EMI system is efficiently described using global transmission and reflection coefficients and Green's functions are used to describe wave diffusion for horizontal and vertical dipole modes. We performed EMI measurements along a 180‐metre‐long transect with two different instrument heights above the soil surface, as well as with different orientations and frequencies. To ensure proper retrieval of the soil apparent electrical conductivity, the reference values were obtained from electrical conductivity data measured from 11 undisturbed soil cores taken along the EMI transect. The apparent electrical conductivity values calculated by applying the proposed model have a good agreement with reference values, however some discrepancies can be observed that are mainly attributed to the presence of local heterogeneities and also errors due to the variations in the height of the EMI instruments above the ground. The proposed method appears to be promising for quantitative retrieval of soil apparent electrical conductivity and resolving calibration issues that are typically encountered using EMI. In addition, the model calibration (antenna transfer functions determination) was successfully accomplished using conductivity values measured from the soil cores.
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Construction and calibration of a field TDR monitoring station
Authors G. Curioni, D.N. Chapman, N. Metje, K.Y. Foo and J.D. CrossABSTRACTTime‐Domain Reflectometry (TDR) has been used extensively in the past thirty years in order to measure soil water content and bulk electrical conductivity (), both in the laboratory and in the field. TDR can be effectively used in combination with geophysical techniques such as Ground Penetrating Radar (GPR) in order to provide information on relative dielectric permittivity and . As part of the Mapping the Underworld project, a TDR monitoring station was constructed with the aim of monitoring the geophysical parameters of the soil in a field case study. A rigorous methodology, utilizing the latest knowledge for calibration and analysis was followed and is thoroughly elucidated in this paper. The reasons behind the choice of the equipment setup are described, with the intention of providing a reference for similar TDR field installations. The precision and accuracy of TDR and the validation of the calibration procedures were assessed with laboratory and field tests. The standard deviation of several TDR measurements in the laboratory was on average smaller than 2% for both apparent permittivity and . The accuracy, expressed as the mean difference to reference values, was on average smaller than 2% and 3% of apparent permittivity and respectively, although higher errors, up to ≈ 5% and ≈ 7.5% respectively, were measured in media with very low apparent permittivity (i.e., air) and at values smaller than 0.0010 S/m. These results demonstrate that with the chosen methodology and setup, TDR can provide reliable data and can be used for long‐term geophysical monitoring. The data provided by TDR monitoring stations could contribute to a data base of geophysical properties for soils. This information may eventually be used to assist the fine tuning of shallow geophysical techniques such as GPR.
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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)