- Home
- A-Z Publications
- Near Surface Geophysics
- Previous Issues
- Volume 4, Issue 1, 2006
Near Surface Geophysics - Volume 4, Issue 1, 2006
Volume 4, Issue 1, 2006
-
-
Electromagnetic propagation features of ground‐penetrating radars for the exploration of Martian subsurface
Authors E. Pettinelli, G. annaroni, E. Mattei, A. Di Matteo, F. Paolucci, A.R. Pisani, A. Cereti, D. Del Vento, P. Burghignoli, A. Galli, A. De Santis and F. BellaABSTRACTIn this work, the effects of magnetic inclusions in a Mars‐like soil are considered with reference to the electromagnetic propagation features of ground‐penetrating radars (GPRs). Low‐frequency and time‐domain techniques, using L‐C‐R meters and TDR instruments, respectively, are implemented in laboratory experimental set‐ups in order to evaluate complex permittivity and permeability and wave velocity for different scenarios of a dielectric background medium (silica) with magnetic inclusions (magnetite). Attenuation and maximum detection ranges have also been evaluated by taking into account a realistic GPR environment, which includes the transmitting/receiving antenna performance and the complex structure of the subsurface. The analysis and the interpretation of these results shed new light on the significant influence of magnetic inclusions on the performance of Martian orbiting and rover‐driven GPRs.
-
-
-
Application of anisotropy borehole radar tomography in Korea
Authors Jung‐Ho Kim, Seong‐Jun Cho, Myeong‐Jong Yi and Motoyuki SatoABSTRACTAlthough the main geology of Korea consists of granite and gneiss, it is not uncommon to encounter anisotropy phenomena in cross‐hole radar tomography even when the basement is crystalline rock. To solve the anisotropy problem, we have developed an anisotropic inversion algorithm, assuming a heterogeneous elliptic anisotropy, to reconstruct three kinds of tomogram: tomograms of maximum and minimum velocities, and of the direction of the symmetry axis. We introduce some case histories of the application of anisotropic radar tomography in Korea. The first two case histories were conducted to construct social infrastructure, and their main objective was to locate cavities in limestone. The last two were performed in a granite and gneiss area. The anisotropy in the granite area was caused by fine fissures aligned in the same direction, while that in the gneiss and limestone areas was caused by the alignment of the constituent minerals. Through these case histories we aim to show that the anisotropic characteristic itself provides additional important information for understanding the internal structure of basement rock.
-
-
-
Realistic FDTD modelling of borehole georadar antenna radiation: methodolgy and application
Authors Jacques R. Ernst, Klaus Holliger, Hansruedi Maurer and Alan G. GreenABSTRACTHigh‐frequency electromagnetic‐wave propagation phenomena associated with borehole georadar experiments are complex. To improve our understanding of the governing physical processes and radiative properties of borehole georadar antenna systems, we have developed a modelling tool based on a finite‐difference time‐domain (FDTD) solution of Maxwell's equations in cylindrical coordinates. The computational domain is bounded by cylindrical symmetry conditions along the left edge of the model and uniaxial perfectly matched layer (UPML) absorbing boundary conditions along the top, bottom and right model edges. An accurate and efficient grid‐refinement technique allows us to account for detailed aspects of borehole georadar antenna systems, slim boreholes and materials with very high dielectric permittivities, such as water. Numerical experiments reveal that the radiation patterns of finite‐size Wu–King‐type antennae and infinitesimal electric dipoles in dry boreholes differ only slightly from the analytic solution of an infinitesimal electric dipole in a homogeneous full‐space. In contrast, there are substantial differences between the radiation patterns of antennae placed in water‐filled boreholes and their analytic full‐space equivalents without boreholes. The effects of placing the antennae in air‐ and water‐filled boreholes are explored using data acquired in crystalline rock and alluvial sediments, respectively. In both cases, simulations based on realistic transmitter antennae located in boreholes and spatially corrected receiver radiation patterns provide better agreement between the observed and modelled data than simulations based on infinitesimal transmitter and receiver dipoles.
-
-
-
Buried target signature extraction from ground‐penetrating radar signal: measurements and simulations
More LessABSTRACTGround‐penetrating radar (GPR) proves to be a very valuable tool in the field of humanitarian de‐mining, especially for the detection of plastic land‐mines. Recently, a monostatic stepped‐frequen‐cy continuous‐wave (SFCW) GPR, together with a conceptual model of the radar‐antenna‐soil system, has been developed for the characterization of the electromagnetic parameters of soil, i.e. dielectric permittivity (), magnetic permeability () and electric conductivity (). This approach is extended here to the extraction of the GPR signal and to modelling the signatures of buried targets. The equivalence principle is used to decompose the GPR signal into its soil and target‐in‐soil components, as well as to model the radar‐soil‐target system. It permits the soil contribution to be subtracted from the total GPR signal to provide the signature of the buried target. This signature is compared to simulations. For a proof of the concept, the GPR return signal from a buried metal sphere has been simulated using the Method of Moments and it shows good agreement with its measured counterpart. We also have extracted clean frequency‐ and time‐domain signatures of a PMN‐2 plastic mine embedded in a multilayered medium, subject to various water contents. The method is also applied to a B‐scan above a buried conducting cylinder. Finally, a study of the main sources of errors in the extraction of the signature of a buried target shows that mistakes in antenna height measurement lead to errors more important than those due to misestimating the relative dielectric permittivity of the soil.
-
-
-
Three‐dimensional multicomponent georadar imaging of sedimentary structures
Authors Rita Streich, Jan van der Kruk and Alan G. GreenABSTRACTMulticomponent georadar methods that involve recording data using several different antenna configurations and orientations should provide better constrained information about the subsurface than conventional single‐component techniques. We test this hypothesis using three‐dimensional (3D) multicomponent data recently acquired across braided river sediments deposited adjacent to the Alpine Fault in New Zealand. Strong reflections were observed in the two pairs of co‐ and cross‐polarized data sets. Relatively high reflection amplitudes in the cross‐polarized data were a consequence of the dip and oblique orientation of most sedimentary structures relative to the antennae. Many differences in structural information content and reflection amplitude observed in the different components could be explained on the basis of rather simple wavefield models. To obtain reliable subsurface information, a novel 3D multicomponent imaging (migration) algorithm was applied to the data. This algorithm accounted for the effects of antenna orientation, antenna offset and far‐field electromagnetic radiation patterns. In addition to correcting the dips, positions and lengths of reflections, the multicomponent imaging also adjusted their relative amplitudes according to the far‐field wavefield model. The contributions of the cross‐polarized data to the images were particularly important in our data set, because they selectively enhanced dipping reflections oriented at oblique angles to the antenna axes. Phase characteristics of reflections with certain orientations and dips were not recovered consistently, indicating a need for further investigation.
-
-
-
Delineating the near‐surface geometry of the fracture system affecting the Valley of Querétaro, Mexico: Correlation of GPR signatures and physical properties of sediments
Authors Dora C. Carreon‐Freyre and Mariano CercaABSTRACTWe present a combined ground‐penetrating radar (GPR) and geotechnical investigation of a fault‐fracture system that affects the Valley of Querétaro. The main fracture, known as Falla Central (FC), is aligned with N–S‐orientated regional faults suggesting that the buried fault scarps influence the geometry and propagation of fractures. Although the origin of fracturing is closely related to geological factors, in the urbanized area of Querétaro the mechanical and hydraulic equilibrium in the subsoil is also perturbed by anthropogenic activities, such as overexploitation of groundwater and overloading of compressible ground. In order to delineate the fracturing geometry, we obtained several vertical GPR profiles, perpendicular and oblique to the main trace of the fracture, with two different prospecting frequencies, 900 and 300 MHz. Processing of radar profiles consisted of background removal, topographic correction and AGC amplitude correction to enhance stratigraphic‐related records. The near‐surface stratigraphy consists of partially saturated fluvio‐lacustrine granular and pyroclastic deposits. Detailed measurements of specific gravity, grain‐size, plasticity, water content, and electrical conductivity were performed on samples collected from two shallow trenches in order to relate physical changes in the sedimentary sequence to the recorded electrical contrasts. The analysis of the GPR profiles, plus the stratigraphic record, enabled us to identify variations in the deformation of layers, and changes in direction, width and vertical displacement of fractures. Our results suggest that the propagation of the fracture is influenced by the conditions of deposition of the geological materials and by the anthropogenic activity.
-
-
-
Testing ground‐penetrating radar for resolving facies architecture changes ‐ a radar stratigraphic and sedimentological analysis along a 30 km profile on the Karup Outwash Plain, Denmark
Authors Ingelise Møller and Henrik VosgerauABSTRACTKnowledge of the spatial distribution of lithofacies, obtained through detailed modelling of the movement of water and solutes, may improve precise predictions of an area's vulnerability towards contaminants. This calls for information on lithofacies variability on several scales. The possibilities for upscaling lithofacies distribution from excavation scale to field scale or to an even larger scale with the use of ground‐penetrating radar (GPR) is tested on one of the largest outwash plains in Denmark: the Karup Outwash plain in the western part of Jutland. The testing involves GPR surveys at 16 locations along a 30 km long profile following the ancient mean flow direction of the braided‐river system, as well as combined detailed sedimentological investigations at selected locations. The GPR testing is divided into several phases of which this paper reports on the first two.
In the first phase, GPR surveys and sedimentary investigations at gravel‐pits and excavation sites are carried out at the same location with the aim of determining whether lithofacies can be related to GPR reflection sequences either directly or through facies architecture models. Studies of outcropping glaciofluvial deposits at a gravel‐pit and a shallow excavation in the proximal and intermediate parts of the outwash plain, respectively, show that the sand‐dominated outwash plain typically contains lithofacies types with bed thicknesses of up to 0.5 m and successions of beds of thicknesses up to 1 m. GPR surveys carried out at the same locations show that the observed lithofacies types typically have dimensions that, in a vertical direction, are the same as or less than the resolution obtained by a 200 MHz GPR system. In such sand‐dominated outwash deposits, the GPR reflections may more often be caused by the bedding structures in successions of beds and of larger erosion surfaces separating different lithofacies, rather than by the lithofacies boundaries themselves.
In the second phase, tests are conducted to determine whether facies architecture changes along the ancient mean flow direction in the outwash plain can be recognized using radar stratigraphic analysis. Radar stratigraphic analysis, so far limited to two GPR sections orientated parallel and perpendicular to the ancient mean flow direction at 15 locations, reveals a systematic change in the pattern of radar sequence boundaries and the occurrence of radar facies. The changes in radar sequences seem to reflect changes in lithology and sedimentary structures related to the overall fall in the depositional energy from the proximal to the distal part of the outwash plain. The results clearly indicate that radar stratigraphic analysis reveals realistic facies architecture models for the outwash plain, provided densely spaced GPR data are acquired and locally calibrated against excavations and coreholes.
It is concluded that GPR can be used for upscaling lithofacies distributions in sedimentological settings like the present Karup Outwash Plain.
-
-
-
Radar ‘lensing’ by a small river: Can a layer of surface water improve the signal?
Authors David C. Nobes, Henrik Rother, Jan van der Kruk and Harry M. JolABSTRACTA radar survey was carried out on a gravel bar between the Hope and Boyle Rivers near the Hanmer Basin in the central South Island of New Zealand. One of the profiles crossed a shallow stream, where the depth of penetration and resolution both appeared to improve. The water's high dielectric permittivity, and consequent slower velocity, causes the signal to be directed more vertically into the subsurface. The superior resolution is also due in part to the slower average velocity; the water table is at or above the ground surface in the stream channel. Spectral analysis shows that there is a small increase in the amplitude of the higher frequencies in the stream portion of the profile, so that the improved resolution appears to be real. Multiple reflections from the stream may also contribute to the spectral broadening. The apparent increased depth of penetration is partly due to the increased time required for echoes to return at the slower velocity, but the larger two‐way traveltime does yield a marginally greater depth of penetration.
-
Volumes & issues
-
Volume 22 (2024)
-
Volume 21 (2023)
-
Volume 20 (2022)
-
Volume 19 (2021)
-
Volume 18 (2020)
-
Volume 17 (2019)
-
Volume 16 (2018)
-
Volume 15 (2017)
-
Volume 14 (2015 - 2016)
-
Volume 13 (2015)
-
Volume 12 (2013 - 2014)
-
Volume 11 (2013)
-
Volume 10 (2012)
-
Volume 9 (2011)
-
Volume 8 (2010)
-
Volume 7 (2009)
-
Volume 6 (2008)
-
Volume 5 (2007)
-
Volume 4 (2006)
-
Volume 3 (2005)
-
Volume 2 (2004)
-
Volume 1 (2003)