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Characterization of the shallow subsurface (0.25 to 10 m) is of growing importance for engineering activities, solutions of environmental problems, and archaeological investigations. Ground-penetrating radar (GPR) is an appropriate technique considering the depth range of interest, the strength of electric and magnetic contrasts between different subsurface layers and buried objects, and the required resolution. GPR surveys can detect subsurface structures by recording electromagnetic reflections from discontinuities. The detectability of objects and the delineation of subsurface structures increases with increasing wave velocity and conductivity differences between the object and its surroundings or between adjacent layers. However, unwanted reflections from objects above the surface influence the images. Shielded antennas can be used to avoid strong reflections from these objects. The data thus obtained are, however, more difficult to interpret. The fundamentals of GPR and two different acquisition setups for a GPR system are discussed. Basic interpretation tools for travel-time and velocity estimation are described, and finally, case studies are presented, followed by conclusions. 相似文献
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Jürg Hunziker Evert Slob Yuanzhong Fan Roel Snieder Kees Wapenaar 《Geophysical Prospecting》2012,60(5):974-994
We use numerically modelled data sets to investigate the sensitivity of electromagnetic interferometry by multidimensional deconvolution to spatial receiver sampling. Interferometry by multidimensional deconvolution retrieves the reflection response below the receivers after decomposition of the fields into upward and downward decaying fields and deconvolving the upward decaying field by the downward decaying field. Thereby the medium above the receiver level is replaced with a homogeneous half‐space, the sources are redatumed to the receiver level and the direct field is removed. Consequently, in a marine setting the retrieved reflection response is independent of any effect of the water layer and the air above. A drawback of interferometry by multidimensional deconvolution is a possibly unstable matrix inversion, which is necessary to retrieve the reflection response. Additionally, in order to correctly separate the upward and the downward decaying fields, the electromagnetic fields need to be sampled properly. We show that the largest possible receiver spacing depends on two parameters: the vertical distance between the source and the receivers and the length of the source. The receiver spacing should not exceed the larger of these two parameters. Besides these two parameters, the presence of inhomogeneities close to the receivers may also require a dense receiver sampling. We show that by using the synthetic aperture concept, an elongated source can be created from conventionally acquired data in order to overcome these strict sampling criteria. Finally, we show that interferometry may work under real‐world conditions with random noise and receiver orientation and positioning errors. 相似文献
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Modelling and inversion of controlled‐source electromagnetic (CSEM) fields requires accurate interpolation of modelled results near strong resistivity contrasts. There, simple linear interpolation may produce large errors, whereas higher‐order interpolation may lead to oscillatory behaviour in the interpolated result. We propose to use the essentially non‐oscillatory, piecewise polynomial interpolation scheme designed for piecewise smooth functions that contains discontinuities in the function itself or in its first or higher derivatives. The scheme uses a non‐linear adaptive algorithm to select a set of interpolation points that represent the smoothest part of the function among the sets of neighbouring points. We present numerical examples to demonstrate the usefulness of the scheme. The first example shows that the essentially non‐oscillatory interpolation (ENO) scheme better captures an isolated discontinuity. In the second example, we consider the case of sampling the electric field computed by a finite‐volume CSEM code at a receiver location. In this example, the ENO interpolation performs quite well. However, the overall error is dominated by the discretization error. The other examples consider the comparison between sampling with essentially non‐oscillatory interpolation and existing interpolation schemes. In these examples, essentially non‐oscillatory interpolation provides more accurate results than standard interpolation, especially near discontinuities. 相似文献
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Reduction of reflections from above surface objects in GPR data 总被引：2，自引：0，他引：2
During a ground-penetrating radar (GPR) survey, special attention must be paid to objects located above the earth's surface. Due to the low-loss character of electromagnetic propagation in air and high velocity, above-surface reflections or diffractions can overwhelm subsurface events, making the interpretation a difficult task. The relative sensitivity of reflections and diffractions originating from above-surface objects is a function of the antenna radiation characteristics, the lateral and vertical dimensions of the objects and their position with respect to the antennas. The largest amplitude reflections and diffractions are expected when the polarization of the electric field is parallel to the long-axis of the object. Near the surface in the E-plane, the electric field is vertically polarized and has a larger amplitude than the horizontally polarized electric field in the H-plane. Numerical modeling of reflections from three above surface objects (a vertical plane and elongated horizontal and vertical objects) demonstrate that the largest amplitude difference occurs when an elongated vertical object is present in the E- or H-plane. The calculated reflection from the elongated vertical object present in the E-plane was 21 times larger than when it was present in the H-plane. In 60-m long field data sets, reflections from interfering trees present in the E-plane were at several positions >15 times larger and on average 6 times larger than when the trees were present in the H-plane. These large amplitude differences indicate that appropriate orientation of the antennas can be used to minimize the effects of above-surface reflections and diffractions. 相似文献
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Giovanni Angelo Meles Lele Zhang Jan Thorbecke Kees Wapenaar Evert Slob 《Geophysical Prospecting》2020,68(6):1834-1846
Seismic images provided by reverse time migration can be contaminated by artefacts associated with the migration of multiples. Multiples can corrupt seismic images, producing both false positives, that is by focusing energy at unphysical interfaces, and false negatives, that is by destructively interfering with primaries. Multiple prediction/primary synthesis methods are usually designed to operate on point source gathers and can therefore be computationally demanding when large problems are considered. A computationally attractive scheme that operates on plane-wave datasets is derived by adapting a data-driven point source gathers method, based on convolutions and cross-correlations of the reflection response with itself, to include plane-wave concepts. As a result, the presented algorithm allows fully data-driven synthesis of primary reflections associated with plane-wave source responses. Once primary plane-wave responses are estimated, they are used for multiple-free imaging via plane-wave reverse time migration. Numerical tests of increasing complexity demonstrate the potential of the proposed algorithm to produce multiple-free images from only a small number of plane-wave datasets. 相似文献
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A. K. Mahajan Siefko Slob Rajiv Ranjan Rob Sporry P. K. Champati ray Cees J. van Westen 《Journal of Seismology》2007,11(4):355-370
The understanding of geotechnical characteristics of near-surface material is of fundamental interest in seismic microzonation.
Shear wave velocity (Vs), one of the most important soil properties for soil response modeling, has been evaluated through
seismic profiling using the multichannel analysis of surface waves in the city of Dehradun situated along the foothills of
northwest Himalaya. Fifty sites in the city have been investigated with survey lines between 72 and 96 m in length. Multiple
1-D and interpolated 2-D profiles have been generated up to a depth of 30–40 m. The Vs were used in the SHAKE2000 software
in combination with seismic input motion of the recent Chamoli earthquake to obtain site response and amplification spectra.
The estimated Vs are higher in the northern part of the study area (i.e., 200–700 m/s from the surface to a depth of about
30 m) as compared to the south and southwestern parts of the city (i.e., 180–400 m/s for the same depth range). The response
spectra suggest that spectral acceleration values for two-story structures are three to eight times higher than peak ground
acceleration at bedrock. The analysis also suggests peak amplification at 3–4, 2–2.5, and 1–1.5 Hz in the northern, central,
and south-southwestern parts of the city, respectively. The spatial distributions of Vs and spectral accelerations provide
valuable information for the seismic microzonation in different parts of the urban area of Dehradun. 相似文献
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