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- Volume 17, Issue 5, 2019
Near Surface Geophysics - Volume 17, Issue 5, 2019
Volume 17, Issue 5, 2019
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Two‐dimensional elastic full‐waveform inversion of Love waves in shallow vertically transversely isotropic media: synthetic reconstruction tests
Authors Valérie Krampe, Yudi Pan and Thomas BohlenABSTRACTIn most shallow‐seismic applications of full‐waveform inversion, the subsurface is assumed to be isotropic, although near‐surface materials may exhibit strong seismic anisotropy. Ignoring anisotropy will lead to inexact solutions when simulating wave propagation or imaging the subsurface using full‐waveform inversion. For shallow structures, vertically transversely isotropic media provide a suitable approximation due to the fine horizontal layering of the sediments. We investigate the effects of anisotropy on surface waves and on shallow‐seismic full‐waveform inversion in vertically transversely isotropic media. The comparisons of seismograms calculated in isotropic and vertically transversely isotropic models show that the sensitivity of full‐waveform inversion towards anisotropy is significantly higher for Love waves than for Rayleigh waves. This observation indicates that it is more promising to perform full‐waveform inversion on Love waves rather than on Rayleigh waves to identify anisotropy of near‐surface materials. Therefore, we performed synthetic two‐dimensional reconstruction tests of anisotropic full‐waveform inversion using only shallow‐seismic Love waves. These tests revealed that the parameters describing vertical transverse isotropy can be accurately reconstructed by full‐waveform inversion. Although the inversion for density is still problematic, this does not affect the results for the seismic velocities in a significant way. The tests on synthetic data thus prove the general applicability and the benefits of an anisotropic inversion of shallow‐seismic Love waves, which can provide a more comprehensive subsurface characterization in shallow anisotropic media.
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Ray‐based reflection traveltime tomography using approximate stationary points
Authors Xiangyue Li, Dong‐Joo Min, Jongha Hwang and Ju‐Won OhABSTRACTReflection traveltime tomography has been used to describe subsurface velocity structures which, in practice, can be used as a background or initial model for pre‐stack depth migration or full waveform inversion. Conventional reflection traveltime tomography is performed by solving an optimization problem based on a ray‐tracing method. As a result, reflection traveltime tomography requires heavy computational efforts to carry out ray tracing and solve a large matrix equation. In addition, like most data‐domain tomography methods, reflection traveltime tomography depends on initial guesses and suffers from non‐uniqueness and uncertainty of solutions. In this research, we propose a deterministic ray‐based reflection traveltime tomography method by applying seismic interferometry. This method does not suffer from the non‐uniqueness problem and does not require a priori information on subsurface media. By adding a virtual layer (whose properties are known) on top of the real surface and applying convolution‐type interferometry, we approximately determine the stationary points (i.e., incident raypaths in the virtual layer). Then, we generate reflection points for a range of assumed velocities and estimate the velocity by considering the number of reflection points and the traveltime difference between the observed and calculated data. The reflection surface can then be recovered by using the estimated velocity. Once the first target layer is resolved, we can recover the whole media by recursively applying the same method to the lower layers. Numerical examples using surface seismic profile data for homogeneous‐layer (with a low‐velocity layer) and inhomogeneous‐layer models and real field data experiments on the Congo data set demonstrate that our method can successfully recover the velocities and depths of subsurface media without initial guesses. However, our method has some limitations for multi‐layer models because the method does not have sufficient reflection points for the deeper layers.
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Using jittered sampling in designing geometry and imaging in shallow 3D seismic surveys
Authors Peng Li and Jian‐Qing ZhangABSTRACTWhen the depth of the shallow three‐dimensional seismic exploration is less than 100 m, one often encounters very low velocities for the target and high frequencies in the data. Following Nyquist–Shannon sampling theorem, the permissible maximum receiver interval can be smaller in this case compared to relatively deeper seismic exploration. This suggests that there are still issues to be addressed in the design of geometry and in data processing in shallow three‐dimensional seismic exploration. This paper addresses these problems by applying the theory of compressed sensing for signal processing to shallow‐seismic geometry designing and data processing. Theoretical research shows that random sampling of data can better reconstruct the wavefield than undersampled data. Random sampled data can transform the coherent aliasing to non‐coherent noise, which turns the seismic data interpretation problem into a data denoising problem. The jittered random sampling method avoids the situation when the spatial data points of a randomly sampled dataset are too concentrated or too sparse. Our proposed approach was tested on simulated and real seismic data. The results show that if the jittered random undersampling method is used in shallow three‐dimensional seismic data acquisition, then a wider range of observation with fewer receivers in the layout is possible. This greatly improves the data collection efficiency in the field. In addition, the random sampling method has more flexibility in the field environment. When using the regular sampling method, an open survey area without large obstacles is needed. However, the random sampling method can be adapted to rugged terrains. When obstacles are encountered, the receiver spacing can be increased appropriately. In open areas, the receiver spacing can be decreased to compensate for the reduced data.
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Rayleigh‐wave dispersion analysis using complex‐vector seismic data
Authors Xinming Qiu, Yun Wang and Chao WangABSTRACTIdentification of different modes of Rayleigh waves is essential in surface‐wave surveys. Multi‐mode Rayleigh waves can provide higher accuracy of the near‐surface structure than the fundamental mode alone. However, some modes or frequencies of Rayleigh waves may be absent in the vertical‐component seismic data. To complement the dispersion information, a method based on complex‐vector seismic data is proposed. We construct the complex vector by setting the radial component and vertical component as the real part and imaginary part, respectively. Then, high‐resolution linear Radon transform is used to obtain the multi‐mode Rayleigh‐wave dispersion image of the complex‐vector seismic data. Based on different dispersion characteristics of the radial and vertical components, the dispersion images of the complex–vector seismic data show better performance against interferences and mode misidentification. Synthetic and field examples demonstrate advantages of the complex‐vector method over the traditional vertical‐component method in spectral bands and dispersion curve mode identification. Therefore, a more robust and accurate near‐surface S‐wave velocity structure can be expected compared to the traditional vertical‐component Rayleigh‐wave method.
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Application of surface waves for detecting lateral variations: buried inclined plane
More LessABSTRACTIn this paper, using synthetic and real data, we tested the capability of surface wave‐based methods for detecting subsoil lateral variations across an inclined slope. Simplified soil structures at different inclination angles were considered following an advanced 2D finite‐element modelling approach. Different values of inclination angle (10°–170° at 10° steps), spatial sampling rate and synthetic array length were used in different subsoil models to see the effects. It was found that, as low inclination angles (smaller than 40° with respect to the horizontal axis) are not detectable using the surface wave methods based on offset‐phase angle (X–ϕ), such methods are not able to correctly recognize the location of possible lateral variations at such inclination angles. On the other hand, for intermediate inclination angles (i.e. between 40° and 140°), the X–ϕ approach was successfully used to determine the exact location of the lateral variations for a wide range of frequencies, thereby opening new perspectives for the application of surface waves for detecting laterally inclined layers.
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VS30 mapping and site characterization in the seismically active intraplate region of Western India: implications for risk mitigation
Authors B. Sairam, A. P. Singh, Vandana Patel, Sumer Chopra and M. Ravi KumarABSTRACTWe carried out shallow subsurface investigations at 600 sites covering all types of geological units in the Northwestern Deccan Volcanic Province of India, using the multichannel analysis of surface waves and suspension PS‐logging methods. The results reveal that VS30 values are in the range of 760–1500 m/s for granites and Deccan traps, thus enabling their classification as B‐class, as per the National Earthquake Hazard Reduction Program recommendations. The Tertiary, Cretaceous, Jurassic and Paleoproterozoic sediments show VS30 values between 360 and 760 m/s, and hence, are assigned C‐class. Further, the Quaternary soils are categorized as D‐class, since they show VS30 in the range of 180–360 m/s. Also, the Holocene tidal flat and Rann sediments are classified as E‐class, since the VS30 values are less than 180 m/s. The observed site response reveals that the D‐ and E‐type soils have significantly higher seismic amplification than that in the B‐ and C‐category soils. We noticed that the buildings on D‐ and E‐classes soils experienced higher damage than those on the B‐ and C‐classes, during the past large earthquakes in the Northwestern Deccan Volcanic Province. Our study suggests that VS30 is a good proxy for soil classifications in the Northwestern Deccan Volcanic Province. We validated our results using available geological, geophysical and geotechnical data. For the first time, a regional site characterization map for the Northwestern Deccan Volcanic Province of India is prepared based on geology and VS30.
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Application of the DC resistivity method in urban geological problems of karstic areas
Authors Zsuzsanna Plank and Dorottya PolgárABSTRACTTerminating industrial activities may have harmful side effects on the infrastructure of a civil society. One effective method of solving environmental and geotechnical problems in urban areas is a direct‐current resistivity survey. In karstic areas, large‐scale geological structures directly affect the near‐surface, small‐scale events. Reliable data on the geological background of harmful events are acquired through suitable combinations of large‐scale and of small‐scale high‐resolution surveys. In this study, the geological background of three urban subsurface‐related problems was investigated in a karstic area: collapse of a sinkhole, suffusion of a basement area, and contamination of an urban creek. In each case, the scientific work aimed at setting up of a suitable model that describes the harmful event and its geological background. For this purpose, a combination of large‐scale and high‐resolution resistivity data was acquired and augmented by borehole information and laboratory analyses. The error analyses of the resulting models suggest that the methodology of dual‐scale geoelectric investigation of urban hazards in karstic areas can provide applicable models for mitigation measures.
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Vector analysis of pole–pole array for determining the 3D boundary of object
Authors Olga Nemtsova, Igor Zhurbin and Anna ZlobinaABSTRACTAn algorithm is proposed for the interpretation of resistivity data that allows the 3D parameters (the boundary in plan view and the depth range) of an anomalous resistivity object in a heterogeneous medium to be determined. The proposed method is based on a vector analysis of the apparent resistivity of soil obtained by a pole–pole survey within the measurement window. In the first stage of the algorithm, in each measurement window, the radius vector is calculated, and as a result, vector images of the main directions of change in the resistivity of the medium are constructed. This allows us to estimate the location of a local anomalous object and to correlate the resistivity of the object with that of the background medium. With a consistent variation in the effective depth of investigation, a set of vector images is formed that characterizes the apparent resistivity distribution in the soil layers. Mathematical analysis of the vector images by using the scalar product allows us to estimate the depth range of the anomalous object. The effectiveness of the proposed algorithm has been proven on synthetic models and in comprehensive investigations of archaeological sites. The proposed method does not allow the true resistivity of an anomalous object to be determined, which is a disadvantage. However, this simple algorithm for processing and analysing shallow electrical prospecting data can be directly used at the preliminary data processing stage during a field survey. First, a pole–pole array can be rapidly operated by a single person; second, the user does not need special knowledge in the field of electrical resistivity data processing. The estimated 3D boundaries of local objects will make it possible to determine the area of interest for further detailed surveying and to justify all the parameters of the measurement technique (distance between electrodes, depth range etc.).
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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)