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  • 1
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 114 (1993), S. 0 
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: A grid search method, analogous to Huygens’ Principle, is employed to find the first-break seismic traveltime field in a seismic model. By defining a set of directions in space, a dynamic directed graph (digraph) containing the first-break time information can be constructed during the minimum traveltime tree-searching process. The dynamic di-graph has far fewer edges than the global graph suggested by previous researchers. An efficient sorting algorithm (heapsort) is adapted to the traveltime data structure by employing an indirect heap strategy. The computational speed can thus be improved several times over other approaches. The minimum traveltime tree algorithm is an efficient and flexible method to simultaneously calculate the first-break time field and the corresponding ray paths. It produces a robust and global result in comparison with traditional ray tracing methods. Later seismic phases can also be handled by imposing constraints on the ray paths. A subgrid technique is better than the normal grid technique in terms of accuracy and efficiency.The construction of the digraph uses the local directional information of ray-paths and therefore local anisotropic information can be naturely incorporated. The first-break time field and its ray paths in an anisotropic model can be calculated as easily as those for an isotropic model.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 109 (1992), S. 0 
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: A new finite-difference formulation is presented for the simulation of P-SV-wave propagation in heterogeneous, isotropic media. In this approach, the wave equations are separated as two sets of equations: one for the displacement fields and the other for potential fields. These two sets of equations involve only first-order spatial derivatives. In terms of potentials, the P-SV-wavefield can be split into P- and S-wave potential fields, which may open an opportunity of simulating P- and S-wave propagation in different ways. By assuming constant density and shear modulus, both P- and S-wave potentials can be expressed in the form of scalar wave equations. Thus, the Lindman ‘free space' boundary condition (Lindman 1975) for scalar waves can be used. An improvement can be made by adding a dissipation zone to absorbing boundaries. For a 2-D model, four equations are used, which is one less than Virieux's (1986) velocity-stress approach. Therefore, the new algorithm is more efficient and requires less computer memory. The calculation is naturally performed in a staggered-grid manner which gives an excellent result for both internal discontinuities and model edges.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 121 (1995), S. 0 
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The aim of geophysical inversion is to infer information about the subsurface model parameters subject to certain constraints and plausibility considerations. It is achieved through a systematic adjustment of model parameters along a solution path to the minimum of an objective function. The objective function is related to both the observations (data) and the a priori information about the subsurface model parameters (model). When different statistics are assumed for the data and the a priori information about the model, different objective functions are constructed and different solution models become inevitable. Traditionally, data residuals are used directly in constructing the objective function. If the residuals are statistically proportional to the data magnitudes, a least-squares (LS) method often leads to a solution model biased towards (i.e. which emphasizes) observations of large magnitude, whereas a least absolute deviation (LAD) method can reduce this data-magnitude-dependent effect. An alternative is to formulate the objective function in terms of relative differences. It completely eliminates the possible solution bias due to the intrinsic magnitude-dependent discrepancies in the objective function. The solution path is altered in such a way that data residuals are reduced in a more uniform manner. The idea is illustrated through a seismic traveltime tomographic inversion. Traveltime residuals are often correlated with ray-path lengths. Greater residuals of long ray paths dominate the objective function in the traditional tomographic formulation. The solution path is thus strongly affected by those long ray paths, and short ray paths play a reduced role in the inversion. The final solution is biased toward longer ray paths. The new alternative formulation eliminates this bias by exploiting the relative-error measure to give equal weights to data of the same quality regardless of their ray-path lengths.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 44 (1996), S. 0 
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: The filter for wave-equation-based water-layer multiple suppression, developed by the authors in the x-t, the linear τ-p, and the f-k domains, is extended to the parabolic τ-2 domain. The multiple reject areas are determined automatically by comparing the energy on traces of the multiple model (which are generated by a wave-extrapolation method from the original data) and the original input data (multiples + primaries) in τ-p space. The advantage of applying the data-adaptive 2D demultiple filter in the parabolic τ-p domain is that the waves are well separated in this domain. The numerical examples demonstrate the effectiveness of such a dereverberation procedure. Filtering of multiples in the parabolic τ-p domain works on both the far-offset and the near-offset traces, while the filtering of multiples in the f-k domain is effective only for the far-offset traces.Tests on a synthetic common-shot-point (CSP) gather show that the demultiple filter is relatively immune to slight errors in the water velocity and water depth which cause arrival time errors of the multiples in the multiple model traces of less than the time dimension (about one quarter of the wavelet length) of the energy summation window of the filter. The multiples in the predicted multiple model traces do not have to be exact replicas of the multiples in the input data, in both a wavelet-shape and traveltime sense. The demultiple filter also works reasonably well for input data contaminated by up to 25% of random noise. A shallow water CSP seismic gather, acquired on the North West Shelf of Australia, demonstrates the effectiveness of the technique on real data.
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  • 5
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Science Ltd
    Geophysical prospecting 45 (1997), S. 0 
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: Seismic first-arrival times from a cross-well experiment are inverted by an iteratively reweighted least squares method for the velocity distribution between boreholes. The inversion is carried out in a multiresolution manner in which long wavelength features are reconstructed first and short wavelength features are gradually introduced into the solution model. As such, the inverse problem is overdetermined at the beginning and becomes mixed-determined as the constraint on the solution is relaxed. The reconstruction procedure is independent of the initial model and very robust. An accurate and efficient first-break traveltime calculation scheme is employed to eliminate the possibility of phase mismatches, which can arise with traditional ray shooting or bending methods. Shot static delays are included with the model parameters as unknowns and are recovered simultaneously with the velocity distribution.
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  • 6
    Publication Date: 2012-03-01
    Description: Elastic-waveform inversions have the potential to provide detailed subsurface images of the elastic parameters (P- and S-wave velocities and density), but acquisition of suitable data sets and their inversion are nontrivial tasks. We explore the information content offered by elastic-waveform data by means of a 2D synthetic study. Comprehensive noise-free data sets that include recordings based on multicomponent (directed) sources and multicomponent (vector) receivers that fully surround the area of interest allow all elastic parameters to be reliably recovered. Results that are almost as good can be achieved with the more commonly used crosshole configuration. If only single-source components (e.g., those oriented perpendicular to the borehole walls) are used, then there is no significant quality degradation of the tomographic images. Crosshole experiments that include pressure sources and multicomponent receivers still allow P- and S-wave velocities to be recovered, but such data sets contain virtually no information about the density. Finally, seismic data collected with omnidirectional pressure sources and pressure receivers contain information about P- and S-wave velocities, but there are pronounced trade-offs between these parameters. This is demonstrated through formal model-resolution analyses. This study concludes that seismic data recorded with pressure sources and 2C receivers offer the best compromise between acquisition efficiency and data-information content.
    Print ISSN: 0016-8033
    Electronic ISSN: 1942-2156
    Topics: Geosciences , Physics
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  • 7
    Publication Date: 2012-01-01
    Description: Seismic waveform-inversion offers opportunities for detailed characterization of the subsurface. However, its full potential can only be exploited when any systematic source and receiver effects are either carefully avoided or appropriately accounted for during the inversions. Repeated crosshole measurements in the Mont Terri (Switzerland) underground laboratory have revealed that receiver coupling may significantly affect the seismic waveforms. More seriously, coupling conditions may vary during the course of a monitoring experiment. To address this problem, we have developed a novel scheme that estimates medium properties, frequency-dependent source functions, and frequency-dependent receiver-coupling factors. We demonstrate the efficacy of the new scheme via a synthetic 2D crosshole experiment in which realistic receiver-coupling factors are incorporated. Because determination of medium parameters and estimation of source functions and receiver-coupling factors are largely separated, the method can be easily adapted to any other waveform-inversion problem, including elastic, anisotropic, 2.5D, or 3D situations.
    Print ISSN: 0016-8033
    Electronic ISSN: 1942-2156
    Topics: Geosciences , Physics
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  • 8
    Publication Date: 2012-04-01
    Description: By utilizing our newly developed multiple arrival (transmitted, reflected, refracted, and mode-converted) tracking algorithm (multistage irregular shortest-path method), in combination with the conjugate gradient method to solve the damped minimum norm, constrained least-squares problem, we present a simultaneous inversion algorithm to recover both the velocity information and the subsurface interface geometry. In the inversion process, we introduce different weighting factors according to the different picking errors for the various seismic arrivals and normalize the two types of model parameters that form the elements of the Jacobian matrix so as to balance the influence on the travel times of the different velocity variations and reflector depths. From numerical tests and by comparison with a subspace inversion algorithm, we show the new approach to be a practical and efficient way to improve the spatial resolution and reduce artifacts in the reconstructed image.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
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  • 9
    Publication Date: 2011-01-01
    Description: Exploiting the information content offered by geoelectric data in an efficient manner requires careful selection of the electrode configurations to be used. This can be achieved using sequential experimental design techniques proposed over the past few years. However, these techniques become impractical when large-scale 2D or 3D experiments have to be designed. Even if sequential experimental design were applicable, acquisition of the resulting data sets would require an unreasonably large effort using traditional multielectrode arrays. We present a new, fully parallelized pole-bipole measuring strategy by which large amounts of data can be acquired swiftly. Furthermore, we introduce a new experimental design concept that is based on “complete” data sets in terms of linear independence. Complete data sets include a relatively small number of basis electrode configurations, from which any other configuration can be reconstructedby superposition. The totality of possible configurations is referred to as the comprehensive data set. We demonstrate the benefits of such reconstructions using eigenvalue analyses for the case of noise-free data. In the presence of realistic noise, such reconstructions lead to unstable results when only four-point (bipole-bipole) configurations are considered. In contrast, complete three-point (pole-bipole) data sets allow more stable reconstructions. Moreover, complete pole-bipole data sets can be acquired very efficiently with a fully parallelized system. Resolution properties of complete pole-bipole data sets are illustrated using both noise-free and noisy synthetic data sets. We also show results from a field survey performed over a buried waste disposal site, which further demonstrates the usefulness of our approach. Although this paper is restricted to 2D examples, it is trivial to extend the concept to 3D surveys, where the advantages of parallelized pole-bipole data acquisition become very significant.
    Print ISSN: 0016-8033
    Electronic ISSN: 1942-2156
    Topics: Geosciences , Physics
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  • 10
    Publication Date: 2011-03-01
    Description: The hydrogeological properties and hydrological responses of a productive aquifer in northeastern Switzerland are investigated. For this purpose, 3D crosshole electrical resistivity tomography (ERT) is used to define the main lithological structures within the aquifer (through static inversion) and to monitor the water infiltration from an adjacent river. During precipitation events and subsequent river flooding, the river water resistivity increases. As a consequence, the electrical characteristics of the infiltrating water can be used as a natural tracer to delineate preferential flow paths and flow velocities. The focus is primarily on the experiment installation, data collection strategy, and the structural characterization of the site and a brief overview of the ERT monitoring results. The monitoring system comprises 18 boreholes each equipped with 10 electrodes straddling the entire thickness of the gravel aquifer. A multichannel resistivity system programmed to cycle through various four-point electrode configurations of the 180 electrodes in a rolling sequence allows for the measurement of approximately 15,500 apparent resistivity values every 7 h on a continuous basis. The 3D static ERT inversion of data acquired under stable hydrological conditions provides a base model for future time-lapse inversion studies and the means to investigate the resolving capability of our acquisition scheme. In particular, it enables definition of the main lithological structures within the aquifer. The final ERT static model delineates a relatively high-resistivity, low-porosity, intermediate-depth layer throughout the investigated aquifer volume that is consistent with results from well logging and seismic and radar tomography models. The next step will be to define and implement an appropriate time-lapse ERT inversion scheme using the river water as a natural tracer. The main challenge will be to separate the superposed time-varying effects of water table height, temperature, and salinity variations associated with the infiltrating water.
    Print ISSN: 0016-8033
    Electronic ISSN: 1942-2156
    Topics: Geosciences , Physics
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