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  • 1
    Monograph available for loan
    Monograph available for loan
    Multi-component VSP analysis for applied seismic anisotropy (2002)
    Oxford [u.a.] : Elsevier
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    Call number: 5/M 02.0480
    In: Handbook of geophysical exploration
    Type of Medium: Monograph available for loan
    Pages: xii, 356 S.
    Edition: 1st. ed.
    ISBN: 0080424392
    Series Statement: Handbook of geophysical exploration : seismic exploration 26
    Classification:
    Seismology
    Language: English
    Location: Reading room
    Branch Library: GFZ Library
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  • 2
    Electronic Resource
    Electronic Resource
    Linear matrix operations for multicomponent seismic processing (1996)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 124 (1996), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Multicomponent seismic data contain overlapping information on the polarization states of distinct body-wave modes, due to the physical process of excitation, propagation and recording. This geometric redundancy should be exploited to provide an accurate separation and estimation of the wavefield attributes in order to understand the medium properly. This may be achieved using linear transforms, originally developed for separating split shear waves in four-component seismic data. These transforms separate the principal time-series components of the wavefield from the ray-path geometry and the orientation of the source and geophone axes for a uniform medium: they are deterministic and can be easily implemented.Here we reformulate the linear transforms by introducing simple geometry and medium-independent matrix operators. Although for ‘ideal’ experiments the technique may offer nothing new to the estimation of polarization that eigenanalysis cannot offer, nevertheless the formulation avoids the need to consult mathematical libraries and is useful in the interpretation of the wavefield when various inevitable acquisition-related errors dominate. Some typical problems in processing four-component data, such as the interpretation of data matrix asymmetry due to misorientations of the acquisition components and non-orthogonal polarizations for the wave components, may be easily treated and identified using a common framework with this condensed matrix form. In addition, the operation is extended to similar geometric problems in six- and nine-component data. Synthetic and field nine-component data examples are presented to illustrate the application of the matrix operations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1996.tb06363.x
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  • 3
    Electronic Resource
    Electronic Resource
    Automatic picking of seismic arrivals in local earthquake data using an artificial neural network (1995)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 120 (1995), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: A preliminary study is performed to test the ability of an artificial neural network (ANN) to detect and pick seismic arrivals from local earthquake data. This is achieved using three-component recordings by utilizing the vector modulus of these seismic records as the network input. A discriminant function, F(t), determined from the output of the trained ANN, is then employed to define the arrival onset. 877 pre-triggered recordings from two stations in a local earthquake network are analysed by an ANN trained with only nine P waves and nine noise segments. The data have a range of magnitudes (ML) from -0.3 to 1.0, and signal-to-noise ratios from 1 to 200. Comparing the results with manual picks, the ANN can accurately detect 93.9 per cent of the P waves and also 90.3 per cent of the S waves with a F(t) threshold set at 0.6 (maximum is 1.0). These statistics do not include false alarms due to other non-seismic signals or unusable records due to excessive noise. In 17.2 per cent of the cases the ANN detected false alarms prior to the event. Determining the onset times by using the local maximum of F(t), we find that 75.4 per cent of the P-wave estimates and 66.7 per cent of the S-wave estimates are within one sample increment (10 ms) of the reference data picked manually. Only 7.7 per cent of the P-wave estimates and 11.8 per cent of the S-wave estimates are inaccurate by more than five sample increments (50 ms). The majority of these records have distinct local P and S waves. The ANN also works for seismograms with low signal-to-noise ratios, where visual examination is difficult. The examples show the adaptive nature of the ANN, and that its ability to pick may be improved by adding or adjusting the training data. The ANN has potential as a tool to pick arrivals automatically. This algorithm has been adopted as a component in the early stages of our development of an automated subsystem to analyse local earthquake data. Further potential applications for the neural network include editing of poor traces (before present algorithm) and rejection of false alarms (after this present algorithm).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1995.tb01851.x
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  • 4
    Electronic Resource
    Electronic Resource
    Multicomponent near-surface correction for land VSP data (1995)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 121 (1995), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Multicomponent seismic data collected using directional sources are degraded by the wave excitation process due to inaccurate control of the ground motion. unequal activation strengths or ground couplings between differently oriented sources, and misalignment of the pad. These acquisition uncertainties are exacerbated by the complicated near-surface scattering present in most seismic areas. Neither group of effects should be neglected in multicomponent analyses that make use of relative wavefield attributes derived from compressional and shear waves. These effects prevent analysis of the direct and reflected waves using procedures based on standard scalar techniques or a prima facia interpretation of the vector wavefield properties, even for the seemingly straightforward case of a near-offset vertical seismic profile (VSP). Near-surface correction, using a simple matrix operator designed from the shallowest recordings, alleviates many of these interpretational difficulties in near-offset VSP data. Results from application of this technique to direct waves from a nine-component VSP shot at the Conoco test-site facility, Oklahoma, are encouraging. The technique corrects for unexpected compressional-wave energy from shear-wave vibrators and collapses near-surface multiples, thus facilitating further processing for the upgoing wavefield. The method provides a simple and effective processing step for routine application to near-offset VSP analyses.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1995.tb03528.x
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  • 5
    Electronic Resource
    Electronic Resource
    Inverting shear-wave polarizations for anisotropy using three-component offset VSPs: synthetic seismograms (1991)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 107 (1991), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: A numerical algorithm is presented for interpreting shear-wave polarization directions, from offset VSP data, as subsurface anisotropy. The technique compares observed (synthetic) horizontal polarization angles with expected values for a range of physically plausible anisotropic models with different orientations. The comparison is implemented in a database inversion scheme. The anisotropic models are formed by combining the effective anisotropy of parallel, aligned, vertical cracks with matrix anisotropy due to thin-layering or lithology. Different models are defined by varying the crack and matrix properties. The technique is applied to synthetic offset VSP data for a multilayered anisotropic structure, generated by multi-azimuth sources. Analysis of the results indicates that polarization measurements from non-zero offsets with at least two and preferably three different azimuths are required in order to reasonably constrain the final inversion solutions. The parameters which are resolved depend upon the depth of the subsurface zone of interest. Crack strike is well resolved in the deeper layers for which directions of propagation are sub-vertical, whereas other model parameters such as aspect ratio and crack content can only be resolved for wider ranges of incidence angle. The key to a successful inversion appears to lie in the selection of propagation paths through the relatively unique patterns of disruption created by singularities.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1991.tb01417.x
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  • 6
    Electronic Resource
    Electronic Resource
    The problem of water-column multiples for processing converted S waves in marine VSP data (1994)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 119 (1994), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Seismic data from marine VSPs contain converted S, waves which can be used to provide information on the internal structure of the subsurface rocks. to make the best analysis of these waves it is necessary to choose a suitable offset to maximize the converted energy. Numerical modelling with a hard sea-bottom shows that even for the moderate offsets which provide the best conversions at the sea-bed in a shallow-water environment, it is not possible to jointly compensate the P and S waves for their effective seismic signatures using a single common scalar function. This is due to a difference in the periodicity of the water-column multiples associated with the two wavetypes. the difference becomes more pronounced for a large Poisson's ratio in the sea-bed sediments. Standard processing on the three-component recordings cannot be adequately performed except for subvertical incidence. Consequently, P and S waves must be separated before signature deconvolution, or else a more sophisticated multicomponent operator must be deployed. In the process of acquisition design, the detailed modelling required to understand the multicomponent response must be weighed up against the offset desired for optimal conversion.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1994.tb04031.x
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  • 7
    Electronic Resource
    Electronic Resource
    Inversion for subsurface anisotropy using estimates of shear-wave splitting (1991)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 107 (1991), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Inversion of the parameters characterizing shear-wave splitting (polarizations, time delays, and differential attenuation) provides a way of obtaining a good starting model for full-wave anisotropic modelling, and improves the convergence to a final waveform match. This is necessary as the shear waves behave in a highly non-linear fashion with respect to anisotropy. Here we discuss the extent to which polarization and time-delay information, estimated from split shear waveforms, may supply details of this starting model. 3-D patterns of these parameters for different anisotropic models are considered, to investigate the potential of the inversion scheme. Guidelines are formed for the best arrangements of sources and geophones, to provide optimum resolution of an anisotropic model. The study indicates that in most cases different anisotropic models cannot be adequately distinguished using vertical seismic profiles (VSPs), reflection profiles, and cross-hole surveys in which separate polarization or time-delay measurements at one azimuth are made. Better results are obtained by combining polarization and time-delays estimates, preferably from more than one azimuth. The type of model parameters which can be resolved depends crucially upon the geometry of the acquisition system, and to a lesser extent on the background velocity structure. Individual model parameters are more difficult to extract, but some details appear (in principle) to be resolvable for certain types of survey. VSPs are most suitable to monitor crack strike and density of parallel, vertical, aligned cracks, and the dip of the fine layering contributing to layer-induced anisotropy. Cross-hole experiments are best for measuring the dip of the cracks. The resolution of the crack content and aspect ratio are determined principally by the position of line singularities, and either an offset VSP, or an adapted cross-hole survey may be adequate for resolving these properties if the line is shot at an appropriate azimuth with respect to the strike of the cracks. The ratio of crack-induced anisotropy to matrix anisotropy in sedimentary basins can be evaluated by measuring the positions of the point singularities in recordings of polarization and time-delay patterns using a near-offset VSP.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1991.tb01418.x
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  • 8
    Electronic Resource
    Electronic Resource
    EXAMINATION OF A SPECTRAL METHOD FOR MEASURING THE EFFECTS OF ANISOTROPY (1991)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 39 (1991), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: The most diagnostic effect of anisotropy on shear waves is shear-wave splitting. This phenomenon creates a distinctive signature in the 3D particle motion. Methods to extract the effects of anisotropy from shear-wave data attempt to measure details of this motion. Many techniques have been published recently which process the shear waves in the time or frequency domain. Here we examine the way in which information on the interference effects between the split shear waves is contained within the frequency domain, and suggest some criteria which may be used in future processing algorithms. The time-delay between the split shear waves, and the polarization direction of the leading shear wave can be converted into easily measured features from analysis of the Fourier spectrum of the shear-wave signal on each component of motion. These features arise in the spectral interference patterns which are formed by the interaction between the two closely-spaced and similar waveforms. The interference patterns are interpreted for synthetic and observed seismogram data.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1991.tb00335.x
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  • 9
    Electronic Resource
    Electronic Resource
    OPTIMAL ESTIMATION OF CRACK-STRIKE (1992)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 40 (1992), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: The accuracy of estimating crack-strike from the algebraic equivalent of a popular technique, the dual source cumulative technique (DCT), for analysing shear-wave splitting in seismic experiments is evaluated for earth models permeated by different alignments of micro-cracks. A complementary analysis is performed using another analysis procedure, the dual-independent source-geophone technique (DIT), to investigate any benefits of the alternative formulation. The investigation considers synthetic vertical seismic profile (VSP) and reflection data for an earth model with two layers over a half-space, and three different classes of crack-strike variation with depth: uniform crack-strike, an abrupt change of crack-strike between the upper and lower layer, and a continuous increase over both layers. The synthetic data for zero-offset and near-offset VSPs and a reflection profile are computed using a full-wave modelling package in which equivalent anisotropic media simulate distributions of aligned vertical, parallel, water-filled microcracks. Estimates from the two techniques agree for the constant crack-strike model, but differ for the VSP data with crack-strike changes. The asymptotic behaviour of the two angular parameters θG and θS from DIT suggest that it may be used to determine crack-strike under appropriate circumstances in these VSPs, when the time-delay between the split shear-waves for the layer of interest exceeds the peak period of the wavelet. In this limit, θG tends to follow the crack-strike change with θS tending to a constant value, whereas DCT will give a misleading value between the upper and lower crack-strike. Although the behaviour of DIT is not understood in all cases, θG and θS values from the VSP data always appear to diverge near the point where an abrupt crack-strike change takes place. This could be used as a qualitative indicator for layer stripping. Both techniques agree for the reflection data as the recorded data matrix is necessarily symmetric, but still give misleading results for deeper layers in the presence of crack-strike changes. This study suggests that more care should be taken when designing and analysing experimental configurations for detecting crack properties in reservoir rocks, to consider the response and resolution limits of the analysis techniques. A note of caution is offered to those who directly interpret polarization estimates as crack-strike.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1992.tb00556.x
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  • 10
    Electronic Resource
    Electronic Resource
    Cross-matching with interpreted warping of 3D streamer and 3D ocean-bottom-cable data at Valhall for time-lapse assessment (2005)
    PO Box 1354, 9600 Garsington Road , Oxford OX4 2XG , UK . : Blackwell Science Ltd
    Geophysical prospecting 53 (2005), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: Legacy streamer data and newer 3D ocean-bottom-cable data are cross-matched and analysed for time-lapse analysis of geomechanical changes due to production in the Valhall Field. The issues relating to time-lapse analysis using two such distinctly different data sets are addressed to provide an optimal cross-matching workflow that includes 3D warping. Additionally an assessment of the differences between the imaging using single-azimuth streamer and multi-azimuth ocean-bottom-cable data is provided. The 3D warping utilized in the cross-matching procedure is sensitive to acquisition and processing differences but is also found to provide valuable insight into the geometrical changes that occur in the subsurface due to production. As such, this work also provides a demonstration of the use of high-resolution 3D interpreted warping to resolve the 3D heterogeneity of the compaction and subsidence. This is an important tool for Valhall, and possibly other fields, where compaction and subsidence (and monitoring thereof) are key factors in the reservoir management since the predominant observed production-induced changes are compaction of the soft, high-porosity chalk reservoir, due to pore-pressure reduction, and the resultant overburden subsidence. Such reservoir compaction could have significant implications for production by changing permeabilities and production rates. Furthermore the subsidence effects could impact upon subsea installations and well-bore stability. Geomechanical studies that have previously been used to model such subsidence and compaction are only constrained by observed surface displacements and measured reservoir pressure changes, with the geological overburden being largely neglected. The approaches suggested herein provide the potential for monitoring and assessment in three dimensions, including the probable heterogeneity and shearing, that is needed for full understanding of reservoir compaction and the resultant effects on the overburden to, for example, mitigate well-bore failures.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.2004.00470.x
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