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  • 1
    Monograph available for loan
    Monograph available for loan
    The art of being a scientist : a guide for graduate students and their mentors (2009)
    Cambridge [u.a.] : Cambridge Univ. Press
    Details Availability
    Call number: AWI S6-14-0059 ; 2/N 14.0263
    Description / Table of Contents: "This is a hands-on guide for graduate students and other young researchers wishing to perfect the practical skills that are needed for a successful career in research. By teaching junior scientists to develop effective research habits, the book helps make the experience of graduate study a more efficient, effective and rewarding one. Many graduate students learn these skills "on the job", often by doing them poorly at first, with the result that much valuable time can be lost; this book will help prevent that. The authors have taught a graduate course on the topics covered in this book for many years, and provide a sample curriculum for instructors in graduate schools who wish to teach a similar course. ... The wealth of advice offered in this book is invaluable to students, junior researchers and their mentors in all fields of science, engineering and the humanities."
    Type of Medium: Monograph available for loan
    Pages: X, 286 S. : Ill., graph. Darst.
    Edition: 1. publ. 2009, 5th print. 2013
    ISBN: 9780521743525
    Classification:
    E.7.
    Language: English
    Note: Contents: 1 Introduction. - 2 What is science?. - 3 Choices, choices, choices. - 4 The adviser and thesis committee. - 5 Questions drive research. - 6 Giving direction to our work. - 7 Turning challenges into opportunities. - 8 Ethics of research. - 9 Using the scientific literature. - 10 Communication. - 11 Publishing a paper. - 12 Time management. - 13 Writing proposals. - 14 The scientific career. - 15 Applying for a job. - 16 Concluding remarks. - Appendix A. Futher reading. - Appendix B. A sample curriculum. - Appendix C. The Refer and BibTeX format. - References. - About the authors. - Index.
    Location: AWI Reading room
    Location: Reading room
    Branch Library: AWI Library
    Branch Library: GFZ Library
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  • 2
    Electronic Resource
    Electronic Resource
    Error in shear-wave polarization and time splitting (2004)
    PO Box 1354, 9600 Garsington Road , Oxford OX4 2XG , UK . : Blackwell Publishing Ltd
    Geophysical prospecting 52 (2004), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: Shear-wave polarization and time delay are attributes commonly used for fracture detection and characterization. In time-lapse analysis these parameters can be used as indicators of changes in the fracture orientation and density. Indeed, changes in fracture characteristics provide key information for increased reservoir characterization and exploitation. However, relative to the data uncertainty, is the comparison of these parameters over time statistically meaningful? We present the uncertainty in shear-wave polarization and time delay as a function of acquisition uncertainties, such as receiver and source misorientation, miscoupling and band-limited random noise. This study is applied to a time-lapse borehole seismic survey, recorded in Vacuum Field, New Mexico. From the estimated uncertainties for each survey, the uncertainty in the difference between the two surveys is 31° for the shear-wave polarization angle and 4 ms for the shear-wave time delay. Any changes in these parameters greater than these error estimates can be interpreted with confidence. This analysis can be applied to any time-lapse measurement to provide an interval of confidence in the interpretation of shear-wave polarization angles and time splitting.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1365-2478.2003.00404.x
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  • 3
    Electronic Resource
    Electronic Resource
    A unified approach to ray bending, ray perturbation and paraxial ray theories (1993)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 115 (1993), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: In the past, perturbation theory has been formulated for the case that either a slowness model was perturbed and the effect of this perturbation on rays was determined, or for the case where the slowness was fixed and where estimates of the ray position were deformed towards the true ray. In this paper both problems are combined in a single perturbation theory. The theory also accommodates arbitrary perturbations to the endpoints of rays and leads to a simple linear differential equation for the ray perturbation. Expressions are derived for the second-order perturbation of the traveltime. This quantity describes the effect of the ray perturbation on the traveltime and of the bias in the traveltime due to the fact that the reference curve need not be a true ray. The second-order traveltime perturbation can be evaluated efficiently by a single integration along the reference curve. In contrast to formalisms using ray-centred coordinates, endpoints perturbations in an arbitrary direction are allowed. This is of importance in tomographic inversions which incorporate earthquake relocations. The cross-term between the slowness perturbations and the source relocations is derived explicitly. The fact that the reference curve does not need to be a true ray in the reference medium allows for an iterative application of ray perturbation theory. The use of the second-order traveltime perturbation allows one to correct for the bias in the traveltime due to the fact that the reference curve is not a ray. A proof is given that the equation for the ray perturbation is consistent with earlier results derived in ray-centred coordinates and the relation with the ray bending theory of Julian & Gubbins (1977) is established. For a fixed-slowness model and for fixed-ray endpoints the two theories are equivalent except at isolated points, this is illustrated with an analogy from classical mechanics. This difference, which results in superior numerical properties for the new algorithm, is illustrated by several numerical examples.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1993.tb01199.x
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  • 4
    Electronic Resource
    Electronic Resource
    Waveform inversions and the significance of surface-wave mode coupling (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: If body-wave phases in laterally heterogeneous media are modelled by surface-wave mode summation, mode coupling is required to bring out the ray character of the body-wave phases. However, it is frequently assumed that in laterally heterogeneous media surface-wave modes propagate without interacting with other modes. The effects of neglecting surface-wave mode coupling on the results from waveform inversions including body-wave arrivals have been studied by performing inversions with and without taking surface-wave mode coupling into account. The two inversion schemes are similar to the Partitioned Waveform Inversion (Nolet 1990) but differ in the approximations used to compute the synthetics: WKBJ, which neglects mode coupling, or SEA (Marquering & Snieder 1995), which does take mode coupling into account.Synthetic inversion experiments show that neglecting mode coupling can lead to biases in the deeper part of the model. Their most striking feature is that for ‘real’ shallow lateral heterogeneities, artificial anomalies with an opposite sign may show up in the deeper part of the model. When mode coupling is taken into account, as in the SEA inversion scheme, these biases do not occur.We have also studied possible bias effects as the result of using WKBJ in the EUR-S91 model of Zielhuis & Nolet (1994), which is characterized by strong velocity anomalies extending as deep as the transition zone. One of the most striking features in this model is the region of low velocities at larger depths beneath the fast Russian platform on the eastern side of the Tornquist–Teisseyre Zone (TTZ). Nolet & Zielhuis (1994) suggested that these low velocities are an indication for injection of water in the mantle by an earlier subduction. Our results show that the neglect of mode coupling indeed does bias the model towards a larger reversal of velocities with depth, and heterogeneities show up even where the S ray does not sample the mantle. However, low velocities deep below the eastern part of the TTZ persist even when mode coupling is taken into account.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1996.tb06369.x
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  • 5
    Electronic Resource
    Electronic Resource
    Solving large linear inverse problems by projection (1990)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 103 (1990), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: As originally formulated by Backus & Gilbert (1970), ill-posed linear inverse problems possess a unique minimum norm solution, and a locally averaged property of the model may be estimated with a resolution that is a monotonic function of its variance. Application of Backus–Gilbert theory requires the inversion of an N x N matrix, where N is the number of data, and therefore becomes cumbersome for large N. In this paper we show how Lanczos iteration may be used to project the original linear problem on a problem of much smaller size in order to obtain an approximation to the Backus–Gilbert solution without the need of matrix inversion. To calculate the resolution in the projected system one only needs to invert a symmetric tridiagonal matrix.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1990.tb01792.x
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  • 6
    Electronic Resource
    Electronic Resource
    The reflection and transmission of plane P- and S-waves by a continuously stratified band: a new approach using invariant imbedding (1989)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 96 (1989), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The reflection and transmission of plane P- and S-waves by a laterally homogeneous band is discussed. A dyadic representation of a ‘plane wave Green's tensor’ is derived, which is used to describe the reflection and transmission of plane waves by a thin homogeneous layer in the first Born approximation. From this, the reflection and transmission by an arbitrarily thick continuously stratified band is derived using invariant imbedding. We derive an exact set of matrix Ricatti equations which describe the reflection and transmission of plane waves by the laterally homogeneous band. These equations remain regular at turning points, and incorporate both homogeneous and inhomogeneous waves within the heterogeneity. It is not necessary for the band to be stratified; the density and the elasticity tensor of the band may have an arbitrary depth dependence. It is shown that in case the band is a smooth heterogeneity without turning points, its only effect is a phase shift of the transmitted wave. In a numerical example for the analogue case of 1-D scattering in quantum mechanics the behaviour of homogeneous and inhomogeneous (tunneling) waves is illustrated.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1989.tb06006.x
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  • 7
    Electronic Resource
    Electronic Resource
    Resolving a low-velocity zone with surface-wave data (1994)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 118 (1994), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The main purpose of seismic inversion is the retrieval of seismic velocities and densities in the earth. Inversion of body-wave traveltimes cannot uniquely determine the seismic velocities as a function of depth when a low-velocity layer is present. It is generally assumed that surface waves do not suffer from the same non-uniqueness. The issue is addressed whether it is possible to remove the non-uniqueness of traveltime inversions with a realistic set of surface-wave data. This requires the exact determination of the velocity distribution within and around the low-velocity zone. Waveforms, phase velocities as well as group velocities are investigated qualitatively. Synthetic Love-wave phase and group velocities are actually inverted. Waveforms are shown to be sensitive to the exact velocity distribution of a low-velocity layer, but it is concluded that the removal of the non-uniqueness with the use of waveforms is difficult because the differences of the waveforms are generally small and because complications such as lateral heterogeneity and poorly known source parameters reduce the accuracy of waveform inversions. The inversions of Love phase and group velocities indicate that it is difficult to determine the velocity distribution of a low-velocity layer in a statistically significant way with a realistic set of dispersion data. Group velocities are shown to be more sensitive to the low-velocity structure than phase velocities. Unfortunately, group-velocity data suffers from a practical non-uniqueness because in general only fundamental-mode group velocities can be measured. It is concluded that the non-uniqueness in the detailed structure caused by a low-velocity layer cannot readily be resolved by using surface-wave dispersion data.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1994.tb03965.x
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  • 8
    Electronic Resource
    Electronic Resource
    Surface-wave mode coupling for efficient forward modelling and inversion of body-wave phases (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: The importance of surface-wave mode coupling in the modelling of body-wave phases by surface-wave mode summation is studied by means of sensitivity kernels obtained with the Born approximation and exact solutions of the Invariant Imbedding Technique. It is shown that, independent of the character of the lateral heterogeneity, surface-wave mode coupling is required to model body-wave phase perturbations and that neglecting intermode coupling, as in the WKBJ method for surface waves, can lead to large biases. Because methods which describe surfacewave mode coupling in an exact fashion are computationally too expensive to use in inversion schemes, the Scalar Exponent Approximation (SEA) is presented, which is a computationally efficient method and takes mode coupling into account. Since, instead of Earth normal modes, surface-wave modes are used, the summation over the angular order l is carried out analytically. This means that the number of modes and mode interactions needed is significantly reduced which assures an efficient manner of modelling. It is shown that the SEA is accurate in modelling body-wave phase perturbations for geophysically realistic configurations. Because, in contrast to the WKBJ sensitivity kernels, mode coupling introduces sensitivity kernels which also depend on the position along the source-receiver path, the SEA requires a larger model parameter set in inversions. A procedure is presented which reorganizes the model parameter set and leads to a reduced set of physically relevant model parameters. The combination of the SEA and the reorganization of the model parameters can be used efficiently in large-scale 3-D inversions which incorporate the important effects of surface-wave mode coupling.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1995.tb05920.x
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  • 9
    Electronic Resource
    Electronic Resource
    Wavefield smoothing and the effect of rough velocity perturbations on arrival times and amplitudes (1996)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 125 (1996), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Geometric ray theory is an extremely efficient tool for modelling wave propagation through heterogeneous media. Its use is, however, only justified when the inhomogeneity satisfies certain smoothness criteria. These criteria are often not satisfied, for example in wave propagation through turbulent media. In this paper, the effect of velocity perturbations on the phase and amplitude of transient wavefields is investigated for the situation that the velocity perturbation is not necessarily smooth enough to justify the use of ray theory. It is shown that the phase and amplitude perturbations of transient arrivals can to first order be written as weighted averages of the velocity perturbation over the first Fresnel zone. The resulting averaging integrals are derived for a homogeneous reference medium as well as for inhomogeneous reference media where the equations of dynamic ray tracing need to be invoked. The use of the averaging integrals is illustrated with a numerical example. This example also shows that the derived averaging integrals form a useful starting point for further approximations. The fact that the delay time due to the velocity perturbation can be expressed as a weighted average over the first Fresnel zone explains the success of tomographic inversions schemes that are based on ray theory in situations where ray theory is strictly not justified; in that situation one merely collapses the true sensitivity function over the first Fresnel zone to a line integral along a geometric ray.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1996.tb06024.x
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  • 10
    Electronic Resource
    Electronic Resource
    Small-scale sublithospheric continental mantle deformation: constraints from SKS splitting observations (1995)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 123 (1995), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: It is generally believed that azimuthal seismic anisotropy is an important property of the subcontinental mantle, and that this is a relatively small-scale phenomenon compared with anisotropy of the oceanic mantle. During recent years, shear-wave splitting has come to be viewed as the most direct diagnostic of azimuthal anisotropy. The magnitude of the splitting delay times can often rule out a crustal source of anisotropy; however, it is not yet clear whether the source of anisotropy lies within the lithosphere or the sublithosphere. It is essential to know the scale of lateral variations and the distribution of anisotropy with depth in order to understand the origin of this phenomenon. Here we describe a way to constrain the depth of the anisotropy, by studying the lateral variation of the splitting parameters at neighbouring seismic stations. We use SKS and SKKS phases recorded at the NARS-NL array, a relatively dense network of broad-band stations separated by about 50 km. Fresnel zones at different depths are calculated for these phases. The depth of the anisotropy is constrained by the criterion that Fresnel zones corresponding to different splitting observations should not overlap. Variability in the splitting measurements for events with different directions of approach recorded at one station, and for single events recorded at various stations, provides evidence for a non-homogeneously distributed source of anisotropy, located below the array at different depths. We show that this variability in the measurements cannot be due to effects of crustal scattering. Our results indicate that the anisotropy is partly located in the sublithospheric mantle at about 400 km depth. This suggests that the deformation in this region is subject to significant lateral variations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1995.tb06864.x
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