ALBERT

All Library Books, journals and Electronic Records Telegrafenberg

Your email was sent successfully. Check your inbox.

An error occurred while sending the email. Please try again.

Proceed reservation?

Export
  • Keywords: seismology ; reflection profiles ; seismic waves ; seismic tomography ; Australia
    Notes: Deep Crustal Seismic Reflection Profiling: Australia 1978–2015 presents the full suite of reflection profiles penetrating the whole crust carried in Australia by Geoscience Australia and various partners. The set of reflection data comprises over 16,000 km of coverage across the whole continent, and provides an insight into the variations in crustal architecture in the varied geological domains. Each reflection profile is presented at approximately true scale with up to 220 km of profile per page and overlap between pages. Each reflection section is accompanied by a geological strip map showing the configuration of the line superimposed on 1:1M geology. The compilation includes a suite of large-scale reflection transects groups of 1,000 km or more that link across major geological provinces, and an extensive bibliography of reports and relevant publications.
    Pages: V, 224 S.
    Edition: 2nd ed.
    ISBN: 9781760460846
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 27 (1979), S. 0 
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: A new approach is presented for the suppression of multiples reflected at the surface of a horizontally layered fluid or elastic medium, recorded at non-zero offsets from the source. The scheme used is to extract the effect of the free surface in the frequency-wavenumber domain and then to replace this surface by a non-reflecting boundary. The multiple suppression operator requires a detailed knowledge of the source time function and the elastic properties of the medium between the source and the surface.For a stratified fluid or a liquid layer overlying a stratified elastic medium, complete multiple suppression can be achieved with noise free data. If only the vertical component is available for an elastic medium an approximate approach may be used which removes most of the multiple energy. Good results may be achieved with this multiple suppression scheme in the presence of noise. The method is designed to be used before records are stacked in a CDP gather.
    Type of Medium: Electronic Resource
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: A common example of a large-scale non-linear inverse problem is the inversion of seismic waveforms. Techniques used to solve this type of problem usually involve finding the minimum of some misfit function between observations and theoretical predictions. As the size of the problem increases, techniques requiring the inversion of large matrices become very cumbersome. Considerable storage and computational effort are required to perform the inversion and to avoid stability problems. Consequently methods which do not require any large-scale matrix inversion have proved to be very popular. Currently, descent type algorithms are in widespread use. Usually at each iteration a descent direction is derived from the gradient of the misfit function and an improvement is made to an existing model based on this, and perhaps previous descent directions.A common feature in nearly all geophysically relevant problems is the existence of separate parameter types in the inversion, i.e. unknowns of different dimension and character. However, this fundamental difference in parameter types is not reflected in the inversion algorithms used. Usually gradient methods either mix parameter types together and take little notice of the individual character or assume some knowledge of their relative importance within the inversion process.We propose a new strategy for the non-linear inversion of multi-offset reflection data. The paper is entirely theoretical and its aim is to show how a technique which has been applied in reflection tomography and to the inversion of arrival times for 3D structure, may be used in the waveform case. Specifically we show how to extend the algorithm presented by Tarantola to incorporate the subspace scheme. The proposed strategy involves no large-scale matrix inversion but pays particular attention to different parameter types in the inversion.We use the formulae of Tarantola to state the problem as one of optimization and derive the same descent vectors. The new technique splits the descent vector so that each part depends on a different parameter type, and proceeds to minimize the misfit function within the sub-space defined by these individual descent vectors. In this way, optimal use is made of the descent vector components, i.e. one finds the combination which produces the greatest reduction in the misfit function based on a local linearization of the problem within the subspace. This is not the case with other gradient methods. By solving a linearized problem in the chosen subspace, at each iteration one need only invert a small well-conditioned matrix (the projection of the full Hessian on to the subspace). The method is a hybrid between gradient and matrix inversion methods. The proposed algorithm requires the same gradient vectors to be determined as in the algorithm of Tarantola, although its primary aim is to make better use of those calculations in minimizing the objective function.
    Type of Medium: Electronic Resource
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: An operator development of the seismic wavefield is used to generate descriptions of the propagation processes contributing to the main regional seismic phases Pn, Pg, Sn, Lg. These operator forms are valid for laterally heterogeneous crust and mantle models and include the major processes of interconversion between wavetypes. These representations of the regional phases are used to examine the theoretical basis for discriminants between earthquakes and underground explosions based on the relative amplitudes of P and S phases. The ratio of Sn to Pn amplitude looks promising as a high frequency discriminant. However, the ratio of Lg to Pn amplitudes is not as useful because of the complex nature of the propagation characteristics of Lg.
    Type of Medium: Electronic Resource
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: This paper shows how the performance of a fully non-linear earthquake location scheme can be improved by taking advantage of problem-specific information in the location procedure. The genetic algorithm is best viewed as a method of parameter space sampling that can be used for optimization problems. It has been applied successfully in regional and teleseismic earthquake location when the network geometry is favourable. However, on a series of test events with unfavourable network geometries the performance of the genetic algorithm is found to be poor.We introduce a method to separate the spatial and temporal parameters in such a way that problems related to the strong trade-off between depth and origin time are avoided. Our modified algorithm has been applied to several test events. Performance over the unmodified algorithm is improved substantially and the computational cost is reduced. The algorithm is better suited to the determination of hypocentral location whether using arrival times, array information (slowness and azimuth) or a combination of both.A second type of modification is introduced which exploits the weak correlation between the epicentral parameters and depth. This algorithm also improves performance over the standard genetic algorithm search, except in circumstances where the depth and epicentre are not weakly correlated, which occurs when the azimuthal coverage is very poor, or when azimuth and slowness information are incorporated. On a shallow nuclear explosion with only teleseismic P arrivals available, the algorithm consistently converged to a depth very close to the true depth, indicating superior depth estimation for shallow earthquake locations over the unmodified algorithm.
    Type of Medium: Electronic Resource
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Many techniques for the solution of seismic-wave propagation problems depend on the representation of the seismic wavefield in terms of a linear combination of basis functions, as for example in Fourier or Gaussian Beam expansions. A common formal representation encompasses such methods when a preferred coordinate is isolated to track the propagation path. Different techniques can be classified by the dependence of the basis functions on this preferred coordinate. The common representation provides useful insight into the relation between apparently disparate methods and can guide the development of computational techniques. This common framework allows the development of generalized propagator methods and a compact formulation of reflection and transmission problems. A general perturbation approach can be used either to add heterogeneity to an existing structure or to restore features, such as coupling between P and S waves, which have been ignored in an approximate development.
    Type of Medium: Electronic Resource
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: New empirical traveltime curves for the major seismic phases have been derived from the catalogues of the International Seismological Centre by relocating events by using P readings, depth phases and the iasp91 traveltimes, and then re-associating phase picks. A smoothed set of traveltime tables is extracted by a robust procedure which gives estimates of the variance of the traveltimes for each phase branch. This set of smoothed empirical times is then used to construct a range of radial velocity profiles, which are assessed against a number of different measures of the level of fit between the empirical times and the predictions of the models. These measures are constructed from weighted sums of L2 misfits for individual phases. The weights are chosen to provide a measure of the probable reliability of the picks for the different phases.A preferred model, ak135, is proposed which gives a significantly better fit to a broad range of phases than is provided by the iasp91 and sp6 models. The differences in velocity between ak135 and these models are generally quite small except at the boundary of the inner core, where reduced velocity gradients are needed to achieve satisfactory performance for PKP differential time data.The potential resolution of velocity structure has been assessed with the aid of a non-linear search procedure in which 5000 models have been generated in bounds about ak135. Msfit calculations are performed for each of the phases in the empirical traveltime sets, and the models are then sorted using different overall measures of misfit. The best 100 models for each criterion are displayed in a model density plot which indicates the consistency of the different models. The interaction of information from different phases can be analysed by comparing the different misfit measures. Structure in the mantle is well resolved except at the base, and ak135 provides a good representation of core velocities.
    Type of Medium: Electronic Resource
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Temporary array deployments of short-period seismometers in northern Australia have been used to build up composite record sections for waves interacting with the upper mantle. Stable measures of the seismic wavefield are provided by stacking the complex envelopes of all the seismic waveforms falling in a 10km distance interval away from the source.Two groups of sources (a) along the Flores Arc, Indonesia with propagation under northwestern Australia, and (b) in New Guinea with paths to the NNE of the array, have been used to construct composite record sections for both P and SV waves over the distance range 1300–2800 km. the timing and amplitude distributions for P waves from the two regions show noticeable differences. Detailed modelling of the record sections yields velocity models with significant variation in velocity for the two sets of propagation paths for which the midpoints are separated by about 1000km.The short-period SV-wave sections indicate efficient propagation of highfrequency S waves in a lithosphere extending down to 210km. Arrivals from the deeper mantle cannot be correlated with confidence because of a loss in high-frequency content revealed by broad-band observations. This requires a significant attenuation zone for S beneath 210 km.
    Type of Medium: Electronic Resource
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Earthquakes in oceanic areas are normally located using traveltime tables which are representative of continental paths, since most seismic stations lie on continents. It should therefore be possible to improve such locations by employing a set of traveltimes more appropriate to paths from oceanic events to continental stations.A comparison has therefore been made between locations for a number of oceanic events using the recent iasp91 global traveltimes and the times for the pac91 model derived from observations of events in the Pacific. Although there were often significant differences in the location estimates for the two models, these were often no larger than the shifts induced by changing the misfit criterion used for determining the location.For events in purely oceanic regions such as Tonga and the Marianas with little nearby continent, the results from the pac91 model either provided a significantly better fit to the data or produced depth estimates in close accord with independent constraints (e.g. centroid moment tensor locations). In these cases the use of a specific set of ‘oceanic’ traveltimes can be recommended. However for marginal zones and island arcs, the situation is less clear and it is probably best to employ the global traveltime set with the use of additional phases to improve depth estimates.
    Type of Medium: Electronic Resource
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Many methods for the analysis of long-period and broad-band S-waveforms depend on a representation of the seismic wavefield in terms of a sum of modes of a reference structure. In addition it is frequently assumed that the propagation of the modal contributions from source to receiver may be regarded as independent. This assumption may not be warranted if there is significant heterogeneity in seismic properties along the propagation path.The interaction between modal components in different styles of heterogeneity model for the upper mantle is examined using a coupled mode propagation technique (Kennett 1984) which allows direct construction of reflection and transmission matrices with full allowance for intermode interactions for 2-D heterogeneity structures.The first type of structure considered has been proposed to explain amplitude and traveltime anomalies in body wave studies of upper mantle phases. This heterogeneity has an amplitude of about 1 per cent and is distributed with a horizontal scale of around 300-400km and a vertical scale which increases from 70km in the uppermost mantle to 200 km at 900 km depth. Horizontally travelling S-waves are hardly affected by this class of heterogeneity for frequencies less than 0.07 Hz.The second heterogeneity model was based on the WEPL3 model proposed by Nolet (1990) from waveform inversion for the structure under the NARS array in western Europe. The heterogeneity reaches 5 per cent deviation from the reference model PREMC in organized regions 700 km or more in length. For this structure surface wave modes with group velocity below 4.2kms-’ can be regarded as propagating independently up to 0.020Hz. The body wave group of modes with higher group velocity succumbs to significant interaction above 0.040 Hz. The frequency limit for largely independent propagation for the body wave group of modes can be extended to about 0.05 Hz for a velocity model with up to 1 per cent additional variability superimposed on WEPL3. Such a composite heterogeneity model would be consistent with both body wave and surface wave behaviour.The errors introduced into the analysis methods by ignoring mode interactions above these frequency limits will depend on the distribution of energy across the modes imposed by the source, and the criterion used for waveform matching between observed and theoretical seismograms.Theoretical seismograms for the heterogeneity structures based on WEPL3 including full allowance for intermode coupling show a distinct phase shift for the fundamental mode when compared with the corresponding calculations for the reference model PREMC: as is indeed observed at the NARS stations. The parts of the seismograms which show the largest influence from the presence of lateral heterogeneity are those which depend on interference phenomena, such as Su. The disruption of the phase patterns can have a profound influence on the appearance of the waveforms. The presence of small-scale heterogeneity has very little influence on the seismograms below 0.035 Hz but becomes more important as the frequency increases, especially for the body wave phases.
    Type of Medium: Electronic Resource
    Location Call Number Expected Availability
    BibTip Others were also interested in ...