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11

Electronic Resource

Multiple scattering of surface waves from discrete obstacles (1992)

Bostock, M. G. ; Kennett, B. L. N.

Oxford, UK : Blackwell Publishing Ltd

Geophysical journal international 108 (1992), S. 0

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ISSN: 1365-246X

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Geosciences

Notes: The surface wave T-matrix formulation which describes scattering from a discrete obstacle embedded in a stratified medium is extended to accommodate scattering from two or more obstacles. By exploiting the translation properties of the vector cylindrical wave functions and employing the definition of the surface wave T-matrix for a single obstacle it is possible to construct a composite T-matrix for a two-obstacle configuration in terms of the individual T-matrices. This procedure can be extended, recursively, to incorporate N obstacles. The scattered field contributions from each obstacle including the entire hierarchy of multiple scattering interactions are clearly identifiable in the resulting expression.The formulation is applied to several two-obstacle configurations over a range of ka to investigate the implications of multiple scattering interactions for regional phases such as Lg propagating in the Earth's crust and upper mantle. The results indicate that the significance of multiple scattering is dependent upon the size of the scatterers, their separation and orientation with respect to the incident wave. Multiple scattering is less pronounced for small scatterers (ka 〈 1.0) and is most significant at separations under a wavelength. For obstacles of ka∼ 1.0, pair interactions result in deviations from the zeroth-order field of less than 10 per cent in the forward scattered power. The effects of multiple scattering are limited to still smaller separations for point scatterers; however the nature of their multipole representations suggests that scattered Love waves may be generated more efficiently through pair interactions than Rayleigh waves in the low-frequency limit. Consideration of multiple scattering becomes essential as the size of the obstacle increases. Large obstacles (ka∼ 10.0) behave as lenses by focusing the majority of scattered energy along a narrow corridor about the forward direction. If two obstacles are aligned parallel to the direction of the incident wave the power in the first-order pair interaction is comparable to the total power scattered from a single obstacle for a wide range of separations.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1111/j.1365-246X.1992.tb00838.x

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12

Electronic Resource

The effect of 3-D structure on Lg propagation patterns (1990)

Bostock, M. G. ; Kennett, B. L. N.

Oxford, UK : Blackwell Publishing Ltd

Geophysical journal international 101 (1990), S. 0

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ISSN: 1365-246X

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Geosciences

Notes: The Lg phase is one of the most prominent features of many regional seismograms and its amplitude is known to be sensitive to major changes in crustal structure. This sensitivity can be exploited to investigate the nature of crustal heterogeneity, but also reduces the utility of the analysis of the Lg phase for characterizing seismic sources.The method of ray diagrams, based on the constructive interference of multiple S reflections, gives a visual measure of the interaction of guided wavetrains with variations in crustal structure. The technique is extended to 3-D heterogeneity and by representing the S ray systems in plan view through their basement reflection points, a semi-quantitative measure of the disruption of Lg at different phase velocities can be made. In addition, the tilt of the ray systems can be used as an indication of the degree of interconversion between Rayleigh and Love wave energy.Two simple crustal models are examined to investigate the effects of oblique incidence and transverse topographic gradients on multiple S reflections using ray diagrams. The method is then applied to several real earth models of the crust in central Asia using topographic data and assuming isostasy. Interpretations of the ray diagrams for paths across the major tectonic features compare well with many of the observations from previous studies (Ruzaikan et al. 1977; Ni & Barazangi 1983), and suggest that the gross characteristics of Lg propagation can be attributed to the shape of the crustal waveguide.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1111/j.1365-246X.1990.tb06574.x

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An investigation, of the upper mantle beneath NW Australia using a hybrid seismograph array (1990)

Bowman, J. R. ; Kennett, B. L. N.

Oxford, UK : Blackwell Publishing Ltd

Geophysical journal international 101 (1990), S. 0

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ISSN: 1365-246X

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Geosciences

Notes: A large array of short-period, portable seismographs was operated in the Northern Territory of Australia for 3 months in 1986 in order to record earthquakes in the island arcs to the north and east of Australia. The array consisted of 18 digital recorders, 28 analogue tape recorders and the 20 stations of the permanent Warramunga (WRA) seismic array. An unusual aspect of this experiment was the variable station spacing and apertures of the different elements of the hybrid array. The WRA and portable digital arrays had station spacing of 2.2 km and 5–13 km, respectively, and allowed confident identification of phases returned from the upper mantle, whilst the analogue array had station spacing of 40 km and allowed us to track mantle phases across its 500 km aperture.Seismograms from 17 shallow earthquakes (mb 4.3–5.1) in the Indonesian arc are used to investigate the P-wave velocity structure of the upper mantle beneath NW Australia. We combine seismograms from these events into a composite record section covering ranges from 1000 to 2600 km. Strong phases following the first P-wave arrivals in this composite record section clearly indicate the presence of significant structure in the upper mantle. Lateral heterogeneity in the upper mantle causes the timing and amplitudes of mantle phases to vary across the array and among earthquakes. In order to minimize the effects of lateral heterogeneity, we filter and stack the data and concentrate on features in the data that are seen for many individual seismograms and for several earthquakes. We calculate WKBJ and reflectivity seismograms in order to construct a vertical velocity profile that fits the observed traveltimes and waveforms in an average sense. Our preferred model NWB-1 includes second-order velocity discontinuities of 3.7 per cent near a depth of 200 km, 6.1 per cent near 400 km and 3.7 per cent near 620 km in order to reproduce the amplitudes of the later phases. The low amplitudes of the first arrivals in the range from 1600 to 2200 km require either a low-velocity zone below 230 km depth or a low velocity gradient between 230 km and the 400 km discontinuity. Model NWB-1 is smoother than some models that have been previously proposed for this region which may have mapped lateral heterogeneity into vertical velocity profiles.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1111/j.1365-246X.1990.tb06578.x

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Boundary value ray tracing in a heterogeneous medium: a simple and versatile algorithm (1990)

Sambridge, M. S. ; Kennett, B. L. N.

Oxford, UK : Blackwell Publishing Ltd

Geophysical journal international 101 (1990), S. 0

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ISSN: 1365-246X

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Geosciences

Notes: Traveltime calculations in 3-D velocity models have become more commonplace during the past decade or so. Many schemes have been developed to deal with the initial value problem, which consists of tracing rays from a known source position and trajectory usually towards some distant surface. Less attention has been given to the more difficult problem of boundary value ray tracing in 3-D. In this case, source and receiver positions are known and one, or more, minimum time paths are sought between fixed endpoints.A new technique for boundary value ray tracing is proposed. The scheme uses a common numerical integration technique for solving the initial value problem and iteratively updates the take-off angles until the ray passes through the receiver. This type of ‘shooting’ technique is made efficient by using expressions describing the geometrical spreading of the wavefront to determine the relationship between the ray position at any time and the take-off angles from the source. The use of numerical integration allows the method to be compatible with a wide variety of structures. These include models with velocity varying smoothly as a function of position and those with arbitrarily orientated surfaces of discontinuity. An examination of traveltime accuracy is given as well as a discussion of efficiency for a few classes of velocity model.To improve upon the first guess pair of take-off angles, a small-scale non-linear inverse problem must be solved. The difference between the receiver position and the arrival point of a ray, on a plane through the receiver, describe a mis-match surface as a function of the two take-off angles of the ray. The shape of this surface can possess local minima and multiple ‘global’ minima even for relatively simple 1-D velocity models. Its study provides some insight into the non-linearities of a small-scale geophysical inverse problem.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1111/j.1365-246X.1990.tb00765.x

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15

Electronic Resource

Imaging Earth (1993)

Kennett, B. L. N.

[s.l.] : Nature Publishing Group

Nature 365 (1993), S. 306-306

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ISSN: 1476-4687

Source: Nature Archives 1869 - 2009

Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics

Notes: [Auszug] METHODS of determining the three-dimensional distribution of seismic wave speeds in the Earth have come to be known as 'seismic tomography' by analo-gy with medical imaging techniques. But the illumination of the Earth by earth-quake and man-made tremors is not near-ly as uniform as the ...

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1038/365306b0

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