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  • 1
    Electronic Resource
    Electronic Resource
    A PARAMETRIC APPROACH TO THE COMPRESSION OF SEISMIC SIGNALS BY FREQUENCY TRANSFORMATION (1980)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 28 (1980), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: The compression of seismic signals which have propagated through a dispersive medium can be achieved by a frequency-domain transformation. This transformation is formally related to the dispersion characteristic of the medium and is independent of phase and group velocities. By defining a suitable cost-function to measure the degree of dispersion of a time- signal, an iterative technique can be employed to find that transformation which gives minimum dispersion.In this paper, we assume that the inverse of the dispersion characteristic can be adequately approximated by a finite polynomial in the region of maximum signal energy density. The coefficients of this polynomial are the parameters of dispersion of the medium. These parameters can be estimated both in the presence of noise and in the case of signals made up of multiple arrivals.The techniques developed in this paper are applied to seismic signals which have been recorded underground by a set of geophones designed to detect and map discontinuities in coal-seams. Results of dispersion estimation and signal compression are discussed for data collected in the Blackshale seam, Pye Hill Colliery, near Derby, in the United Kingdom.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1980.tb01242.x
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  • 2
    Electronic Resource
    Electronic Resource
    Controlled Direction Reception Filtering of P- and S-Waves In τ-P Space (1990)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 100 (1990), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: This paper presents a modified form of polarization position correlation (PPC) operator which can be used to separate P- and S-waves in a multicomponent seismic profile. the essence of the method (in seeking S-wave extinction) is to form a dot product between the signal vector and the slowness vector during projection of the seismic section into τ-p space, using the P-wave velocity profile measured along the array. the dot product (in effect) is a linear controlled direction reception filter (CDR type 1) which selectively passes only P arrivals.The second step is to use the converse rotation operator, during the forward transform, to compute both the P-wave ω-p‘pass plane’ and the orthogonal P-wave ‘extinction plane’. the two together are needed in order to preserve a measure of the total energy falling within any ω-p pixel in the original time sections. the extinction plane on its own gives a measure of the success achieved by the CDRI filter in isolating P-wave energy in a pixel on the pass plane. the best measure of this success is given by performing a cross-spectral matrix analysis of the two ω-p planes on a pixel-by-pixel basis (summing over a window dω×dp). the ratio of the eigenvalues yields the rectilinearity of polarization. A 2-D gain function based on rectilinearity may be used as a non-linear boost function in order to enhance strongly polarized P-wave pixels in the ω-p pass plane, prior to inverse RADON transformation.The success of this method in achieving wavefield separation and background noise reduction is illustrated with synthetic and physical model seismic data.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1990.tb02482.x
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  • 3
    Electronic Resource
    Electronic Resource
    VELOCITY ANALYSIS OF THE SH-CHANNEL WAVE IN THE SCHWALBACH SEAM AT ENSDORF COLLIERY (1988)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 36 (1988), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: An in-seam fan shoot was conducted in 1981 over a 300 m × 500 m panel of the Schwalbach seam at Ensdorf by a team from Prakla-Seismos AG of Hannover under contract to Saarbergwerke AG, Saarbrücken. The object was to study SH-mode propagation in the coal seam waveguide. The high quality dataset retrieved provides a general and valuable test bed with which to compare the performance of in-seam seismic velocity analysers.Five different dispersion analysers are demonstrated using the Schwalbach data. They are all based on the a priori assumption of coal seam homogeneity and isotropy. Space or time windows limit the resolution of the Fourier moving-window analyser, the migration based phase-velocity analyser, and the double Fourier transformer. The other two analysers, the maximum entropy moving-window analyser and the phase-moveout analyser, achieve noise-limited super-resolution by predicting the probable behaviour of the wavefield outside the window.The coal seam's characteristics conform to those predicted for a simplified model based on proposals by Elsen, Rüter and Schott of Westfälische Berggewerkschaftskasse, Bochum. The slight discrepancy between theoretical and actual dispersion characteristics could be reduced by increasing the model's complexity. However, there would be no material gain without testing the validity of the signal processing assumptions of seam isotropy and homogeneity.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1988.tb02165.x
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  • 4
    Electronic Resource
    Electronic Resource
    A CROSS-HOLE AND FACE-TO-BOREHOLE IN-SEAM SEISMIC EXPERIMENT AT INVINCIBLE COLLIERY, AUSTRALIA (1986)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 34 (1986), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: An experimental cross-hole and face-to-borehole in-seam seismic survey was carried out at Invincible Colliery in the Western Coalfield of New South Wales. Objectives were to determine propagation characteristics of the Lithgow seam and to establish the infrastructure for seismic mapping hole-to-hole in Australia.The seam supports leaky P-, S- and P-SV-modes. These modes propagate with group velocities (at 60 Hz) of 3.1, 1.5 and 1.2 km/s respectively. Particle motion polarization is well developed, as is dispersion of the SH-mode. Attenuation rates are high. The seam is lossy (Q of approximately 20). Two prominent structures were mapped by mode conversion. One is believed to be a fracture zone, the other a zone of intense roof thrusting.The old workings and a minor strike-slip fault, which intersected raypaths, were found to be relatively transparent to P- and S-waves at 60 Hz.Telemetry delay and shot-break timing errors of the exploder box are significant. The resulting traveltime scatter is reduced by means of a least-squares “statics” procedure. The group velocity estimation algorithm (based on Fourier transform) yields dispersion characteristics which can be matched with theoretical results for a simple model of a coal seam waveguide.The experiment demonstrates the capability to retrieve in-seam seismic data of diagnostic quality over an appreciable distance (2 km). The experience gained in both survey layout and data processing will be beneficial to future seam wave surveying of Australian coal mines.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1986.tb00451.x
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  • 5
    Electronic Resource
    Electronic Resource
    MULTICOMPONENT COMMON-RECEIVER GATHER MIGRATION OF SINGLE-LEVEL WALK-AWAY SEISMIC PROFILES (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: Seismic data are usually separated into P-waves and S-waves before being put through a scalar (acoustic) migration. The relationship between polarization and moveout is exploited to design filters that extract the desired wavetype. While these filters can always be applied to shot records, they can only be applied to a triaxial common-receiver gather in special cases since the moveout of scattered energy on the receiver gather relates to path differences between the surface shots and the scatterer while the polarization is determined by the path from scatterer to downhole geophone. Without the ability to separate wavefields before migration, a ‘vector scalar’ or an elastic migration becomes a necessity.Here the propagation of the elastic wavefield for a given mode (e.g. P-S) is approximated by two scalar (acoustic) propagation steps in a ‘vector scalar’ migration. ‘Vector’ in that multicomponent data is migrated and 'scalar’ in that each propagation step is based on a scalar wave equation for the appropriate mode. It is assumed that interaction between the wavefields occurs only once in the far-field of both the source and receiver. Extraction of the P, SV and SH wavefields can be achieved within the depth migration (if one assumes isotropy in the neighbourhood of the downhole receiver) by a projection onto the polarization for the desired mode. Since the polarization of scattered energy is only a function of scatterer position and receiver position (and not source position), the projection may be taken outside the migration integral in the special case of the depth migration of a common-receiver gather. The extraction of the desired mode is then performed for each depth migration bin after the separate scalar migration of each receiver gather component.This multicomponent migration of triaxial receiver gathers is conveniently implemented with a hybrid split-step Fourier-excitation-time imaging condition depth migration. The raytracing to get the excitation-time imaging condition also provides the expected polarization for the post-migration projection. The same downward extrapolated wavefield can be used for both the P-P and P-S migrations, providing a flexible and efficient route to the migration of multicomponent data.The technique is illustrated on a synthetic example and a single-level Walk-away Seismic Profile (WSP) from the southern North Sea. The field data produced images showing a P-P reflector below the geophone and localized P-P and P-S scatterers at the level of the geo-phone. These scatterers, which lie outside the zone of specular illumination, are interpreted as faults in the base Zechstein/top Rotliegendes interface.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1991.tb00357.x
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  • 6
    Electronic Resource
    Electronic Resource
    A new imaging proportional counter using a Penning gas improves energy resolution (1984)
    [s.l.] : Nature Publishing Group
    Nature 309 (1984), S. 532-534 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] The energy resolution of a PC is limited by both Fano and avalanche statistics. The FWHM resolution is given by where w is the mean energy to produce one ion pair, F the Fano factor, / the avalanche factor, g a broadening factor depending on the small scale grid or wire geometry, and E is the ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/309532a0
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  • 7
    Electronic Resource
    Electronic Resource
    Light from electron avalanches and background rejection in X-ray astronomy (1980)
    [s.l.] : Nature Publishing Group
    Nature 287 (1980), S. 808-810 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] An X-ray absorbed in the region between grid and the thin window produces a localized ionization cloud. The electrons in this cloud drift to the grid under the influence of a small electric field. A high electric field exists between the grid and the anode, accelerating the electrons to energies at ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/287808a0
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  • 8
    Electronic Resource
    Electronic Resource
    Thermal Expansion of Pyrolytic Graphite and its Variation due to Non-Alignment of the Crystallites (1964)
    [s.l.] : Nature Publishing Group
    Nature 203 (1964), S. 58-60 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] PYROLYTIC graphite, normally prepared at about 2,200 C, shows a high degree of preferred orientation. However, subsequent heat treatment improves the alignment until, at 3,600 C, the material resembles a mosaic single crystal1. Now the thermal expansion of pyrolytic graphite is a property of some ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/203058a0
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  • 9
    Electronic Resource
    Electronic Resource
    Variation with Temperature of Young's Modulus of Polycrystalline Graphite (1960)
    [s.l.] : Nature Publishing Group
    Nature 188 (1960), S. 33-35 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] IT is well known that at 1,000 C. the elastic moduli of artificial graphites rise with temperature1,2. It is also often assumed that this positive variation with temperature exists down to room temperature, although there appear to be no actual measurements to confirm this. Some fairly precise ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/188033a0
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  • 10
    Electronic Resource
    Electronic Resource
    Soft X-ray reflectivity measurements of the mirrors flown on the Ariel-6 satellite (1983)
    Springer
    Astrophysics and space science 89 (1983), S. 325-339 
    Details
    ISSN: 1572-946X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract This paper describes the detailed calibration at soft X-ray energies (0.1–2.0 keV; 125 Å-6 Å) of gold coated, paraboloidal X-ray mirrors, four of which were subsequently flow on the Ariel-6 satellite. Uncertainties in the attitude of the satellite together with an apparent reduction in sensitivity of the soft X-ray experiment necessitated observations using the Crab Nebula as a reference. These showed that a dramatic reduction in the reflection efficiencies of all four mirrors had occurred, almost certainly after or during launch. An initial recalibration of the mirrors using the Crab observation is described.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00655985
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