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  • 1
    Electronic Resource
    Electronic Resource
    Fault proximity surveys in coal using guided seismic waves—Numerical modelling (1992)
    Springer
    Pure and applied geophysics 139 (1992), S. 215-239 
    Details
    ISSN: 1420-9136
    Keywords: Guided seismic waves ; fault detection ; mode conversion ; coal
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Guided seismic waves are generated at lateral discontinuities in coal seams as an integral part of the mechanics of scattering downgoing body waves from a surface energy source. Numerical and laboratory model studies undertaken for both thin and thick coal seam waveguides show that small faults generate recognizable diffraction patterns by convertingP andS waves into channel wave arrivals at in-seam geophones on a walkawayVSP profile. These results suggest that in-seam geophones can be used during subsequent surface reflection surveys so as to improve mine profitability by optimising longwall layout in faulted ground.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00876328
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  • 2
    Electronic Resource
    Electronic Resource
    The correlation of seismic events on multicomponent data in the presence of coherent noise (1993)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 113 (1993), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: A numerical analysis has been undertaken to show the potential benefits and relevance of eigen-decomposition of covariance matrices formed using multicomponent seismic data from two stations. It is well known that a single-station covariance matrix provides polarization estimates of the particle motion at a given point in space. A two-station (binocular) covariance matrix is shown to provide similar information at two points in space, as well as indicating the correlation between the two stations. By choosing a suitable reference wavelet, correlating events at differing stations may be identified. A major advantage of the procedure is that it can successfully identify overlapping arrivals (in the time domain) when a single-station analysis breaks down completely. The binocular analysis can be sequentially applied to a multistation seismic gather to identify a correlating event. The technique is applied to synthetic gathers with differing signal-to-noise ratios and the picked events compared with a single-station analysis. It is then successfully applied to the data from a physical scale-model experiment.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1993.tb00891.x
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  • 3
    Electronic Resource
    Electronic Resource
    Physical and numerical model study of diffraction effects on seismic profiles over simple structures (1992)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 108 (1992), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: 2-D scale models and synthetic seismograms have been used to study diffraction artifacts and interpretation pitfalls on seismic profiles over two classes of simple-shaped structures: a vertical fault model and a rectangular mound model.The fault throw was varied from one eighth of a wavelength (λ/8) to two wavelengths (2λ). For throws greater than λ/2, the structure is resolved in the x-t domain but diffractions which are present convey misleading structure. Amplitude and wavelet anomalies are observed for small fault displacement (λ/2) which serve to detect but not to define the exact fault position and extent. Migration improves but does not necessarily overcome the problem.Mound structures having heights of λ/8 to 2λ, and length of one half to twice the Fresnel zone radius, were investigated. Lateral resolution is improved as the length of the structure increases, but only for mounds which are vertically resolved (height greater than λ/2). For low relief mounds, resolution deteriorates with increasing mound length. The resulting edge diffractions, which interfere with the reflection events, serve to detect and identify the target. Amplitude anomalies are observed across the structures, but it is not possible (without calibration) to interpret the edges of the model. Even for thick mounds the diffraction patterns complicate the seismic picture and can lead to false interpretation.The migration aperture needs to be about 10 times the Fresnel zone radius to produce satisfactory results. However, for vertically unresolved structures, migration may not significantly help. Complicating factors with post-stack migration are lack of knowledge of the true velocity function and inadequacy of the CMP zero-offset equivalence assumption.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1992.tb03479.x
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  • 4
    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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  • 5
    Electronic Resource
    Electronic Resource
    BLOCKING SURFACE WAVES BY A CUT PHYSICAL SEISMIC MODEL RESULTS (1989)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 37 (1989), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: High resolution seismic reflection exploration for minerals places severe demands on field practice so as to maximize the signal-to-noise bandwidth. In particular, all horizontally propagating coherent noise, especially ground roll, must be attenuated.The blocking effect of a trench between source and receiver has been investigated by means of two-dimensional physical seismic model experiments. Rectangular, circular and wedge-shaped saw-cuts of various dimensions were studied. The results show that thin rectangular cuts of depth equal to one-quarter of the Rayleigh wave noise wavelength produce a 12 dB or better improvement in the signal-to-noise ratio. Rayleigh wave attenuation is greater than 30 dB at a cut depth of one wavelength. In the field applications envisaged, this corresponds to trenches up to a few metres deep. The trenches should be filled with foam or loose sand to dampen out mode conversion and diffraction noise. There are obvious practical difficulties of implementing such a technique in routine CMP operations.The technical effectiveness of the saw-cut is illustrated by imaging a deeply-buried small hole (diffractor) in an aluminium plate. Without the saw-cut between source and receiver, the seismic record is dominated by Rayleigh wave noise, masking P-wave arrivals from the target diffractor. However, with a saw-cut of depth three-quarters of a Rayleigh wave wavelength, the improvement is dramatic, making it easy to detect and identify the hole. When scaled to the field situation, this is equivalent to imaging a 6 m tunnel at a depth of 400 m, using a surface trench of depth 2 m to block ground roll.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1989.tb02224.x
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  • 6
    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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  • 7
    Electronic Resource
    Electronic Resource
    VERTICAL SEISMIC PROFILING IN COAL (1985)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 33 (1985), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: The intellection of seismic wave propagation in coal measures demands direct observation of the wavefield progression. Two vertical seismic profiles with high spatial and temporal sampling, were recently recorded in the Sydney Basin coalfields as part of an experimental coal seismic program.Static corrections and interval velocities were obtained by an automated system to determine first kicks and pulse rise times. Upgoing and downgoing waves were separated in the f—k-plane using a novel technique of contour slice filtering. The isolated upgoing waves clearly display reflections from the major coal seams within the stratigraphic sequence. The downgoing wave spectra were subjected to attenuation analysis. The deduced specific quality factor Q for Permian coal measure rocks lies in the range 20–70. Similar estimates were obtained in the time domain from measurements of pulse broadening.Synthetic VSP seismograms, computed using an exact recursive formulation, are an indispensable aid to interpretation. They illustrate the filtering effects of coal seams and sequences, and the effects of the contribution of internal and free-surface multiple reflections in the recorded wavetrains.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1985.tb00773.x
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  • 8
    Electronic Resource
    Electronic Resource
    A BLIND ZONE SOLUTION TO THE PROBLEM OF HIDDEN LAYERS WITHIN A SEQUENCE OF HORIZONTAL OR DIPPING REFRACTORS (1978)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 26 (1978), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: In seismic refraction surveys, in particular those using first arrival recording techniques, the hidden layer problem occurs where energy from a refractor of higher velocity arrives at the surface before energy from an overlying refractor. The maximum thickness of the hidden layer is referred to as the blind zone.Hypothetically, every recorded refractor has an associated blind zone which may or may not contain a hidden layer. For an assumed earth model of plane constant-velocity layers and stepwise increase of velocity with depth, the effect of a blind zone on an interpreted depth section may be evaluated by defining an intercept time for a blind zone of assumed or known velocity and by using standard time-term equations for layer thicknesses and depths.The treatment covers an arbitrary number of blind zones embedded within a multilayer sequence of horizontal or dipping refractors. Model calculations affirm the benefits of this approach compared with previous methods which, in general, have been restricted to the case of two horizontal layers with one intermediate blind zone.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1978.tb01630.x
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  • 9
    Electronic Resource
    Electronic Resource
    COMMENTS ON “THE HIDDEN LAYER PROBLEM IN SEISMIC REFRACTION WORK” (1977)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 25 (1977), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1977.tb01160.x
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  • 10
    Electronic Resource
    Electronic Resource
    Lateral Resolution In Seismic Reflection—A Physical Model Study (1989)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 97 (1989), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Physical seismic model experiments have been undertaken to study the detectability and lateral resolution of buried linear reflecting targets (slits in an aluminium plate) which might simulate a range of interesting geological structures, e.g. fracture zones, fault blocks, reefs, facies changes, cavities. the reflector length was varied from one-thirtieth to over 20 wavelengths. the depth of burial was 17 wavelengths.Even the smallest target yielded discernible diffractions on receivers located in the shadow zone of the reflector. the lateral extent of the reflector can be accurately determined in the subwavelength range by utilizing together the amplitude, frequency, moveout and polarity information contained in the composite diffraction/reflection signals. As the reflector length increases, the amplitude and travel time increase, but the predominant frequency decreases. the dominant polarity of the interference waveform changes as the reflector length exceeds one wavelength.For targets extended horizontally more than one wavelength, the time difference between the two edge-diffracted waves is a good measure of reflector extent. In the case of an inclined reflector, the moveout pattern of the two diffractions was asymmetrical, forming a convergence zone (and higher amplitude) on the downdip side of the shotpoint. the offset at which the diffraction wavelets merge serves to define the reflector dip. the reflector length can be estimated from the time separation between the two weaker diffractions observed on receivers located updip of the shot, after appropriate correction for target inclination.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1989.tb00495.x
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