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- Volume 34, Issue 8, 1986
Geophysical Prospecting - Volume 34, Issue 8, 1986
Volume 34, Issue 8, 1986
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SYNTHESIS OF ACOUSTIC WELL‐LOGGING WAVEFORMS ON AN IRREGULAR GRID*
Authors M. SALERNO, G. ORLANDI, G. MARTINELLI and P. BURRASCANOABSTRACTThe need for synthetic seismograms arises in several acoustic well‐logging contexts. We propose a numerical algorithm for computing the seismograms, characterized by a very reduced computational cost. Unlike previous contributions appearing in the technical literature, the proposed algorithm is based on a non‐uniform sampling step and on linear interpolation for computing the integrals involved in the synthesis.
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EXTENDED MARINE ARRAYS VERSUS SIMULATED EXTENDED ARRAYS*
More LessABSTRACTTwo factors are responsible for the fact that an extended marine source array performs better than a point source:
1. a higher degree of transmission of the radiated seismic energy through the water‐sediment interface providing a better penetration;
2. filtering effects.
The higher degree of transmission is due to: (a) the directivity of extended sources, (b) the lower reflection coefficient at the water‐sediment interface for seismic waves radiated from an extended source array than for spherical seismic waves radiated from a point source, (c) the lower amplitude decay of the pulses from an extended source than from a point source. In addition, signature characteristic of an extended source array and Fresnel zone of waves generated by such a source differ from those corresponding to a point source.
The propagating wavelet radiated from a point source array may not be, in a sedimentary sequence below the sea‐floor, the linear combination of wavelets emitted from point sources. In such cases, there is a noticeable difference between the performance of a field‐implemented source array and that of the corresponding simulated source array. The performances of the field‐implemented and simulated extended receiver arrays can be identical if the recording system is adequate and the processing technique appropriate.
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LOVE‐TYPE SEAM‐WAVES IN WASHOUT MODELS OF COAL SEAMS*
Authors M. BREITZKE and L. DRESENABSTRACTThe propagation of Love seam‐waves across washouts of coal seams was studied by calculating synthetic seismograms with a finite‐difference method. Seam interruption, seam end and seam thinning models were investigated. The horizontal offset, the dip of the discontinuities and the degree of erosion served as variable parameters. Maximum displacement amplitudes, relative spectral amplitudes and phase and group slowness curves were extracted from the synthetic seismograms.
Both seam interruption and seam thinning reduce the maximum displacement amplitudes of the transmitted Love seam‐waves. The degree of amplitude reduction depends on the horizontal offset and the degree of erosion. It is four times greater for a total seam interruption than for an equivalent seam thinning with a horizontal offset of four times the seam thickness. In a seam cut vertically, the impedance contrast between the coal and the washout filling determines the maximum displacement amplitudes of the reflected Love seam‐waves. They diminish by a maximum factor of four in oblique interruption zone discontinuities with a dip of maximum 27°, and by a maximum factor of ten in a seam thinning with a degree of erosion of at least 22%.
The analysis of the relative spectral amplitudes indicates a preferential transmission of those phases with frequencies below, and a preferential reflection of those phases with frequencies above the first mode Airy‐phase. The relative spectral amplitudes of the reflected Love seam‐waves show a distinct interference pattern of the waves reflected at both interruption zone discontinuities.
The dispersion analysis reveals a flattening of the phase and group slowness curves with increasing frequencies, horizontal offset and degrees of erosion.
These results imply that a detection of washouts in‐mine will be possible in a frequency range including at least the first mode Airy‐phase. An interference pattern and a flattening of the dispersion curve indicate a washout rather than other seam obstructions and leads to an estimate of the washout dimension.
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EXPERIMENTAL STUDIES OF ELASTIC‐WAVE PROPAGATION IN A COLUMNAR‐JOINTED ROCK MASS*
Authors M.S. KING, L.R. MYER and J.J. REZOWALLIABSTRACTResults are presented of a series of cross‐hole acoustic measurements made between four horizontal boreholes drilled from a near‐surface underground opening situated in a basaltic rock mass. The objectives of the program were to assess the extent of blast damage around the opening, and to evaluate the rock mass characteristics and their spatial variation around the opening. The acoustic velocity and attenuation data are indicative of an anisotropic, jointed rock mass, with a greater intensity of jointing along travel paths in the horizontal than the vertical direction. Low acoustic P‐ and S‐velocities are indicative of blast damage and of zones of intense jointing or fracturing. In this case blast damage extends to approximately 1.5 m from the face. Attenuation data appear to be less sensitive in distinguishing between the blast‐damaged zone and intense vertical jointing and fracturing in the virgin rock mass. Taken together with field data, laboratory measurements of P‐ and S‐wave velocities on intact core samples suggest that the rock mass is probably water saturated.
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CONSIDERATIONS IN ESTIMATING THE MINIMUM‐PHASE PROPERTIES OF SAMPLED DATA*
More LessABSTRACTAnalysis of the phase spectra from the signatures, impulse responses and other wavelets observed in seismic data leads to the construction of equivalent minimum‐phase functions. The accuracy of such computations using digitally sampled data is questioned with special reference to Texas Instruments DFS IV and DFS V recording filters. Results vary with the lengths and sample rates of the time functions, and further errors may be introduced when implementing the Hilbert transform. Such problems are related to poor resolution in the low amplitude areas of the spectrum. Techniques for correction are described. With appropriate shaping a reasonably accurate phase spectrum may be computed for the minimum‐phase function. The generation of minimum‐phase wavelets within the processing sequence is briefly discussed.
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COMPLETE INVERSION OF ZERO‐OFFSET SEISMIC DATA*
By B. URSINABSTRACTThe one‐dimensional seismic inverse problem consists of recovering the acoustic impedance (or reflectivity function) as a function of traveltime from the reflection response of a horizontally layered medium excited by a plane‐wave impulsive source. Most seismic sources behave like point sources, and the data must be corrected for geometrical spreading before the inversion procedure is applied. This correction is usually not exact because the geometrical spreading is different for primary and multiple reflections.
An improved algorithm is proposed which takes the geometrical spreading from a point source into account. The zero‐offset reflection response from a stack of homogeneous layers of variable thickness is used to compute the thickness, velocity and density of each layer. This is possible because the geometrical spreading contains additional information about the velocities.
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SOME NEW MAGNETIC TRANSFORMATIONS*
More LessAbstractAll magnetic transformations are governed by one simple differential relation between the observed and the transformed quantities. A magnetic map for any component, at any location, and for any given direction of magnetization can be converted into one for which any one, two, or all three parameters differ.
Three new magnetic transformations are introduced: (i) reduction to equator, (ii) orthogonal reduction, and (iii) elimination of remanence. The first eliminates (or minimizes) the asymmetry and the lateral shift of the measured total field anomalies, exactly as in Baranov's reduction to pole. The second produces perfect asymmetry so that a symmetrical target lies vertically below the zero anomaly point, midway between the maximum and minimum. When remanence is a contributing factor, the direction of resultant magnetization must be known a priori in all cases, except for transformation of one component into another in the same area.
Explicit working formulae are presented for reduction to equator and pole, and orthogonal reduction.
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ORIGIN OF THE ANOMALIES OF GRAVITY AND ITS VERTICAL GRADIENT OVER CAVITIES IN BRITTLE ROCK*
More LessABSTRACTInvestigations show that the depth range of the gravity method for detecting cavities is considerably greater than expected on the basis of theoretical calculations that consider only the depth and shape of the cavity.
The cavity generates its own gravity field that is the sum of the activity of the cavity itself and the density changes caused in the surrounding rocks by the process of their destruction as a result of the cavity's expansion. The latter factor is decisive for the cavity's detection by the gravity method in several cases.
The extent of the zones of change density in the surrounding rocks and their actual shape depend on many parameters. Thus, the determination of the depth of the cavity cannot be made by comparing measured and theoretical curves of gravity anomalies. The location of the center of gravity of the area disturbed by the cavity or upper boundary of its development towards the ground surface can be determined by denoting the singular point of gravity as a result of the downward analytical continuation of Ag.
The Strakhov‐Grigorieva‐Lapina (1977) method fulfils the necessary conditions in the field.
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NONLINEAR COMPLEX RESISTIVITY OF SOME NICKEL SULPHIDES FROM WESTERN AUSTRALIA*
Authors S.H. HALL and G.R. OLHOEFTABSTRACTThe electrochemical transfer characteristics of nickel sulphides from six West Australian localities are examined by the nonlinear complex resistivity (nlcr) method. Ores are classified on the basis of their state of oxidation (supergene, transition zone and primary ores) in an equivalent mine water electrolyte; evidence is found for a systematic change of electrical response with oxidation that is symptomatic of electrochemical reactions varying from diffusion to charge‐transfer controlled processes. The analysis is based on the assumption that the Randies’ circuit is an effective model of a rock's electrical response in its natural electrolyte. Both complex impedance plane graphs and computer inversion are used to study the model. Harmonic distortion is a useful aid in this interpretation.
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GEOPHYSICAL INVESTIGATION OF A FAULT ZONE–CASE HISTORY FROM ILE‐IFE, SOUTHWEST NIGERIA*
Authors M.O. OLORUNFEMI, V.O. OLAREWAJU and M. AVCIABSTRACTA fault zone, inferred as a major linear structure from aerial photographs of the University of Ife Campus, has been investigated.
Results of a multi‐method geophysical survey indicate that the zone is characterized by relatively low resistivity and high magnetic effect. They are characteristic of a sheared and perhaps saturated fault zone with magnetic mineral infillings along its plane.
The near symmetry of the magnetic anomalies over the fault zone may not be indicative of a step faulting. The geoelectric sections show no indication of any significant displacement. The above supports results of a previous study that the fault is a strike‐slip fault.
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Volumes & issues
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Volume 72 (2023 - 2024)
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Volume 71 (2022 - 2023)
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Volume 70 (2021 - 2022)
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Volume 69 (2021)
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Volume 68 (2020)
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Volume 67 (2019)
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Volume 66 (2018)
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Volume 65 (2017)
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Volume 64 (2015 - 2016)
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Volume 63 (2015)
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Volume 62 (2014)
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Volume 61 (2013)
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Volume 60 (2012)
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Volume 59 (2011)
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Volume 58 (2010)
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Volume 57 (2009)
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Volume 56 (2008)
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Volume 55 (2007)
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Volume 54 (2006)
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Volume 53 (2005)
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Volume 52 (2004)
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Volume 51 (2003)
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Volume 50 (2002)
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Volume 49 (2001)
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Volume 48 (2000)
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Volume 47 (1999)
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Volume 46 (1998)
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Volume 45 (1997)
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Volume 44 (1996)
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Volume 43 (1995)
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Volume 42 (1994)
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Volume 41 (1993)
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Volume 40 (1992)
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Volume 39 (1991)
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Volume 38 (1990)
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Volume 37 (1989)
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Volume 36 (1988)
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Volume 35 (1987)
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Volume 34 (1986)
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Volume 33 (1985)
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Volume 32 (1984)
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Volume 31 (1983)
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Volume 30 (1982)
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Volume 29 (1981)
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Volume 28 (1980)
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Volume 27 (1979)
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Volume 26 (1978)
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Volume 25 (1977)
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Volume 24 (1976)
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Volume 23 (1975)
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Volume 22 (1974)
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Volume 21 (1973)
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Volume 20 (1972)
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Volume 19 (1971)
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Volume 18 (1970)
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Volume 17 (1969)
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Volume 16 (1968)
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Volume 15 (1967)
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Volume 14 (1966)
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Volume 13 (1965)
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Volume 12 (1964)
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Volume 11 (1963)
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Volume 10 (1962)
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Volume 9 (1961)
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Volume 8 (1960)
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Volume 7 (1959)
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Volume 6 (1958)
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Volume 5 (1957)
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Volume 4 (1956)
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Volume 3 (1955)
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Volume 2 (1954)
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Volume 1 (1953)