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- Volume 32, Issue 3, 1984
Geophysical Prospecting - Volume 32, Issue 3, 1984
Volume 32, Issue 3, 1984
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INVERSION METHOD IN THE SLANT STACK DOMAIN USING AMPLITUDES OF REFLECTION ARRIVALS*
Authors Ph.M. CARRION, J. T. KUO and P. L. STOFFAAbstractAlmost all ray‐tracing methods ignore the analysis of the amplitudes of seismic arrivals and therefore utilize only half of the available information. We propose a method which is a combination of ray‐tracing imaging and transformation of the amplitudes of wide‐aperture data.
Seismic data in the conventional X‐T domain are first transformed to the domain of intercept time τ and ray parameter p to recover the plane wave response. The next step is the derivation of a series of plane wave reflection coefficients, which are mapped as a function of τ and p. The reflection coefficients R(τ, p) for two arbitrarily chosen traces can then be used in our inversion method to derive a slowness‐depth and a density‐depth profile. It is shown that the inclusion of amplitudes of seismic arrivals (in this method, we consider the acoustic case) makes the inverse method highly stable and accurate. In a horizontally stratified medium one can recover separate profiles of velocity and density. Since this method utilizes large‐offset data, it can be used for separate recovery of velocity and density to a greater depth.
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ON THE DETERMINATION OF THE DOWNGOING P‐WAVES RADIATED BY THE VERTICAL SEISMIC VIBRATOR*
By M. H. SAFARAbstractIt is well recognized that in order to realize the full potential of the Vibroseis technique, one needs to ensure accurate phase locking and a meaningful cross‐correlation. To achieve these two important objectives we require an accurate estimate of the compressional stress wave radiated by the vibrator into the ground.
In this paper a simple method (subject of a patent application) is developed for predicting the compressional stress waves radiated by a vertical vibrator. The main feature of the proposed method is that it involves the field measurement of the acceleration of the reaction mass and the baseplate, respectively.
The method is illustrated by computing the compressional stress waves generated by a typical vertical vibrator radiating into ice, chalk, sand, and mud. It is shown that for a seismic vibrator radiating into hard ground the pressure of the downgoing P‐wave is 180° out of phase with the baseplate velocity. It is also shown that when the driving force of the seismic vibrator has a flat amplitude spectrum, the amplitude spectrum of the downgoing P‐wave falls off by 6 dB/octave towards low frequencies.
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PHASE DISTORTION DUE TO ABSORPTION IN SEISMOGRAMS AND VSP*
Authors G. P. ANGELERI and E. LOINGERAbstractAbsorption of seismic energy in the earth reduces amplitudes and changes phases of the propagating seismic waves. Amplitudes are usually recovered according to an estimated exponential decay curve, while phase distortions are generally disregarded. Therefore, accurate processing of seismic data requires a careful investigation of the relationship between absorption and phases.
In this paper a procedure is suggested to achieve this goal, and some related topics are worked out. A method is outlined for computing synthetic seismograms and vertical seismic profiles with phase distortion due to absorption.
The algorithm works in the frequency domain, and it provides for absorption according to the usual model of exponential decay of amplitude with distance. The absorption coefficient is a linear function of frequency and is related to the quality factor Q of the rocks. Complex seismic velocities are introduced and minimum‐phase delay due to absorption is assumed for all cases considered.
Methods for estimating Q profiles from seismic well surveys and seismic data are described. Comparison between field and synthetic data shows the effectiveness and benefits of the procedure. Some applications of the method to phase distortion recovery and wavelet processing are presented.
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INFLUENCE OF AMPLITUDE AND PHASE ERRORS ON MIGRATION RESULTS*
Authors L. F. VAN DER WAL and A. J. BERKHOUTAbstractRandom amplitude and phase errors in seismic input data introduce a coherent distribution of migration half‐circles or “smiles”, the occurrence of which may cause a significant decrease of signal‐to‐noise ratio. In addition, the effect of quantization errors is discussed for different wordlengths, used both during acquisition and during data processing. Results of sign‐bit recordings are shown.
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INTERVAL VELOCITY ANALYSIS BY WAVE EXTRAPOLATION*
Authors J. GAZDAG and P. SGUAZZEROAbstractA method for interval velocity analysis is formulated on the basis of wavefield extrapolation, i.e., on the basis of wave‐equation migration. When this scheme is applied to multioffset seismic sections or to an ensemble of CMP gathers, it allows for the proper treatment of dipping events. The underlying assumptions are that local velocities should be derived from data associated with events within the interval under consideration. To minimize the effect of the region above the layer of interest, the data are first extrapolated to the top of the analysis interval. Subsequent analysis of these data then pertains to the events within this interval. Velocity estimation consists of repeated wavefield extrapolations through the analysis interval using a set of trial velocities. The optimal velocity is chosen on the basis of coherency measures designed to express the collective phase agreement among a set of offset Fourier modes. The reliability of this approach to interval velocity estimation is demonstrated on synthetic multi‐offset data.
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THE EFFECT OF A SUPERPARAMAGNETIC LAYER ON THE TRANSIENT ELECTROMAGNETIC RESPONSE OF A GROUND*
By T. LEEAbstractA thin superparamagnetic layer on the earth's surface greatly affects the transient electromagnetic response of a conducting ground. The effect of the layer is most evident for singleloop transient electromagnetic data where transient voltages decay as 1/t. Even when a separate transmitter and receiver are used, the effect of the superparamagnetic layer is still pronounced. In this case the effect of the 1/t term in the equation is much less. More dominant now is a 1/t2 term. The effect of the superparamagnetism can readily be seen in the analytical expressions for the apparent resistivities. If the presence of the superparamagnetic layer is not recognized, then the apparent resistivities decrease with time rather than approach the true value of the host rock.
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Volumes & issues
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Volume 72 (2023 - 2024)
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Volume 70 (2021 - 2022)
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Volume 69 (2021)
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Volume 53 (2005)
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Volume 49 (2001)
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Volume 46 (1998)
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Volume 45 (1997)
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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)