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- Volume 43, Issue 4, 1995
Geophysical Prospecting - Volume 43, Issue 4, 1995
Volume 43, Issue 4, 1995
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Long period multiple suppression by predictive deconvolution in the x–t domainl1
Authors M. Turhan Taner, Ronan F. O'Doherty and Fulton KoehlerAbctractThere are two forms of systematic error in conventional deconvolution as applied to the problem of suppressing multiples with periodicities longer than a hundred milliseconds. One of these is the windowing effect due to the assumption that a true autocorrelation function can be computed from a finite portion of data. The second form of error concerns the assumption of periodicity, which is strictly true only at zero offset for a 1D medium. The seriousness of these errors increased with the lengthening of the multiple period.
This paper describes and illustrates a rigorous 2D solution to the predictive deconvolution equations that overcomes both of the systematic errors of conventional 1D approaches. This method is applicable to both the simple or trapped system and to the complex or peg‐leg system of multiples. It does not require that the design window be six to ten times larger compared to the operator dimensions and it is accurate over a wide range of propagation angles. The formulation is kept strictly in the sense of the classical theory of prediction. The solution of normal equations are obtained by a modified conjugate gradient method of solution developed by Koehler. In this algorithm, the normal equations are not modified by the autocorrelation approximation.
As with all linear methods, approximate stationary attitude in the multiple generating process is assumed. This method has not been tested in areas where large changes in the characteristic of the multiple‐generating mechanism occur within a seismic spread length.
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Phase inversion deconvolution for long and short period multiples attenuationl1
Authors Eugene Lichrnan and E. John NorthwoodAbctractA new approach to deconvolution has been developed to improve the attenuation of multiple energy. This approach to deconvolution is unique in that it not only eliminates the usual assumptions of a minimum phase lag wavelet and a random distribution of impulses, but also overcomes the noise limitation of the homomorphic deconvolution and its inherent instability to phase computation.
We attempt to analyse the continuous alteration of the acoustic waveform during the propagation through a linear system. Based on the results of this analysis, the surface‐related measurements are described as a convolution of the impulse response of the system with the non‐stationary forward wavelet which includes all multiple terms generated within the system.
The amplitude spectrum of the forward wavelet is recovered from the amplitude spectrum of the recorded signal, using the difference between the rate of decay of the source wavelet and the duration of the measurement.
The phase spectrum of the forward wavelet is estimated using the Hilbert transform and the fact that the mixed phase lag wavelet can be presented as a convolution of the minimum and maximum phase lag wavelets.
The multiples are discriminated from primaries by comparison of the phase spectrum of the seismic signal and the inverse of the forward wavelet. Therefore, the technique is called phase inversion deconvolution (PID). This approach requires no velocity information in order to recognize and attenuate multiple energy. Therefore, primary energy is recovered in the near‐offset region where the velocity differential between primary and multiple energies is very small.
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Harmonic resonance structure and chaotic dynamics in the earth‐vibrator system1
By David WalkerAbstractSource‐generated energy in seismic vibrator records includes ultraharmonics, subharmonics, ultra‐subharmonics and possibly chaotic oscillatory behaviour. Nonlinear behaviours can be modelled using a ‘hard‐spring’ form of the Duffing equation. Modelling indicates that a qualitatively similar harmonic resonance structure is present for a broad range of possible mathematical descriptions. Qualitative global system behaviours may be examined without knowledge of actual earth parameters. Non‐linear resonances become stronger, relative to fundamental sweep frequencies, as the driving force increases or damping decreases. System response energy levels are highest when non‐linear resonances are strong. The presence of chaotic energy can indicate the highest energy state of a system reponse. Field data examples are consistent with behaviours predicted by modelling. Conventional correlation and stack uses a fraction of the energy produced in the earth‐vibrator system. A correlation and filtering process that uses a representation of the source dynamics based on the system response can reduce signal degradation due to non‐linear resonance.
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Imaging beneath an opaque basaltic layer using densely sampled wide‐angle OBS data1
Authors C. Samson, P.J. Barton and J. KarwatowskiAbstractA combined reflection/refraction (wide‐angle) seismic survey was conducted on the continental shelf north‐west of Britain, using a conventional streamer with an airgun source, and static ocean‐bottom seismometers (OBS) to record wide‐angle energy. The shallow structure down to a basaltic layer was reasonably well imaged on the stacked reflection section. The basalts, however, proved to be opaque to the conventional reflection method and prevented the imaging of deeper horizons, where an important velocity inversion was anticipated. This paper reports on the processing, modelling and interpretation of the densely sampled wide‐angle OBS data that were coincident with the reflection profile. Eleven OBS instruments were deployed along a 75 km line and recorded signal from a powerful 149 litre (9100 in.3) airgun array fired every 50 m. Data processing was performed using a standard industrial reflection seismic software package prior to first‐arrival picking. Processing steps included geometry definition, trace summation and display of the data using various scaling algorithms. An initial model was constructed from 1D velocity‐time profiles digitized every 4 km along the stacked section. First arrival traveltime modelling rapidly converged to a detailed model of the structure of the top 5 km of the crust. Modelling revealed the existence of a buried low‐velocity Mesozoic sedimentary basin, of a prominent basement horst and of a normal fault penetrating to the basement.
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Quantitative evaluation of crosshole seismic reflection images using physical model data1
Authors Peter S. Rowbotharn and Neil R. GoultyAbstractPhysical models give us a known geometry with which to compare our processed reflection images and therefore our imaging techniques. We show how this comparison may be quantified in order to evaluate processed images properly. A crosshole data set was acquired through a model interrogated at ultrasonic frequencies using Durham University's physical modelling system. Various reflectivity images were obtained using processing sequences which include deconvolution, wavefield separation and migration. An error‐energy scheme was used to assess the quality of these images, by comparing them against a best‐fit depth model. A synthetic data set was also used to evaluate the imaging capability of the crosshole geometry and the effectiveness of the different processing schemes.
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Anisotropic velocity analysis1
By C.M. SayersAbstractResults from walkaway VSP and shale laboratory experiments show that shale anisotropy can be significantly anelliptic. Heterogeneity and anellipticity both lead to non‐hyperbolic moveout curves and the resulting ambiguity in velocity analysis is investigated for the case of a factorizable anisotropic medium with a linear dependence of velocity on depth. More information can be obtained if there are several reflectors. The method of Dellinger et al. for anisotropic velocity analysis in layered transversely isotropic media is examined and is shown to be restricted to media having relatively small anellipticity. A new scheme, based on an expansion of the inverse‐squared group velocity in spherical harmonics, is presented. This scheme can be used for larger anellipticity, and is applicable for horizontal layers having monoclinic symmetry with the symmetry plane parallel to the layers. The method is applied to invert the results of anisotropic ray tracing on a model Sand/shale sequence. For transversely isotropic media with small anisotropy, the scheme reduces to the method of Byun et al. and Byun and Corrigan. The expansion in spherical harmonics allows the P‐phase slowness surface of each layer to be determined in analytic form from the layer parameters obtained by inversion without the need to assume that the anisotropy is weak.
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Error estimation and optimization of gravity surveys1
More LessAbstractGravity data are often acquired over long periods of time using different instruments and various survey techniques, resulting in data sets of non‐uniform accuracy. As station locations are inhomogeneously distributed, gravity values are interpolated on to a regular grid to allow further processing, such as computing horizontal or vertical gradients. Some interpolation techniques can estimate the interpolation error. Although estimation of the error due to interpolation is of importance, it is more useful to estimate the maximum gravity anomaly that may have gone undetected by a survey. This is equivalent to the determination of the maximum mass whose gravity anomaly will be undetected at any station location, given the data accuracy at each station. Assuming that the maximum density contrast present in the survey area is known or can be reasonably assumed from a knowledge of the geology, the proposed procedure is as follows: at every grid node, the maximum mass whose gravity anomaly does not disturb any of the surrounding observed gravity values by more than their accuracies is determined. A finite vertical cylinder is used as the mass model in the computations. The resulting map gives the maximum detection error and, as such, it is a worst‐case scenario. Moreover, the map can be used to optimize future gravity surveys: new stations should be located at, or near, map maxima. The technique is applied to a set of gravity observations obtained from different surveys made over a period of more than 40 years in the Abitibi Greenstone Belt in eastern Canada.
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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 18 (1970 - 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 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)