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- Volume 39, Issue 5, 1991
Geophysical Prospecting - Volume 39, Issue 5, 1991
Volume 39, Issue 5, 1991
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STACKING OF P‐SV SEISMIC REFLECTION DATA USING DIP MOVEOUT1
More LessAbstractThe moveout of P‐SV mode‐converted seismic reflection events in a common‐midpoint gather is non‐hyperbolic. This is true even if the medium has constant P‐ and SV‐wave velocities. Furthermore, reflection‐point smear occurs even along horizontal reflectors. These effects reduce the resolution of the zero‐offset stack. In such a medium, the generalization of the dip moveout transformation to P‐SV data can be calculated analytically. The resulting P‐SV dip moveout operators solve the problem of reflection‐point smear, and image any reflector regardless of dip or depth. The viability of this technique is demonstrated on synthetic and field data.
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A SIMULATED ANNEALING APPROACH TO SEISMIC MODEL OPTIMIZATION WITH SPARSE PRIOR INFORMATION1
Authors KLAUS MOSEGAARD and PETER D. VESTERGAARDAbstractIt is well known that seismic inversion based on local model optimization methods, such as iterative use of linear optimization, may fail when prior information is sparse. Where the seismic events corresponding to reflectors of interest remain to be identified, a global optimization technique is required.
We investigate the use of a global, stochastic optimization method, that of simulated annealing, to solve the seismic trace inversion problem, in which the two‐way traveltimes and reflection coefficients are to be determined. The simulated annealing method is based on an analogy between the model‐algorithm system and a statistical mechanical system. We exploit this analogy to produce improved annealing schedules. It is shown that even in cases of virtually no prior information about two‐way traveltimes and reflection coefficients, the method is capable of producing reliable results.
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INVERSION OF POST‐STACK SEISMIC DATA USING SIMULATED ANNEALING1
Authors PETER D. VESTERGAARD and KLAUS MOSEGAARDAbstractModel‐based inversion of seismic reflection data is a global optimization problem when prior information is sparse. We investigate the use of an efficient, global, stochastic optimization method, that of simulated annealing, for determining the two‐way traveltimes and the reflection coefficients.
We exploit the advantage of an ensemble approach to the inversion of full‐scale target zones on 2D seismic sections.
In our ensemble approach, several copies of the model‐algorithm system are run in parallel. In this way, estimation of true ensemble statistics for the process is made possible, and improved annealing schedules can be produced.
It is shown that the method can produce reliable results efficiently in the 2D case, even when prior information is sparse.
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WAVELET ESTIMATION USING THE MULTITAPER METHOD1
By A. T. WALDENAbstractAn accurate estimate of the seismic wavelet on a seismic section is extremely important for interpretation of fine details on the section and for estimation of acoustic impedance. In the absence of well‐control, the recognized best approach to wavelet estimation is to use the technique of multiple coherence analysis to estimate the coherent signal and its amplitude spectrum, and thence construct the seismic wavelet under the minimum‐phase assumption.
The construction of the minimum‐phase wavelet is critically dependent on the decay of the spectrum at the low‐frequency end. Traditional methods of cross‐spectral estimation, such as frequency smoothing using a Papoulis window, suffer from substantial side‐lobe leakage in the areas of the spectrum where there is a large change of power over a relatively small frequency range. The low‐frequency end of the seismic spectrum (less than 4 Hz) decays rapidly to zero. Side‐lobe leakage causes poor estimates of the low‐frequency decay, resulting in degraded wavelet estimates.
Thomson's multitaper method of cross‐spectral estimation which suffers little from side‐lobe leakage is applied here, and compared with the result of using frequency smoothing with the Papoulis window. The multitaper method seems much less prone to estimating spuriously high coherences at very low frequencies. The wavelet estimated by the multitaper approach from the data used here is equivalent to imposing a low‐frequency roll‐off of some 48 dB/oct (below 3.91 Hz) on the amplitude spectrum. Using Papoulis smoothing the equivalent roll‐off is only about 36 dB/oct. Thus the multitaper method gives a low‐frequency decay rate of the amplitude spectrum which is some 4 times greater than for Papoulis smoothing. It also gives more consistent results across the section. Furthermore, the wavelet obtained using the multi‐taper method and seismic data only (with no reference to well data) has more attractive physical characteristics when compared with a wavelet extracted using well data, than does an estimate using traditional smoothing.
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JOINT INVERSION OF SEISMIC AND GEOELECTRIC DATA RECORDED IN AN UNDERGROUND COAL MINE1
Authors M. DOBRÓKA, Á. GYULAI, T. ORMOS, J. CSÓKÁS and L. DRESENAbstractUntil the present time the ‘ rock‐coal‐rock’ layer sequence and offsets in coal‐seams in underground coal mines have been detected with the aid of seismic waves and geoelectric measurements. In order to determine the geometrical and petrophysical parameters of the coal‐seam situation, the data recorded using seismic and geoelectric methods have been inverted independently. In consequence, the inversion of partially inaccurate data resulted in a certain degree of ambiguity. This paper presents the first results of a joint inversion scheme to process underground vertical seismic profiling data, geolectric resistivity and resistance data.
The joint inversion algorithm makes use of the damped least‐squares method and its weighted version to solve the linearized set of equations for the seismic and geolectric unknowns. In order to estimate the accuracy and reliability of the derived geometrical and petrophysical layer parameters, both a model covariance matrix and a correlation matrix are calculated. The weighted least‐squares algorithm is based on the method of most frequent values (MFV). The weight factors depend on the difference between measured data and those calculated by an iteration process.
The joint inversion algorithm is tested by means of synthetic data. Compared to the damped least‐squares algorithm, the MFV inversion leads to smaller estimation errors as well as lower sensitivities due to the choice of the initial model. It is shown that, compared to an independent inversion, the correlation between the model parameters is definitely reduced, while the accuracy of the parameter estimation is appreciably increased by the joint inversion process. Thus the ambiguity is significantly reduced.
Finally, the joint inversion algorithm using the MFV method is applied to underground field data. The model parameters can be derived with a sufficient degree of accuracy, even in the case of noisy data.
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EXAMINATION OF A SPECTRAL METHOD FOR MEASURING THE EFFECTS OF ANISOTROPY1
Authors COLIN MACBETH and STUART CRAMPINAbstractThe most diagnostic effect of anisotropy on shear waves is shear‐wave splitting. This phenomenon creates a distinctive signature in the 3D particle motion. Methods to extract the effects of anisotropy from shear‐wave data attempt to measure details of this motion. Many techniques have been published recently which process the shear waves in the time or frequency domain. Here we examine the way in which information on the interference effects between the split shear waves is contained within the frequency domain, and suggest some criteria which may be used in future processing algorithms. The time‐delay between the split shear waves, and the polarization direction of the leading shear wave can be converted into easily measured features from analysis of the Fourier spectrum of the shear‐wave signal on each component of motion. These features arise in the spectral interference patterns which are formed by the interaction between the two closely‐spaced and similar waveforms. The interference patterns are interpreted for synthetic and observed seismogram data.
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A SEARCH FOR DIRECT HYDROCARBON INDICATORS IN THE FORMBY AREA1
Authors J. P. BUSBY, R. J. PEART, C. A. GREEN, R. D. OGILVY and J. P. WILLIAMSONAbstractIt has been proposed that the high‐frequency, low‐amplitude magnetic anomalies found over some hydrocarbon deposits are due to long‐term microseepage of hydrocarbons into iron‐rich sedimentary roof rocks, with subsequent precipitation of diagenetic magnetite or pyrrhotite at or near the water‐table. Aerogeophysical data sets, comprising both magnetic and gamma‐ray spectrometer measurements, over the Formby oil‐field, Lancashire, U.K., have been analysed for hydrocarbon‐related anomalies. Detailed ground magnetic traverses were also made to investigate some of the aeromagnetic anomalies.
No hydrocarbon‐induced magnetic anomalies were detected. The majority of the high‐frequency events occurring in the aeromagnetic data correlated with cultural features, others were attributed to artefacts of the data processing. In particular there were no extensive areas of high‐frequency, low‐amplitude anomalies as might be expected from authigenic magnetic minerals.
Borehole chippings from inside and outside the oil‐field were examined. High magnetic susceptibilities were due to contamination during drilling. Magnetite, which was identified by mineralogical analysis, was found to be of detrital origin. If any diagenetic magnetic material was present it was in such small quantities as to be insignificant.
The oil‐field did not possess a characteristic gamma‐ray spectrometric signature. Anomalies were found to be related to hydrological conditions and to the distribution of surficial deposits. Attempts to suppress the influence of the drift geology had only limited success.
It is concluded that the effectiveness of high‐resolution aeromagnetic onshore surveys for direct hydrocarbon detection has yet to be established. In particular, the anomaly found over the Formby oil‐field was caused by the cumulative effect of borehole casing. Similar cultural contamination by oil‐field equipment may explain some of the anomalies discovered over hydrocarbon deposits in North America. It is also unlikely that the spectrometric method can be applied diagnostically in any but the most simple and drift‐free geological environments.
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Volume 72 (2023 - 2024)
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Volume 69 (2021)
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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)