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- Volume 39, Issue 1, 1991
Geophysical Prospecting - Volume 39, Issue 1, 1991
Volume 39, Issue 1, 1991
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THE DETERMINATION OF THE SEISMIC QUALITY FACTOR Q FROM VSP DATA: A COMPARISON OF DIFFERENT COMPUTATIONAL METHODS1
By RAINER TONNAbstractTen methods for the computation of attenuation have been investigated, namely: amplitude decay, analytical signal, wavelet modelling, phase modelling, frequency modelling, rise‐time, pulse amplitude, matching technique, spectral modelling and spectral ratio. In particular, we have studied the reliability of each of these methods in estimating correct values of Q using three synthetic VSP seismograms for plane P‐waves with different noise contents.
The investigations proved that no single method is generally superior. Rather, some methods are more suitable than others in specific situations depending on recording, noise or geology. The analytical signal method has been demonstrated to be superior if true amplitude recordings are available. Otherwise spectral modelling or, in the ‘ noise‐free’ case the spectral ratio method, is optimal.
Finally, two field VSPs in sediments are investigated. Only in the case of the highest quality VSP can significant information be deduced from the computed attenuation.
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MODELLING 1D WAVE PROPAGATION IN A SYSTEM OF ABSORBING LAYERS1
By T. M. McGEEAbstractThe purpose of this paper is to derive relationships useful for the analysis of shallow transmission and reflection seismic data. Since the effects of absorption cause such data to be non‐stationary, the derived results do not rely on frequency‐domain representations being independent of the time origin. Rather, results are expressed in the z‐domain and are suitable for implementation in the time domain. In order to give an unambiguous system of notation a classic approach to the problem of 1D wave propagation in a layered system is fully described. Absorption is included in a general but realistic way and the implications of representing it digitally are considered in detail. Computational efficiency is improved by grouping the layers in blocks of similar absorptive effect. The transmissivity and reflectivity of systems composed of such blocks are obtained and a dereverberation filter identified. The filter is described succinctly in terms of a non‐linear recursion relationship. The sequences obtained by convolving the filter with the transmissivity and reflectivity are discussed in detail. The results are illustrated by nominally realistic synthetic examples computed in the time domain.
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PULSE DISTORTION IN CAUSTIC REGIONS1
Authors L.‐J. GELIUS, J. J. STAMNES and H. HEIERAbstractIt is well known that there is a phase shift of π associated with the propagation of a seismic pulse through a focus. But this simple behaviour is true only if the two points at which the pulse is observed lie sufficiently far from the focus, one on either side. Near the focus matters are more complicated and the shape of the pulse may be severely distorted because of diffraction. To examine this problem we consider a pulse generated by a point source and focused by a spherical reflector. Using a general combined method of ray tracing and diffraction, we compute seismograms for receivers lying both inside and outside the focal region, and determine how the shape of the pulse changes on propagation through the focus. To assist the physical interpretation of the numerical results, we also present relevant exact and asymptotic results for comparison.
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RESOLUTION ANALYSIS OF BAND‐LIMITED AND OFFSET‐LIMITED SEISMIC DATA FOR PLANE‐LAYERED SUBSURFACE MODELS1
Authors E. P. F. VAN RIJSSEN and G. C. HERMANAbstractA singular‐value decomposition technique is presented for quantifying the information content of band‐limited and offset‐limited seismic reflection data for the case of plane‐layered subsurface models. With the aid of this method, vertical resolution, tuning effects and ambiguities between different types of lithological parameters can be analysed simultaneously. The method is applied in a model study for quantifying the effects of offset‐limitation and the presence of different wavetypes on the ability to discriminate different types of parameters.
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APPLICATION OF THE PARTIAL KARHUNEN‐LOÈVE TRANSFORM TO SUPPRESS RANDOM NOISE IN SEISMIC SECTIONS1
More LessAbstractThe Karhunen‐Loève (K–L) transform is an effective technique for suppressing spatially uncorrelated noise, but because of its high computational cost, fast transforms, such as the Fourier transform, have been more favoured. Two techniques that combine to make the K–L transform feasible for seismic data processing are discussed. The first technique filters the data for limited dips. For each dip, linear moveout is applied to the seismic sections so that events with this dip are made flat. By interpolation, we can include dips that are fractions of a sample/trace. After linear moveout, zero‐lag K–L filtering is applied followed, by inverse linear moveout; the results from all dips are added to form the final filtered data. The second technique is blocking, in which the seismic section is divided into blocks small enough for each block to be processed using relatively small matrices; the processed blocks are assembled to form the final filtered section. Using a combination of these techniques, seismic sections can be filtered at a reasonable cost using the K‐L transform.
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TRANSFORMATION OF LINE‐SOURCE RESISTIVITY DATA TO POINT‐SOURCE DATA AND VICE VERSA USING THE MATRIX METHOD1
By SRI NIWASAbstractInvestigations have shown the existence of a linear relationship between point‐source resistivity data and line‐source resistivity data through a matrix operator, which paves the way for the efficient transformation of line‐source data to the corresponding point‐source data and vice versa. The power of these equations has been established by computational examples. The relationship will be useful in the modelling and inversion of resistivity data from 2D structures.
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SEISMO‐ELECTRIC EXPLORATION: EXPECTED SIGNAL AMPLITUDES1
Authors R. D. RUSSELL and A. S. BARKER JrAbstractFor more than 20 years, Soviet scientists have published papers and registered patents describing the conversion of seismic to electromagnetic energy in geological environments and the detection of the electric or magnetic signals as a method of geophysical exploration. Because of the potential importance of a reliable geophysical technique for locating quartz veins, we have been conducting extensive laboratory and field tests of the phenomena.
For the purposes of designing appropriate field tests we need to know approximate signal magnitudes, but little has been published on them. The present paper describes a simplified model from which order‐of‐magnitude estimates of expected electric and magnetic signal strengths can be made with sufficient accuracy for such purposes. For mathematical convenience we model the target as a homogeneous sphere in which the seismic input induces uniform, time‐varying electric polarization. More realistic configurations can be described by linear superposition of the potentials of appropriate sub‐elements.
True piezo‐electricity is, by definition, linear. Therefore, responses should have the same frequency content as the seismic input. Combining the low‐frequency form of our results with the assumption that the entire thickness of a target vein responds in phase to a seismic excitation, we obtain the following estimates of the maximum electric and magnetic fields at a distance r from the target:
where p is the resistivity and μ0 the dielectric constant of the ground in which E is measured, V is the effective volume of the target, P is the polarization of the vein, s is the seismic stress at the target, and a is the effective piezo‐electric coefficient. Signals observed in experiments at the Erickson Mine, British Columbia, gave electric fields in acceptable agreement with our theoretical predictions.We conclude by considering plausible relationships for the high‐frequency signals observed from sulfide minerals, assuming that they represent the release of stored stress triggered by the seismic arrival.
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FIELD RESULTS WITH DOWNWARD CONTINUATION TECHNIQUE IN INDUCED POLARIZATION PROFILING USING POINT ELECTRODES1
Authors A. APPARAO, T. GANGADHARA RAO and V. SUBRAHMANYA SARMAAbstractThe method of downward continuation is well known to those working in gravity, magnetic, SP and low‐frequency electromagnetic exploration. It is demonstrated that the method of continuation can also be usefully employed in the interpretation of induced polarization gradient profiling using point electrodes to determine target depth. The apparent resistance Ra and chargeability Ma measurements obtained with point electrode excitation of the ground have been used to compute the values of (Ra)l and (Ma)l that would be obtained with a linear array.
Continuation of the apparent polarizability profile thus obtained with the linear array gives a value for the depth of the target which agrees closely with that obtained by the continuation of the SP profile. On the other hand, continuation of the profile of the secondary transient signal (VS)L alone, yields a depth of the target which is in agreement with the borehole information. However, it is seen that the secondary transient voltage profiling response splits into two anomalies which fall on either side of the SP and/or (Ma)l anomaly centre, and does not coincide with that of the latter.
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Volume 46 (1998)
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Volume 42 (1994)
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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 4 (1956)
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Volume 3 (1955)
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Volume 2 (1954)
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Volume 1 (1953)