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- Volume 31, Issue 2, 1983
Geophysical Prospecting - Volume 31, Issue 2, 1983
Volume 31, Issue 2, 1983
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HIGH‐RESOLUTION PREDICTION OF ACOUSTIC IMPEDANCES BELOW BOTTOM‐OF‐HOLE*
Authors B. B. TAL‐VIRSKY and A. A. TABAKOVAbstractPrediction of acoustic impedances below bottom‐of‐hole developed in the paper is based on VSP data processing. The pulse form and reflections are deduced from records on the vertical array by the method of subtraction. A deterministic prediction error and a wave‐shaping Wiener filter are used to transform the pulse train and reflections into a short symmetrical pulse. Additional broadening of the pulse spectrum is achieved by stacking of seismograms from shots with various dominant frequencies after zero‐phase transformation of downgoing waves.
The inversion of seismograms is made after ideal spiking by means of a subtraction procedure. Acoustic impedances deduced are closely related to lithology of rocks below bottom‐of‐hole.
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ON NUMERICAL METHODS FOR MIGRATION IN LAYERED MEDIA*
More LessAbstractThree current migration methods derived from the wave‐equation model and based on approximations by Fourier series, finite‐differences, and a combination of both (mixed method) are reviewed. Each method is examined in the context of horizontally layered media, where the velocity of wave propagation varies with depth alone. It is shown how, under such conditions, the special properties of the linear equations obtained from finite‐difference and mixed approximations can be exploited for computational efficiency. A numerical algorithm is described which substantially reduces the complexity and the computational cost of the solution. This algorithm is directly applicable to vector processing.
The description of each method includes derivation, dispersion relation, stability properties, and applicable numerical algorithms. Comparative analyses show that, for the same computational cost, the finite‐difference approximation has poor accuracy characteristics and should be used only when the boundary conditions are difficult to express in the frequency domain. The results are illustrated with numerical experiments on synthetic data.
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REFLECTION OF ELASTIC WAVES FROM PERIODICALLY STRATIFIED MEDIA WITH INTERFACIAL SLIP*
More LessAbstractA periodically stratified elastic medium can be replaced by an equivalent homogeneous transverse isotropic medium in the long wavelength limit. The case of a homogeneous medium with equally spaced parallel interfaces along which there is imperfect bonding is a special instance of such a medium. Slowness surfaces are derived for all plane wave modes through the equivalent medium and reflection coefficients for a half‐space of such a medium are found. The slowness surface for the SH mode is an ellipsoid. The exact solution for the reflection of SH‐waves from a half‐space with parallel slip interfaces is found following the matrix method of K. Gilbert applied to elastic waves. Explicit results are derived and in the long wavelength limit, shown to approach the results for waves in the equivalent homogeneous medium. Under certain conditions, a half‐space of a medium with parallel slip interfaces has a reflection coefficient independent of the angle of incidence and thus acts like an acoustic reducing mirror. The method for the reflection of P‐ and SV‐waves is fully outlined, and reflection coefficients are shown for a particular example. The solution requires finding the eigenvalues of a 4 × 4 transfer matrix, each eigenvalue being associated with a particular wave. At higher frequencies, unexpected eigenvalues are found corresponding to refracted waves for which shear and compressional parameters are completely coupled. The two eigenvalues corresponding to the transmitted wavefield give amplitude decay perpendicular to the stratification along with up‐ and downgoing phase propagation in some other direction.
Much of this work was performed while the author was at the Department of Geophysics and Planetary Sciences, Tel‐Aviv University, Ramat‐Aviv, Israel. The author is grateful for illuminating discussions with K. Helbig and K. Gilbert.
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A THEORY OF ACOUSTIC DIFFRACTORS APPLIED TO 2‐D MODELS*
Authors S. M. DEREGOWSKI and S. M. BROWNAbstractFinite‐offset seismic reflection modeling of acoustic waves, propagating in a two‐dimensional depth section of arbitrary complexity, is discussed. The procedure developed employs the principles of simplified (far‐field) diffractor theory and ray tracing. Each reflector is represented by a set of discrete secondary sources or diffractors and the wavefield associated with each diffractor is calculated directly in the time domain by ray tracing. Reflections and diffractions are subsequently built up by the numerical superposition of these wavefields. This superposition is nondispersive for all frequencies for which the Fresnel zones are large compared with the diffractor separation.
All primary travel paths connecting the shot to diffractor and diffractor to geophone are accounted for together with phase changes induced by focal events. The method allows the modeling of arbitrary trace gathers for energy originating from selected reflectors. The nonsequential nature of the algorithm makes it suited to machines capable of carrying out many similar operations in parallel or concurrently. Diffractor theory also provides physical insight into wave scattering and focusing. In particular, the half‐differential waveform associated with a line diffractor leads to an explanation of the 90° phase lead induced by a cylindrical focus and, similarly, the full differential waveform of a point diffractor can be used to explain the 180° phase shift induced by a point focus.
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THE USE OF AIRGUN PRESSURE PULSES FOR CALIBRATING A LOW‐FREQUENCY STANDARD HYDROPHONE*
By M. H. SAFARAbstractA new technique is developed for calibrating a low‐frequency hydrophone. The technique involves the use of pressure pulses radiated by the Par 0.65 liter airgun when fired at a fixed depth but with various values of initial chamber pressure. The sensitivity of a low‐frequency hydrophone, when determined by the proposed technique, is found to be in agreement with that obtained by using the so‐called “impulse method”.
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TEST RESULTS OF A NEW TYPE OF EFFICIENT SMALL AIRGUN ARRAY*
By M. H. SAFARAbstractRecently the author developed and demonstrated (Safar 1980) an efficient method for operating the airgun. The method involves the generation of a short seismic pulse from the pressure bubble pulses radiated by an airgun when fired several times at the same optimum depth but with different chamber pressures.
The purpose of this paper is to present and discuss the test results obtained when implementing the same method using a two‐dimensional airgun array. The array consists of seven 0.65 liter airguns fired simultaneously at the same depth but with different chamber pressures.
It is shown that the far‐field pressure pulse radiated by the seven 0.65 liter airgun array is similar to that radiated by the Flexichoc seismic source. It is concluded that the proposed airgun array can be used as a subarray to form an extremely powerful super‐long array suitable for deep seismic exploration.
The author would like to thank the Chairman and Board of Directors of the British Petroleum Co. Ltd for permission to publish this paper. Thanks are also due to Mike Symes and Lovell Cox for carrying out the field tests and Seismograph Service (England) Ltd for providing the airguns.
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PROBLEMS IN THE USE OF GEOPHYSICAL INFORMATION WHEN INTERPRETING THE DATA OF SPACECRAFT OBSERVATIONS*
Authors V. A. BOGOSLOVSKY, E. B. IL'INA, E. N. KUZMINA and A. A. OGILVYAbstractObservations made from space have become almost a standard method of geologic investigation. However, interpretation of space photographs is not always unambiguous. Particularly, the nature and spatial location of various objects photographed from space remain obscure. It is especially difficult to investigate territories where basement rocks are overlain by sedimentary cover.
From the example of investigations carried out in the central part of the Russian Platform it is shown how separate lineaments and ovoids reflect the peculiar features of geologic, hydrogeologic and geophysical structure of the region. It has been established that—within the sedimentary cover—they correspond usually to zones of increased fissuredness and are associated with low‐amplitude tectonic dislocations.
The authors concentrate on the elucidation of the transfer of deep geologic information to the surface. The sedimentary cover is regarded as a communication channel which makes it possible to employ the concepts of information theory for assessing its “transparency”. The contrast due to heterogeneities of the sedimentary cover in space photographs and, consequently, the degree of their manifestation in geologic and physical fields, depends on the water content of the rocks composing the section. Along with hydrogeologic characteristics, geo‐electric parameters, such as specific electric resistivity, longitudinal conductivity and transverse resistivity, can serve as criteria of the degree of rock moistening.
When the location of lineaments does not coincide with the plane axes of geophysical anomalies, an inclination of the fault plane can be supposed. The angle of inclination is characterized by the deviation of the lineament from the axis of the anomaly.
The data resulting from the investigation of selected standard structures were used to develop a classification based on the degree of their expression in hydrodynamic, geoelectric, gravitational, and magnetic fields and depending on the spatial position and peculiarities of geologic arrangement. For geophysical investigations of unknown structures detected by space observations, this classification can be used as the basis for solving inverse problems by the methods of mathematical statistics.
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REGIONAL GEOTHERMAL EXPLORATION IN EGYPT*
Authors P. MORGAN, FOUAD K. BOULOS and C. A. SWANBERGAbstractAlthough Egypt is not characterized by abundant Cenozoic igneous activity, its location in the northeastern corner of the African plate suggests that it may possess geothermal resources, especially along its eastern margin. Regional geothermal exploration has been carried out in Egypt using the thermal gradient/heat flow technique and groundwater temperature/chemistry technique. In the thermal gradient/heat flow study, existing oil‐well bottom‐hole temperature data as well as subsurface temperature measurements in existing boreholes were utilized before special thermal gradient holes were drilled. Groundwater temperature and chemistry data were used to extend the geographic range of the direct subsurface thermal measurements. On a very modest budget, a regional thermal high has been discovered along the eastern margin of Egypt, and a local thermal anomaly has been discovered in this zone. Published geological information suggests that the sandstones of the Nubian Formation may be a suitable reservoir for geothermal fluids. The new data indicate that temperatures of 150°C or higher may be found in this reservoir in the Gulf of Suez and Red Sea coastal zone where it lies at a depth of 4 km and deeper.
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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)