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- Volume 33, Issue 1, 1985
Geophysical Prospecting - Volume 33, Issue 1, 1985
Volume 33, Issue 1, 1985
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MIGRATION STRATEGY*
Authors J. W. J. HOSKEN and S. M. DEREGOWSKIABSTRACTThe geophysical data processor today has on offer a great variety of tools for the inversion of seismic reflection data to estimate geological structure. The major subset of these comprises migration procedures, which span a wide range of sophistication and cost in terms both of computation time and manual effort on the part of interpreters and processing staff. The choice of an over‐powerful process can be very wasteful, but on the other hand too naive a migration procedure can lead to wrong interpretations which are much more costly still.
Complete inversion procedures which aim to delineate all changes in rock densities and elastic properties in the subsurface are still in the imaginative stages of research. Not even the most sophisticated migration procedure in current use with real data, however, provides a complete inversion, but all depend in some measure on prior knowledge of the velocity structure of the section of the earth traversed by the seismic energy. Such knowledge may be very approximate at first, but each inversion should, through the skill of the interpreter, allow him to revise his velocity model and, up to some limit imposed by the quality and ambiguity of the original data, to improve the next inversion. Paradoxically, he can often be helped by using forward modeling procedures to check the implications of his ideas in the data domain, both in deciding how to update the velocity model and in selecting the most appropriate migration process to use next.
We review here the currently available toolkit of migration and modeling processes and make suggestions as to how each process can fit into a learning strategy which can improve the interpretation as economically as possible and in as many iterative steps as the complexity of the earth's velocity structure makes necessary. An example is shown of the strategy being used in a complex overthrust region.
The authors wish to thank the Chairman and Board of Directors of BP Exploration Co. for permission to publish this paper, and also make acknowledgment to our colleagues whose labours in research and development have made available to our use many of the essential tools required to implement the strategies we describe.
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PRACTICAL ASPECTS OF THE DETERMINATION OF 3‐D STACKING VELOCITIES*
Authors H. J. LEHMANN and W. HOUBAABSTRACTProper stacking of three‐dimensional seismic CDP‐data generally requires the knowledge of normal moveout velocities in all source‐receiver directions contributing to a CDP‐gather. The azimuthal variation of the stacking velocities mainly depends on the dip of the seismic interfaces. For a single dipping plane a simple relation exists between the dip and the azimuthal variation of NMO‐velocity. Varying strike and dip of subsequent reflectors, however, result in a complex dependency of the seismic parameters.
Reliable information on the spatial distribution of the normal moveout (NMO)‐velocity can be derived from a wavefront curvature estimation using a 3‐D ray‐tracing technique. These procedures require additional information, e.g. reflection time gradients or depth maps to show interval velocities between leading interfaces. Moreover, their application to an extended 3‐D data volume is restricted by high costs.
The need for a routine 3‐D procedure resulted in a special data selection to create pseudo 2‐D profiles and to apply existing velocity estimation routines to these profiles. At least three estimates in different directions are necessary to derive the full azimuthal velocity variation, characterized by the large and the small main axis and the orientation of the velocity ellipse.
Errors are estimated by means of computer models. Stacking velocities obtained by mathematical routines (least‐squares fit) and by seismic standard routines (NMO‐correction and correlation) are compared.
Finally, a general 3‐D velocity procedure using cross‐correlation of preliminarily NMO‐corrected traces is proposed.
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SIDESWIPE REFLECTIONS AND OTHER EXTERNAL AND INTERNAL REFLECTIONS FROM SALT PLUGS IN THE NORWEGIAN‐DANISH BASIN*
By J. HOSPERSABSTRACTSideswipe, foreswipe and backswipe reflections are described from salt plugs situated in the Norwegian‐Danish Basin (southern part of the Norwegian sector of the North Sea). These external reflections can be mapped and are shown to define the shoulders of a salt plug.
Physical three‐dimensional models are described which aid in visualizing the various aspects of the external reflection process. Refracted waves which have been internally reflected in a salt plug have also been identified.
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COMPUTATION OF ZERO‐OFFSET VERTICAL SEISMIC PROFILES INCLUDING GEOMETRICAL SPREADING AND ABSORPTION*
Authors B. URSIN and B. ARNTSENABSTRACTSynthetic vertical seismic profiles (VSP) provide a useful tool in the interpretation of VSP data, allowing the interpreter to analyze the propagation of seismic waves in the different layers. A zero‐offset VSP modeling program can also be used as part of an inversion program for estimating the parameters in a layered model of the subsurface.
Proposed methods for computing synthetic VSP are mostly based on plane waves in a horizontally layered elastic or anelastic medium. In order to compare these synthetic VSP with real data a common method is to scale the data with the spherical spreading factor of the primary reflections. This will in most cases lead to artificial enhancement of multiple reflections.
We apply the ray series method to the equations of motion for a linear viscoelastic medium after having done a Fourier transformation with respect to the time variable. This results in a complex eikonal equation which, in general, appears to be difficult to solve. For vertically traveling waves in a horizontally layered viscoelastic medium the solution is easily found to be the integral along the ray of the inverse of the complex propagation velocity. The spherical spreading due to a point source is also complex, and it is equal to the integral along the ray of the complex propagation velocity.
Synthetic data examples illustrate the differences between spherical, cylindrical, and plane waves in elastic and viscoelastic layered media.
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ON THE CALIBRATION OF THE WATER GUN PRESSURE SIGNATURE*
By M. H. SAFARABSTRACTA simple field method was proposed by the author in 1976 for measuring the absolute amplitude of the pressure pulse radiated by marine seismic sources which radiate a bubble pulse. The proposed method involves the recording of the near‐field pressure signature radiated by the water gun using a wide‐band hydrophone. The key feature of the proposed method is that a knowledge of the hydrophone sensitivity and its distance from the water gun are not required.
It is shown that the absolute amplitudes of the pressure pulses radiated by the S80 and P400 water guns obtained using the proposed method are in agreement with those obtained using a Ref‐Tek hydrophone.
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A LABORATORY INVESTIGATION OF THE INDUCED POLARIZATION OF THE TRIASSIC SHERWOOD SANDSTONE OF LANCASHIRE AND ITS HYDROGEOLOGICAL APPLICATIONS*
Authors M. O. OLORUNFEMI and D. H. GRIFFITHSABSTRACTThe time‐domain induced polarization (IP) of saturated Sherwood Sandstone correlates significantly with the intergranular permeability and the matrix conductivity but only at low electrolyte concentrations (< 500 p.p.m. NaCl). An increase in the magnitude of sandstone IP with increase in the valence of the electrolyte cation is pronounced but occurs only at intermediate concentrations, i.e., between 100 and 2500 p.p.m. Surface IP and resistivity depth sounding measurements, supplemented by data from laboratory measurements, can be used to estimate the groundwater conductivity and hence the salinity in a moderate to strongly saline sandstone aquifer.
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COMPUTING THE KERNEL FUNCTION IN RESISTIVITY SOUNDING WITH AN ARBITRARY ELECTRODE CONFIGURATION*
By F. KOHLBECKABSTRACTA power series expansion can be used to obtain the kernel from apparent resistivities for an arbitrary electrode configuration. Three types of function are most appropriate for this purpose. The expansion coefficients can be by a least‐squares method. In this case, ortho‐normalization of the functions is of great advantage. An example with the Wenner configuration is given.
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THE USE OF VERTICAL LINE SOURCES IN ELECTRICAL PROSPECTING FOR HYDROCARBON*
Authors J. P. ROCROI and A. V. KOULIKOVABSTRACTConventional electrical prospecting can be extended to the search for deep‐seated hydrocarbon deposits, by using the steel casings of drill‐holes as vertical line sources. These sources produce at depth a density of current higher than the density created by point sources located at the ground surface. Several tests have shown that the contrast of conductivity between resistive hydrocarbon deposits and the surrounding salt water produces relevant anomalies on a resistivity map obtained with vertical line sources, especially where there exists a superficial masking effect caused by a highly resistive layer. In a survey carried out in the USSR, combined measurements were performed, both with line source and with surface point sources. The detected residual resistivity anomaly roughly delineates the contours of the known hydrocarbon deposit.
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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)