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- Volume 31, Issue 1, 1983
Geophysical Prospecting - Volume 31, Issue 1, 1983
Volume 31, Issue 1, 1983
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EFFICIENT 3‐D MIGRATION IN TWO STEPS*
More LessAbstractThe addition of a third dimension to seismic data acquisition offers significant improvement of our reflection images of geologic structures, but potentially at a substantial increase in data processing cost. Here we study an approach to migration of three‐dimensional (3‐D) stacked data that can considerably reduce the burden of computation and data manipulation. Cascading two‐dimensional (2‐D) migrations in orthogonal lateral directions, we obtain an efficient 3‐D scheme that is exact for homogeneous media. As applied to examples of synthetic data from inhomogeneous media, the scheme introduces errors well below those attributable, in practice, to uncertainties in migration velocity. Application of this two‐step approach, using both Kirchhoff‐summation and finite‐difference algorithms, to a 3‐D seismic land survey over an area of known hydrocarbon traps additionally proves the importance of 3‐D migration in stratigraphic imaging.
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A SIMPLE EXACT METHOD OF 3‐D MIGRATION—THEORY*
Authors H. JAKUBOWICZ and S. LEVINAbstractRecently, a heuristic and empirical analysis of a method of three‐dimensional (3‐D) migration has been presented by Gibson, Larner, and Levin. It consists of sets of two‐dimensional (2‐D) migrations in orthogonal directions. In this paper, we examine this two‐pass method using scalar wave theory and show its action to be identical to that of so‐called full 3‐D migration for constant‐velocity media. The demonstration of equivalence is carried out in both the frequency‐space and frequency‐wavenumber domains, the latter providing a particularly transparent view of the mechanism of the two‐pass approach. We then extend our analysis to consider how various approximate algorithms used in practical implementations affect the validity of our conclusion.
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DOWNHOLE PERIODIC SEISMIC SOURCES*
By H. C. HARDEEAbstractDownhole periodic seismic sources have been proposed as a means for studying fluid zones in hydrothermal/magma systems encountered in deep scientific holes drilled as part of the Continental Scientific Drilling Program. Scientific measurements in these holes will require the use of downhole, nondestructive, high resolution, seismic tools. Downhole periodic seismic sources tested in a shallow geothermal zone showed that a pneumatic oscillator could effectively generate downhole low‐frequency (10–100 Hz) seismic waves and that these waves could effectively penetrate a highly fractured formation. An improved version of these downhole sources, capable of swept‐frequency operation, is currently under development. This type of downhole seismic source has applications in crustal studies and in the evaluation of petroleum, groundwater and geothermal reservoirs.
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BOREHOLE SEISMIC PROFILING AND TUBE WAVE APPLICATIONS IN A DAM SITE INVESTIGATION*
Authors J. P. HENRIET, J. SCHITTEKAT and Ph. HELDENSAbstractContinuous, single‐channel reflection profiling has been carried out in PVC‐lined boreholes, primarily with the aim of ascertaining the position of an old subsurface gas storage tunnel on a proposed dam site.
Tube wave reflection patterns thus generated have been interpreted in terms of sediment rigidity and shear wave velocity, and these results could be compared with some independent data. It is interesting to note that, within the well section penetrating Tertiary clays, the velocity of the hydraulic transients apparently was not affected by the PVC casing, which might be explained by a tight coupling between casing and clay wall. In such situations, tube waves turn out a straightforward tool for the determination of shear wave velocity and the derivation of dynamic elastic moduli of unconsolidated sediments.
Further applications of the study of the distribution of seismic velocities on the dam site dealt with the consolidation history of the clays. A level of abnormally low P‐wave velocities has been detected and interpreted as a gas‐charged horizon which, by its coincidence with the base level of clay diapirs, might be considered to have contributed to clay flowage in past geological times. Data about maximum past burial depth, derived from shear wave velocities, turned out to be in agreement with results from consolidation testing.
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SIGNATURES FROM SINGLE AIRGUNS*
Authors S. VAAGE, K. HAUGLAND and T. UTHEIMAbstractThe far‐field signatures from a comprehensive and systematic airgun pulse test have been analyzed. Empirical relations between the characteristic signature parameters and depth (5–12 m), pressure (100–137 bar = 10–13.7 MPa) and total chamber volume (0.65–9.5 l) have been derived. Also, the influence of using waveshape kits in different positions within the chamber has been tested.
The results indicate that:
- 1 The amplitude is proportional to chamber pressure to the power 3/4.
- 2 The bubble period is nearly independent of the position of the waveshape plate.
- 3 The increase in primary/bubble amplitude ratio is inversely proportional to the chamber volume above the waveshape plate.
- 4 The amplitude is independent of airgun depth.
Suggestions and comments regarding this work from Dr B. Ursin and Dr A. Ziolkowski are appreciated. The field work was supported by the Norwegian Petroleum Directorate through the Continental Shelf Project at the Seismological Observatory, University of Bergen.
An airgun allowing for continuous variation of the chamber volumes was supplied by GECO (Geophysical Company of Norway). The purchase of two airguns was financed by Norske Getty Exploration A/S.
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A LINEAR MODEL FOR VELOCITY ANOMALIES*
By E. LOINGERAbstractVariations of seismic interval velocities within the cable length cause anomalies in the stacking velocity analyses. Utilizing the approximation of rectilinear ray propagation, i.e. supposing that the velocity changes cause time delays only, it is shown that the stacking velocity anomalies are linearly related to the interval velocity variations. In particular, the stacking velocity anomaly is calculated when the interval velocity of an intermediate layer undergoes a stepwise variation. The amplitude of the anomaly increases with the ratio between horizon depth and cable length.
From the forward model, a program for the inversion is derived in order to identify lateral changes of interval velocities from unsmoothed stacking velocity analyses.
Some examples of the application of this technique to synthetic and real data are presented.
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ON THE APPLICATION OF INVERSE THEORY TO GRAVITY INTERPRETATION*
Authors R. E. CHAVEZ and G. D. GARLANDAbstractNumerical investigations have been carried out on suitable interpretation schemes for gravity, based upon Backus‐Gilbert inverse theory. A two‐dimensional linear model consisting of horizontal prisms (grid shape) has been adopted for interpretation of gravity profiles. An inversion technique, which uses a linear approach suitable for rapid computation, and studies with synthetic bodies have led to improvements in the method, mainly by applying weighting factors, which improve the solution in the final stages of computational work. Gravity data from the well‐known Cyprus island positive anomaly were inverted to demonstrate the feasibility of the method.
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EXPLORING THE INTERIOR OF SALT DOMES FROM BOREHOLES*
Authors H. NICKEL, F. SENDER, R. THIERBACH and H. WEICHARTAbstractPresently, salt deposits are used for storage of energy from liquid or gaseous carrier materials. Another application being considered is the storage of radioactive material. In order to avoid possible future environmental risk, extremely high security has to be guaranteed in advance for a long period. Therefore, a very precise exploration is absolutely necessary.
Exploration methods already known—which are applicable during and after drilling of test holes—help in the recognition of structures in the salt only in the close vicinity of the holes. Radiowave electromagnetic methods developed for routine measurements in salt mines provide a penetration of some hundred meters. Appropriate well‐logging equipment has been designed here for measurements from surface boreholes.
Discontinuities around one borehole in the salt can be detected through the electromagnetic reflection method. When measuring between two boreholes, additional reflecting zones can be detected, and attenuation of directly pathed waves can point out material with greater absorption properties between the holes.
Additional attempts to use acoustic waves of nearly the same wavelength proved that it is possible to recognize the direction of signals arriving at the receiver and allow separation of P‐ and S‐waves.
Instrumentation as well as recent results are reported.
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REMOTE‐REFERENCE MAGNETOTELLURICS: EQUIPMENT AND PROCEDURES*
Authors J. CLARKE, T. D. GAMBLE, W. M. GOUBAU, R. H. KOCH and R. F. MIRACKYAbstractDuring the past 3 years, major advances in the magnetotelluric technique have improved the quality of magnetotelluric data to the point where random errors in the impedance tensor and tipper are generally smaller than the uncertainty in their interpretation. The major factor in this improvement has been the introduction of the remote‐reference technique, although the use of ultrasensitive magnetometers and minicomputers for in‐field data processing has also been important. After a review of the remote‐reference technique, this paper describes the equipment and procedures used for remote‐reference magnetotellurics by the authors. Magnetometers using d.c. Superconducting Quantum Interference Devices typically have a sensitivity of 10−14 T Hz−1/2, a dynamic range of 107 in a 1 Hz bandwidth, and a slewing rate of 3 × 10−5.T s−1 at 10 kHz. The electric field measurements use conventional Cu‐CuSO4 electrodes. The remote magnetic reference signals are transmitted to the base station using FM analog telemetry. The data are collected and processed by a minicomputer based on an LSI‐11 microprocessor; the essential results—for example, the apparent resistivities and the tipper components, with their probable errors—are available in the field. Practical details are given of the handling of superconducting devices, low temperature cryostats and liquid helium in the field. Various spurious noise sources are mentioned, and techniques for minimizing their effects are described.
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COMPUTATION OF VLF RESPONSE OVER HALF‐PLANE AND WEDGE MODELS*
By O. OLSSONAbstractA theoretical solution is presented to the problem where a VLF anomaly is generated by a conducting half‐plane or a perfectly conducting wedge below a stratified overburden. The solution is obtained by the use of a scattering matrix for plane‐wave eigenfunctions. VLF anomalies have been computed for different values of the conductance and dip of the half‐plane. The phase of the VLF anomaly due to a conducting half‐plane depends on the conductance and the distance to the half‐plane. Close to the half‐plane the tilt angle and ellipticity are of opposite sign for a perfect conductor, but the ellipticity will change sign for a poor conductor. The VLF anomaly for a perfectly conducting wedge is essentially determined by the position of the upper surface of the wedge, i.e. the anomaly will closely resemble the anomaly of a perfectly conducting half‐plane in the same position as the upper surface of the wedge.
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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)