- Home
- A-Z Publications
- Geophysical Prospecting
- Previous Issues
- Volume 38, Issue 7, 1990
Geophysical Prospecting - Volume 38, Issue 7, 1990
Volume 38, Issue 7, 1990
-
-
ENHANCED MIGRATION OF SEISMIC DATA1
More LessAbstractThe so‐called ‘enhanced migration’ which uses diffraction tomography as the ‘repair tool’ for correction of amplitudes (reflection coefficients) of migrated sections is discussed. As with any linearized procedure, diffraction tomography requires knowledge of the initial model. It is suggested that the initial model is taken as the migrated image. It will be demonstrated that diffraction tomography applied to the data residuals improves the amplitudes of the migrated images. Migration is redefined as the reconstruction of the wavefront sets of distributions (reflection interfaces), and the inversion process as tomographic correction of migrated images.
-
-
-
NUMERICAL MODELLING OF STANDARD AND CONTINUOUS VERTICAL ELECTRICAL SOUNDINGS1
Authors C. E. MOLANO, M. SALAMANCA and R. A. VAN OVERMEERENAbstractAnalytical solutions of vertical electrical soundings (VES) have mostly been applied to groundwater exploration and monitoring groundwater quality on terrains of fairly simple geology and geomorphology on which the electrode arrays are symmetrical (e.g. Schlumberger or Wenner configurations). The sounding interpretation assumes flat topography and horizontally stratified layers. Any deviations from these simple situations may be impossible to interpret analytically. The recently developed GEA‐58 geoelectrical instrument can make continuous soundings along a profile with any colinear electrode configuration.
This paper describes the use of finite‐difference and finite‐element methods to model complex earth resistivity distributions in 2D, in order to calculate apparent resistivity responses to any colinear current electrode distribution in terrains in which the earth resistivities do not vary along the strike. The numerical model results for simple situations are compared with the analytical solutions. In addition, a pseudo‐depth section of apparent resistivities measured in the field with the GEA‐58 is compared with the numerical solution of a real complex resistivity distribution along a cross‐section. The model results show excellent agreement with the corresponding analytical and experimental data.
-
-
-
SEISMIC INTERPRETATION OF UPPER ELK POINT (GIVETIAN) CARBONATE RESERVOIRS OF WESTERN CANADA1
Authors R. J. BROWN, N. L. ANDERSON and L. V. HILLSAbstractThe seismic signatures of three reefs of the Upper Elk Point Subgroup (Givetian Stage) of the Western Canada Sedimentary Basin are documented and analysed on the basis of variations in seismic image of particular lithologic units, lateral amplitude and/or phase changes, structural relief and velocity‐generated relief, as rendered by the reflection data. The effects on seismic signatures of spatial geological variations resulting from such phenomena as differential compaction, reef‐focused salt dissolution, palaeotopography, lateral and vertical facies variations, regional dip, and reservoir morphology are discussed. The usefulness of seismic data in clarifying relationships between reefs and their adjacent sedimentary sections, particularly in cases where well control is sparse, is also considered. Such documentation of seismic signatures from known reefs using geophysical and geological analysis can establish criteria to enable recognition of similar buildups elsewhere.
Three example reefs are presented, each typical of a particular area and environment of W. Canada. The first is from the Winnipegosis Formation of SE Saskatchewan, the second and third from the Rainbow Member and Upper Keg River Reef Member, respectively, of the Keg River Formation of NW Alberta. All three of these carbonate buildups developed in the evaporitic Elk Point Basin. However, the degree of salt encasement and subsequent dissolution varied greatly, as do the resulting seismic effects. For these three reef types, the typical elements of their seismic signatures have been compiled and are here summarized.
-
-
-
3D ACOUSTIC PRESTACK REVERSE‐TIME MIGRATION1
Authors WEN‐FONG CHANG and GEORGE A. McMECHANAbstractA prestack reverse‐time migration algorithm which operates on common‐source gathers, recorded at the Earth's surface, from 3D structures, is conceived, implemented and tested. Reverse‐time extrapolation of the recorded wavefield (a boundary‐value problem), and computation of the excitation‐time imaging condition for each point in a 3D volume (an initial‐value problem), are both performed using a second‐order finite‐difference solution of the full 3D scalar wave equation. The algorithm is illustrated by processing synthetic data for a point diffractor, an oblique wedge, and the French double dome and fault model.
-
-
-
A COMPARISON BETWEEN KIRCHHOFF AND GRT MIGRATION ON VSP DATA1
By P. B. DILLONAbstractA modern approach to migration is to perform wavefield extrapolation, subject to an imaging condition. Correct wavefield extrapolation requires that the boundary conditions at the array of geophones satisfy the wave equation. A sufficient condition is to perform the survey with a single stationary source. Contrary to this condition, many VSPs are conducted in deviated wells, where the source is maintained vertically above the down‐hole geophone at each well station. Such a survey fails to provide the boundary conditions theoretically necessary for wave‐equation migration.
A recently published inversion scheme, referred to as acoustic generalized Radon transform migration (GRT migration), was developed to handle any configuration of sources and geophones, including moving‐source deviated‐well VSP surveys. GRT migration may be viewed as a weighted version of the generalized Kirchhoff migration, derived in this paper from the exploding‐reflector model.
When a VSP‐survey geometry has been specified, GRT migration can be expressed in terms of array parameters, and compared with the equivalent expression for Kirchhoff (wave‐equation) migration. The differences between the two integrals are significant and their effect is demonstrated on VSP data.
-
-
-
AN ADVANCED CMP RAY‐TRACING1
By ZVI KORENAbstractThe ray‐tracing algorithm presented in this paper is based on formulae derived for the common reflecting element (CRE) stacking method. A 2D, smooth, laterally‐varying media is assumed where offset rays and traveltimes are evaluated from normal‐incidence (central) rays. The method uses a second‐order asymmetrical approximation for rays and an additional oblique spherical approximation of the central wavefronts for calculating offset traveltimes. In order to solve the two‐point ray‐tracing problem for the common midpoint (CMP) configuration of source‐receiver pairs located symmetrically around the CMP stations, the central rays are perturbed to satisfy the above‐mentioned asymmetrical distribution. Although the accuracy of the calculations is limited for far offsets, it is still good for distances of the order of the reflecting depths. Since only a few normal‐incidence rays are traced through the medium, the method is very fast and is found to be most attractive for iterative inversions in macromodel estimation.
-
-
-
CONSTRUCTION OF COMPONENT MAPS FROM AEROMAGNETIC TOTAL FIELD ANOMALY MAPS1
Authors L. B. PEDERSEN, T. M. RASMUSSEN and D. DYRELIUSAbstractTotal field anomalies as defined from normal aeromagnetic surveys give good approximations of the anomalous components along the direction of the main geomagnetic field, which is generally much larger than the anomalous field. Using the relations between vertical and horizontal components of the field, the total field anomaly is related to any vertical or horizontal component and the corresponding horizontal and vertical derivatives. An example from the Siljan impact structure indicates that such directional filters may be applied to extract useful information from magnetic maps.
-
-
-
MEASURED UNDERWATER NEAR‐FIELD E‐PATTERNS OF A PULSED, HORIZONTAL DIPOLE ANTENNA IN AIR: COMPARISON WITH THE THEORY OF THE CONTINUOUS WAVE, INFINITESIMAL ELECTRIC DIPOLE1
Authors W. A. WENSINK, G. GREEUW, J. HOFMAN and J. K. VAN DEENAbstractAt Delft Geotechnics the technique of ground‐penetrating radar is in use for the detection of buried objects such as pipes. To enable us to give our ‘measurements in the field’ a more quantitative interpretation than can be deduced from these alone, a series of experiments has been started under well‐defined conditions. A cylindrical vessel containing water simulates wet soil. Mounted horizontally above the water surface is a pulsed triangular half‐wave dipole which is used as a transmitting antenna (TA). It has a carrier‐frequency of about 160 MHz and a pulse repetition‐frequency of about 50 kHz.
A movable receiving dipole (‘probe’) in the water measures the transverse, mutually orthogonal Eφ,‐ and Eθ‐components of the pulses as a function of probe‐position (r, θ, φ) and of the height h of the TA above the water surface. When these pulses are Fourier‐transformed, the transverse electric fields Eφ and Eθ at 200 MHz are obtained. The resulting field patterns are compared with computational results on the basis of the theory of the continuous wave, infinitesimal electric dipole (‘point dipole’). It can be concluded that:
- 1 Far‐field conditions have not fully developed at a depth of about 2.50 m, the largest value of the radius r at which field patterns were measured, although it represents a distance of about 15 wavelengths.
- 2 The attenuation constant of the tapwater used, as deduced from E‐field measurements for θ= 0, 2.50 m < r < 2.75 m, is slightly less than the value measured using a network analyser and air line combination, in agreement with (1).
- 3 Eφ field patterns calculated using the value of the conductivity σ corresponding to the former value of the attenuation constant agree reasonably well with the measured patterns for r≤ 2.50 m and for θ < 20° at all antenna heights considered. Calculated Eφ patterns do not agree so well with the measured patterns when h is close to zero. With increasing height the agreement inproves.
- 4 In accordance with the theory of the point‐dipole, the angular distribution of the radiation patterns of the TA becomes wider as the frequency decreases.
- 5 The normalized underwater pulse‐spectra shift to lower frequencies with increasing r. This can be explained since the attenuation constant of the water rises with rising frequency.
-
Volumes & issues
-
Volume 72 (2023 - 2024)
-
Volume 71 (2022 - 2023)
-
Volume 70 (2021 - 2022)
-
Volume 69 (2021)
-
Volume 68 (2020)
-
Volume 67 (2019)
-
Volume 66 (2018)
-
Volume 65 (2017)
-
Volume 64 (2015 - 2016)
-
Volume 63 (2015)
-
Volume 62 (2014)
-
Volume 61 (2013)
-
Volume 60 (2012)
-
Volume 59 (2011)
-
Volume 58 (2010)
-
Volume 57 (2009)
-
Volume 56 (2008)
-
Volume 55 (2007)
-
Volume 54 (2006)
-
Volume 18 (1970 - 2006)
-
Volume 53 (2005)
-
Volume 52 (2004)
-
Volume 51 (2003)
-
Volume 50 (2002)
-
Volume 49 (2001)
-
Volume 48 (2000)
-
Volume 47 (1999)
-
Volume 46 (1998)
-
Volume 45 (1997)
-
Volume 44 (1996)
-
Volume 43 (1995)
-
Volume 42 (1994)
-
Volume 41 (1993)
-
Volume 40 (1992)
-
Volume 39 (1991)
-
Volume 38 (1990)
-
Volume 37 (1989)
-
Volume 36 (1988)
-
Volume 35 (1987)
-
Volume 34 (1986)
-
Volume 33 (1985)
-
Volume 32 (1984)
-
Volume 31 (1983)
-
Volume 30 (1982)
-
Volume 29 (1981)
-
Volume 28 (1980)
-
Volume 27 (1979)
-
Volume 26 (1978)
-
Volume 25 (1977)
-
Volume 24 (1976)
-
Volume 23 (1975)
-
Volume 22 (1974)
-
Volume 21 (1973)
-
Volume 20 (1972)
-
Volume 19 (1971)
-
Volume 17 (1969)
-
Volume 16 (1968)
-
Volume 15 (1967)
-
Volume 14 (1966)
-
Volume 13 (1965)
-
Volume 12 (1964)
-
Volume 11 (1963)
-
Volume 10 (1962)
-
Volume 9 (1961)
-
Volume 8 (1960)
-
Volume 7 (1959)
-
Volume 6 (1958)
-
Volume 5 (1957)
-
Volume 4 (1956)
-
Volume 3 (1955)
-
Volume 2 (1954)
-
Volume 1 (1953)