- Home
- A-Z Publications
- Geophysical Prospecting
- Previous Issues
- Volume 41, Issue 7, 1993
Geophysical Prospecting - Volume 41, Issue 7, 1993
Volume 41, Issue 7, 1993
-
-
DIFFRACTION BY A WEDGE IN AN ACOUSTIC CONSTANT DENSITY MEDIUM1
Authors PER BERG, FLEMMING IF, PER NIELSEN and OVE SKOVGAARDAbstractA new method is presented for solving the 2D problem of diffraction of a plane wave by a wedge of arbitrary angle in a purely acoustic, constant‐density medium with different constant compressional wave speeds inside and outside the wedge. The diffraction problem is formulated as integral equations, and a wavenumber–frequency representation of the scattered field is obtained. With the aid of the Cagniard–de Hoop method, exact analytical expressions in the space–time domain are obtained for the different wave constituents, i.e. geometric optical scattered waves and edge diffracted waves including head waves. These expressions can be computed to any degree of accuracy within reasonable computation times on a computer, and the semi‐analytical method of solution presented thus constitutes a means of constructing reference solutions for wedge configurations. Such highly accurate reference solutions are of importance for verification of results that include diffraction phenomena modelled by general numerical approximate methods, e.g. finite differences, finite elements and spectral methods. Examples of such applications of the method of solution are given.
-
-
-
APPROXIMATIONS TO SHEAR‐WAVE VELOCITY AND MOVEOUT EQUATIONS IN ANISOTROPIC MEDIA1
Authors XIANG‐YANG LI and STUART CRAMPINAbstractBackus and Crampin derived analytical equations for estimating approximate phase‐velocity variations in symmetry planes in weakly anisotropic media, where the coefficients of the equations are linear combinations of the elastic constants. We examine the application of similar equations to group‐velocity variations in off‐symmetry planes, where the coefficients of the equations are derived numerically. We estimate the accuracy of these equations over a range of anisotropic materials with transverse isotropy with both vertical and horizontal symmetry axes, and with combinations of transverse isotropy yielding orthorhombic symmetry. These modified equations are good approximations for up to 17% shear‐wave anisotropy for propagations in symmetry planes for all waves in all symmetry systems examined, but are valid only for lower shear‐wave anisotropy (up to 11%) in off‐symmetry planes.
We also obtain analytical moveout equations for the reflection of qP‐, qSH‐, and qSV‐ waves from a single interface for off‐symmetry planes in anisotropic symmetry. The moveout equation consists of two terms: a hyperbolic moveout and a residual moveout, where the residual moveout is proportional to the degree of anisotropy and the spread length of the acquisition geometry. Numerical moveout curves are computed for a range of anisotropic materials to verify the analytical moveout equations.
-
-
-
VARIATION OF REFLECTION AND TRANSMISSION COEFFICIENTS WITH CRACK STRIKE AND CRACK DENSITY IN ANISOTROPIC MEDIA1
Authors XIANG‐YANG LI and STUART CRAMPINAbstractRecent observations show that the differential amplitudes between the faster and slower split shear‐waves in reflection surveys contain information about lateral variations of crack density in cracked reservoirs. However, the variation of amplitude with crack geometry when the crack strike changes with depth has not been reported previously. In this paper, we derive expressions for reflection and transmission coefficients of plane split shear‐waves at vertical incidence at an interface separating two cracked (anisotropic) media with different crack strikes. We examine the effects on these coefficients as crack strike and crack density vary. For interfaces with large velocity‐contrasts, the reflection coefficients carry little information about crack geometry, and the effects of crack strike varying with depth are negligible. In such cases, the polarization and time‐delay of the split shear‐waves are the only features which reliably diagnose anisotropy and contain information about the variation of crack strike and density. However, for interfaces with small velocity‐contrasts, the effects of any variation of crack strike with depth cannot be neglected. In such cases, in addition to the polarization and time‐delays of split shear‐waves, both the differential amplitude of faster and slower shear‐waves and the amplitude ratio of the two off‐diagonal elements in the reflected data matrix after separation of split shear‐waves, contain information about the variation of crack strike and crack density. In contrast, effects of crack strike changing with depth on transmitted waves are more sensitive regardless of the velocity‐contrast and the degree of anisotropy.
-
-
-
BOREHOLE EFFECTS ON DOWNHOLE SEISMIC MEASUREMENTS1
Authors CHENGBIN PENG, C. H. CHENG and M. N. TOKSÖZAbstractAn exact formulation for borehole coupling, which is valid for all frequencies and all azimuthally symmetric and non‐symmetric components, is presented. The borehole effects on downhole seismic measurements are studied in detail as functions of frequency, angle of incidence and polarization of an incident wave as well as geophone orientation. We found that correction for the borehole effect on downhole measurements should be made for frequencies above 500 Hz in a hard formation. In a soft formation, if the angle of incidence is well away from the resonance angle for SV incidence, no borehole correction is needed for frequencies below 300 Hz, while for frequencies above 300 Hz, the borehole can cause severe problems in downhole measurements. The borehole can also significantly alter the particle motion direction which implies that horizontal component rotation from data itself is unreliable for experiments with frequencies above 1 kHz in the hard formation and around 500 Hz in the soft formation.
-
-
-
SEISMIC STRATIGRAPHY IN HIGH RESOLUTION SHALLOW MARINE SEISMIC DATA OF THE GEMLIK GULF1
More LessAbstractSeismic stratigraphy and sedimentological studies of the Gemlik Gulf in the Sea of Marmara, Turkey, have been carried out. For this purpose, 19 lines totalling 189 km of excellent quality, high‐resolution seismic data were recorded.
Four major acoustic units were identified in the seismic profiles. Three were sedimentary units: irregular layered, cross‐layered and well‐layered; and the fourth was an acoustic basement which is probably composed of crystalline volcanic rocks.
Some local areas in the Neogene formation contain gas accumulations.
The formation of faults in E–W and N–S directions can be explained by the existence of shear stresses in the Gulf. The bathymetric map shows good accommodation with the shore line as does the tectonic map.
-
-
-
IMAGING CAPABILITY OF CROSS‐HOLE SEISMIC REFLECTION SURVEYS1
Authors P. S. ROWBOTHAM and N. R. GOULTYAbstractIn seismic migration, it is important to sample a range of dips around the local structural dip at each image point. Meaningful images are obtained only where this condition holds. For cross‐hole seismic reflection surveys, the distribution of dips sampled at each image point is controlled principally by the survey geometry, including source and receiver array lengths and their element spacings. Using a real data set as an example, we show how survey geometry can limit imaging capability close to the boreholes and even in the middle of the section between the boreholes.
At the processing stage, effective removal of direct waves and accurate estimation of the velocity field are essential for optimizing image quality. For migration, we propose a generalized Berryhill (GB) scheme which is based on the Kirchhoff integral and takes into account both the near‐field and far‐field terms. This should improve the ability to image close to source and receiver arrays, provided that the element spacing in the nearby array is small enough.
-
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 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 18 (1970)
-
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)