摘要:

AbstractA 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

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1993.tb00886.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractBackus 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 ofqP‐, qSH‐, andqSV‐ 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 analytica

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1993.tb00887.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractRecent 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 t

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1993.tb00888.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractAn 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 formatio

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1993.tb00889.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractSeismic 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 tec

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1993.tb00890.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractIn 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 e

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1993.tb00891.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY