摘要:

AbstractMultichannel filters are used to eliminate coherent noise from surface seismic data, for wavefield separation from VSP stacks, and for signal enhancement. Their success generally depends on the choice of the filter parameters and the domain of application. Multichannel filters can be applied to shots (monitors), common‐receiver traces, CDP traces and stacked sections. Cascaded applications in these domains are currently performed in the seismic industry for better noise suppression and for signal enhancement. One‐step shot‐domain filtering is adequate for some applications. However, in practice, cascaded applications in shot‐and common‐receiver domains usually give better results when the S/N ratio is low. Multichannel filtering after stacking (especially after repeated applications in shot and/or receiver domains) may create undesirable results such as artificial continuations, or smearing and smoothing of small features such as small throw faults and fine stratigraphic details. Consequently, multichannel filtering after stacking must be undertaken with the utmost care and occasionally only as a last resort.Multichannel filters with fan‐shaped responses (linear moveout filters) should be applied after NMO correction. These are the filters commonly used in the seismic industry where they have such names as velocity filters, moveout filters,f‐kfilters and coherency filters. Filtering before NMO correction may result in break‐up and flattening especially of those shallow reflection events with relatively higher curvatures and diffractions. NMO correction is needed prior to wavefield separation from VSP stacks for the same practical reasons outlined above whenever source‐receiver offsets are involved.Creation of artificial lineup and smearing at the outputs of multichannel filters is presently the common practical concern. Optimum multichannel filters with well‐defined pass, reject and transition bands overcome the latter problems when applied before stacking and after NMO correction. The trace dimension of these filters must be kept small to avoid such lineups and the smoothing of small structures. Good results can be obtained with only five traces, but seven traces seems to be a better compromise both in surface and well seismic applications. The so‐calledf‐kfiltering andτ‐pdomain filtering are no exceptions to the above practical considerations.Residual static computations after multichannel filtering also need special consideration. Since multichannel filtering improves spatial continuity, residual static algorithms using local correlation, i.e. nonsurface‐consistent algorithms, may be impractical especially

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1990.tb01832.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

AbstractThe elastic Kirchhoff‐Helmholtz integral expresses the components of the monochromatic displacement vector at any pointAin terms of the displacement field and the stress field at any closed surface surroundingA.By introducing Green's functions for P‐ and S‐waves, the elastic Kirchhoff‐Helmholtz integral is modified such that it expresses either the P‐wave or the S‐wave atAin terms of the elastic wavefield at the closed surface. This modified elastic Kirchhoff‐Helmholtz integral is transformed into one‐way elastic Rayleigh‐type integrals for forward extrapolation of downgoing and upgoing P‐ and S‐waves. We also derive one‐way elastic Rayleigh‐type integrals for inverse extrapolation of downgoing and upgoing P‐ and S‐waves. The one‐way elastic extrapolation operators derived in this paper are the basis for a new prestack mi

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1990.tb01833.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

AbstractSpectral analysis is one of the most ubiquitous signal processing tools used in exploration geophysics. Among many applications, it is used simply to look at the frequency content of seismic traces, to find notches, to estimate wavelets under the minimum‐phase assumption, and to match broadband synthetic seismograms to seismic data.Seismic spectra exhibit very large dynamic ranges, particularly at low frequencies. Estimation of low‐frequency decay is very important for accurate modelling. However, when using traditional spectral estimates incorporating smoothing windows, too much sidelobe energy leaks from high power into low power areas, spoiling our ability to estimate low‐frequency spectral decay. The multitaper method of spectral analysis due to D. Thomson does not employ just a single window, but rather a set of orthogonal data tapers. It is possible to have much less sidelobe contamination, while maintaining a stable estimate.The trace is tapered by each of a subset of the orthogonal tapers, and a raw spectral estimate produced in each case. These are combined to produce a final spectral estimate. The technique can be madeadaptiveby applying different weights to the different raw spectra at different frequencies.A comparison of seismic spectral estimation using this multitaper technique with a traditional approach having the same analysis bandwidth and stability demonstrates the very different estimates of spectral decay in the areas of high dynamic range. The multitaper approach provides estimates with much reduced sidelobe leakage, and hence is a very appealing method for reflection seism

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1990.tb01834.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

AbstractCalculations of emf induced in a vertical axis coil by a current step in a straight wire segment of finite length were carried out using an equivalent dipole placed along the source wire. An approximation is valid for homogeneous and layered earth models. The location of the equivalent dipole is calculated by forcing the early‐ and late‐time asymptotes of the transient sounding curve of the equivalent dipole to match those of the finite wire. This approximation works because the early‐time asymptote of the emf depends on the component of the receiver position which is parallel to the wire while the late‐time asymptote does not. Analytical integration of the early‐time asymptote yields an expression for the equivalent dipole location in closed form.The coil can be placed anywhere in the vicinity of the finite wire. Square or rectangular loop sources can be simulated by one or more finite wire segments depending on the source‐receiver geometry. The equivalent radius calculated for central loop soundings agrees well with the value derived using a circular loop with the same area as the square loop.Results show that acceptable sounding curves can be generated by the equivalent dipole for coils placed as close as 0.5 source lengths from the finite wire segment. Higher accuracy can be obtained by splitting the finite wire into two or more subsegments. Results for layered models are slightly better than homogeneous earth models when the resistivity increases with depth and slightly worse for models with resistivity decreasing with depth. Approximate calculations are about 10 times less expensive than exact calculations depending on the method used for the numerical

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1990.tb01835.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1990.tb01836.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1990.tb01837.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1990.tb01838.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY