On Imaging of Converted Reflections
|Keywords||Converted wave Offset Tongji University Horizontal stacking Method Migration imaging Transfer point Domain conversion Doctoral Dissertation Stack section|
Subsurface imaging is the primary aim of seismic exploration; the theory andtechniques of seismic imaging play a major role in exploration seismology.Multicomponent seismology, as a new developing high technique in recent years,demands some new specialized approaches to obtain high-quality converted-wavesections. Converted-wave data processing is quite different from that of conventionalPP reflections, it has already been one of the latest branches in exploration geophysics.The objective of this dissertation is to develop the theory and practical techniques forconverted reflection imaging.Three alternative imaging algorithms for different geological requirements aredeveloped in this thesis. Firstly, a computationally efficient common-conversion-point(CCP) stacking technique is presented to image nearly horizontal subsurfacestructures; secondly, a kind of converted-wave dip moveout (DMO) correction isproposed to improve stack images of dip reflectors with velocity variations in depth;finally, for more complex velocity models, converted-wave prestack depth migration(PrSDM) has to be considered as the need to obtain correct images.(1) Accurate and efficient CCP stacking techniqueThere are two key points to CCP stacking: CCP gather sorting andnon-hyperbolic time-distance relationship. Some accurate and concise formulas arederived to compute CCPs and their travel-time, based on which an accurate andefficient way to implement CCP stacking is proposed. This implementation consistsof CCP sorting, non-hyperbolic moveout and stack velocity analysis in turn. CCPsorting gets over the periodical variation of fold introduced by conventional CCPbinning method successfully, furthermore, it can also be applied to deal with bothdepth and lateral velocity variations; the derived normal moveout (NMO) formulascan well align reflections event from shallow-middle reflectors at far offsets; reliableshear-wave stack velocity can be explicitly determined by velocity analysis methodintroduced in this thesis. Synthetic and real examples show that application of thesetechniques help. to enhance stacked reflections as well as to preserve amplitude.(2) True amplitude converted-wave DMO correctionFor imaging the reflectors with different slopes in smoothly lateral-variablevelocities, poststack migration preceded by converted-wave DMO turns out to be aneffective processing sequence in practice. Here we present a fast converted-waveDMO algorithm in f-k domain to improve stacked images of dip reflectors. With thisalgorithm, CCP correction for all dips is taken into consideration automatically, which is also computationally efficient and can be applied to depth-variable velocity cases.Since kinematic DMO fails to handle phase and amplitude properly,converted-wave DMO in f-k domain has been investigated. Unfortunately, allpresented f-k domain converted-wave DMO algorithms are based on second-orderapproximate kinematic equations, which bring serious errors for steep dip interfaces.Thus the amplitude characteristics of steep dip reflectors are relatively attenuated asshown in this thesis. Here we propose a true-amplitude converted-wave DMO in f-kdomain based on new derived kinematic equation. Synthetic and real imagingexamples are shown in this dissertation to compare the amplitude preserving ability ofthis method with that of other existing methods.(3) Converted-wave wave-equation prestack depth migrationBoth accuracy and efficiency of wave-equation (WE) prestack depth migration(PrSDM) are dominated by WE propagator and migration algorithm. To handle thetoughest problem in depth imaging-—strong lateral velocity contrast, we transformwave equation of heterogeneous media into that of homogeneous media byintroducing a velocity-adaptable coordinate-transformation (VACT). Following thisidea, a new one-way wave-equation propagator, called VACT propagator, is presented.Unlike existing one-way propagators, VACT propagator can be applied in complexmedia without any dip-angle limitation. Test results of impulse responses andsynthetic data of SEG/EAGE salt model are also shown in this thesis.One-way scalar wave propagators can be used to migrate converted reflections inshot domain for considerably computational cost. Converted-wave WE-PrSDMalgorithm in offset-domain is investigated for the sake of efficiency. During theimplementation of the algorithm, a global reference velocity is introduced, whichdemands a better capability to deal with lateral velocity variations. The VACTpropagator presented here is competent for this job.For both shot-domain and offset-domain converted-wave WE-PrSDM, theirimaging conditions for producing common-imaging-point (CIP) gathers are discussedin detail. CIP gathers provide a powerful platform for migration velocity updating, P-and S-wave combined velocity inversion, and amplitude-versus-offset (AVO) analysisin complex media. Synthetic data tests show that the converted-wave WE-PrSDMtechnique proposed here is correct and practicable.Some of the techniques developed in this dissertation have been put into practicefor real multicomponent seismic data processing successfully.