Signal Enhancement and Impedance Inversion for Highly Cyclically Stratified Sedimentation



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In highly cyclic sedimentation with large impedance contrasts, such as coal beds, peg-leg multiples make seismic ties with primary-only synthetics difficult. If internal multiples are included in synthetic seismograms, excellent correlations exist between the synthetic seismogram and seismic data. However, neither the synthetic seismogram nor the seismic data tie the well-log lithologic boundaries at well locations because the incident wavefield that strikes a lithologic boundary and returns to the surface contains a signal wavelet followed by high-amplitude noise, which are interbed multiple reflections. Two ways are proposed to interpret the synthetic seismograms and seismic data: signal enhancement processing and impedance inversion. Firstly, a processing technique based on time-varying matched filters is presented for removing the effects of peg-leg multiples in near-offset stack data. This method was tested on a synthetic seismic line based on well-log curves from a highly cyclic coal-bed area and the result demonstrated the efficiency of the method: the synthetic seismic line with significant peg-leg multiples resembled the synthetic seismic line with primary-only reflections after processing, though resolution at the bottom of the coal-bed sequence was lowered due to scattering attenuation. A near-stack seismic line from Cooper Basin, Australia was processed to resolve the base of the Permian coal-bed sequence. Secondly, I demonstrated the limitation of conventional generalized linear inversion (GLI) for impedance estimation from seismograms with severe peg-leg multiples. High-frequency coda noise made conventional GLI unstable. Compared with the conventional GLI, which inverts for the whole trace at once, my modified GLI method iteratively stages the inversion for acoustic impedance on post-stack seismic data from a low-frequency band to a frequency band as wide as the original source wavelet. This progressive GLI assumes an effective medium for the entire coal-bed sequence when the wavelength is much longer than the average layer thickness. The inversion at the low-frequency band inverts for the boundaries of the stratified sedimentation while the inversion at the high-frequency band inverts for the details within the bed boundaries. However, because of stratigraphic filtering, the high-frequency impedance fluctuations cannot be inverted at the base of highly stratified sedimentation even with broad-band seismic acquisition.



Coal-bed multiple, Impedance inversion, Stratigraphic filtering