Investigation of and correction for waveform changes arising from NMO stretch
Lim, Un Young 1981-
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NMO-stretch distorts seismic frequency content significantly depending on depths of the target reservoir and offset locations. It causes poor resolution on CMP-stacked sections for image processing, and also creates difficulty in extracting the exact wavelet for AVO analysis and seismic inversion. In order to quantitatively study waveform changes arising from NMO-stretch, two synthetic convolution models are generated with the Zoeppritz equations. For both models, the amount of frequency shift, called ‘stretch-ratio’, are computed to determine waveform changes accurately. In the first model, which generates one reflectivity boundary concerning top of AVO Class 3 gas-saturated bed, general tendencies of the waveform changes are investigated. From comparison of results based on the spectral analyses between waveform changes before and after applying NMO-correction to the model, the progressive reduction of bandwidth and shift toward lower frequencies are detected along with increased offset. The second model is designed to investigate degree of NMO-stretch in the cases of typical AVO Class 3, 2 and 1. In this model, separations between the degrees of NMO-stretch for top of gas-saturated bed and top of brine-saturated bed concerning AVO Class 3, 2 and 1 noticeably increase along with increased offset. This indicates that gas and wet situations of typical AVO Classes could be differentiated by proper investigation of NMO-stretch in far offset ranges of pre-stack seismic data. A strategy using wavelet deconvolution is proposed as a method to correct waveform changes determined from above models. A multi-layered model is also used to test the effectiveness of the wavelet deconvolution-based NMO-stretch correction for realizing a more realistic situation. Additionally, in order to verify the effectiveness of the correction, time-frequency panels of the second model and the multi-layered model are generated with Short Time Fourier Transform, Continuous Wavelet Transform, and Constrained Least Squares Spectral Analysis. Applying the NMO-stretch correction corrects NMO-stretches of the synthetic models and recovers investigated waveforms before applying NMO-correction in the NMO-corrected gathers. An application to a synthetic seismogram based on real data from Hitts Lake Field also verifies the effectiveness of correction. The correction improves the alignments of reflections on targeted boundary and corrects lower frequency contents in far offset ranges.