We introduce a processing technique which minimizes the ‘stretching effects’ of conventional NMO correction. Unlike conventional NMO, the technique implies constant normal moveout (CNMO) for a finite time interval of a seismic trace. The benefits of the proposed method include preservation of higher frequencies and reduction of spectral distortions at far offsets. The need for severe muting after the correction is reduced, allowing longer spreads for stack, velocity and AVO analysis. The proposed technique has been tested on model and real data. The method may improve the resolution of CMP stack and AVO attribute analysis. The only assumptions for this stretch‐free NMO correction are (i) all time samples of a digital reflected wavelet at a particular offset have the same normal moveout, and (ii) reflection records have an interference nature.

SHATILO, A.P., 1992. Seismic phase unwrapping: methods, results, problems. Geophysical Prospecting 40, 21 1-225.Six known methods of seismic phase unwrapping (or phase restoration) are compared. All the methods tested unwrap the phase satisfactorily if the initial function is a simple theoretical wavelet. None of the methods restore the phase of a synthetic trace exactly.An initial validity test of the phase-unwrapping method is that the sum of the restored wavelet phase spectrum and the restored pulse-trace phase spectrum (assuming the convolutional model for the seismic trace) must be equal to the restored phase spectrum of the synthetic trace. Results show that none of the tested methods satisfy this test. Quantitative estimation of the phase-unwrapping accuracy by correlation analysis of the phase deconvolution results separated these methods, according to their efficiency, into three groups. The first group consists of methods using a priori wavelet information. These methods make the wavelet phase estimation more effective than the minimum-phase approach, if the wavelet is non-minimum-phase. The second group consists of methods using the phase increment A4(Au) between two adjacent frequencies. These methods help to decrease the time shift of the initial synthetic trace relative to the model of the medium. At the same time they degrade the trace .correlation with the medium model. The third group consists of methods using an integration of the phase derivative. These methods do not lead to any improvement of the initial seismic trace.The main problem in the phase unwrapping of a seismic trace is the random character of the pulse trace. For this reason methods based on an analysis of the value of A+(Aw) only, or using an adaptive approach (i.e. as Am decreases) are not effective. In addition, methods based on integration of the phase derivative are unreliable, due to errors in numerical integration and differentiation.

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Ultrasonic P-wave attenuation and phase velocity dispersion have been estimated for 29 samples of sandstones and 13 samples of shales from a Glenn Pool oil reservoir using a pulse transmission technique. The measurements were performed under effective pressures from atmospheric to 15 MPa. There is a strong correlation between attenuation coefficient and phase velocity dispersion. Even though the observed attenuation may deviate from a “constant Q” model, it generally agrees with a minimum‐phase prediction. Attenuation in the water‐saturated sandstones increases with porosity and permeability. We found no correlation between the attenuation and clay content within the sandstone subset. Attenuation in the shales is much less than that in the sandstones. This difference may be used in reservoir characterization.

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