Cancelling of multiple reflections by CDP stacking is generally incomplete. When the order of coverage is low, an improvement may be obtained by the use of special filters (e.g. Schneider et al: Geophysics 1965; D'Hoeraene: Geophysics 1966). But when the order of coverage is high, the efficiency of these filters is less obvious. On the other hand, the higher the order of coverage, the higher the efficiency in the solution presented. Schematically, our method includes three steps: Firstly: Searching for the multiple reflections. For this purpose Move Out corrections corresponding to the multiples are applied to each individual record, then CDP stacking is carried out. Secondly: Cleaning up of the individual records. “Decorrections” are firstly applied, then multiples are subtracted. For this, subtraction is weighed versus the energy of the multiples, that is to say versus the correlation between the original record and the pure multiple reflections. Thirdly: Normal processing with the cleaned records which have been obtained. The different steps of this method are illustrated with the help of theoretical examples. Its efficiency improvement in regard to normal CDP stacking is then demonstrated by means of true examples.
Deconvolution and deringing are well known subjects and it is not necessary to state again their objectives nor the basical methods used to reach them. Let us just remember that, generally, among many others, the two following assumptions are made for simplification purposes: —for deconvolution, it is assumed that the recorded seismic signal is constant, meaning that its shape is the same all along the time interval during which the trace is to be deconvolved; —for de‐ringing, it is assumed that the ringing period is constant and that the intensity of the ringing phenomenon is independant of the time. With these two assumptions, a single constant operator can be applied for deconvolving, deringing or both. In most cases, the time variations of the signal or of the ringing are small enough and the error resulting of the application of a constant operator is acceptable. It results into a slight increase of the noise level or into a small residual ringing in the processed trace. When this noise or the residual ringing are too important, the assumption of a constant signal and ringing period must be rejected. This is the case that is examined here according to the following steps: —short definition of the problem; —fast evaluation of some possible solutions; —the selected solution: resulting approximations and how to obviate them, computing method and a remark about the operators; —theoretical example: the efficiency of the process used is evaluated on data in which the results aimed at are known; the influence of the selection of numerical values to be assigned to the parameters is examined; —real cases: comparison of results obtained with the Protee process and with more conventional processes assuming a time invariance or including a weighted composition of several conventional processes each with a different operator.
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