In the context of Franco-Soviet cooperation, a seismic survey was carried out on the Romashkino reservoir in the Tatar Republic (USSR). This survey involved evaluating azimuthal anisotropy parameters from S-wave VSPs recorded on three wells and from a surface seismic line, the latter in order to monitor any lateral variations in the parameters.In this paper we present results which indicate in particular that for the two wells in which oil production has been established, the anisotropy rate appears to be proportional to production figures. This seems to confirm the hypothesis that variations in production are asociated with the presence of subvertical fractures which cause S-wave birefringence. By processing the reflected converted P-S waves of a surface seismic line, it is possible to monitor lateral variations in the anisotropy parameters and well ties effectively confirm differences revealed by the S-wave VSP data.
Several papers presented at the last SEG Convention in Houston by Schneider, Backus et al have shown how important and fruitful it was to obtain a continuous knowledge of the velocity functions and they have solved their problem by a Dynamic Correlation Analysis. Our purpose is to introduce here a method based on the best summation of a set of traces instead of the best correlation. ROBERT GAROTTA AND DOMINIQUE MICHON
Des travaux sismiques en ondes longitudinales et en ondes transversales ont été effectués en Tatarie (U.R.S.S.) avec un objectif d'étude lithologique. Les ondes transversales utilisées étant du type SH, elles étaient engendrées au moyen de tirs spéciaux polarisés et associés par couples. Une opération de traitement spéciale permettait l'élimination des ondes P et l'amplification des SH à partir de chacun des couples de ces tirs polarisés. La coupe en ondes P a été obtenue par des moyens classiques.La coupe sismique obtenue en ondes transversales donne en général une plus grande finesse dans la description des couches géologiques du fait de la plus faible longueur d'onde des SH. L'utilisation, en combinaison, des données de la coupe P et de la coupe SH permet l'obtention d'un modèle assez précis et détaillé de la géologie locale. Les variations de faciès et l'anisotropie de vitesse peuvent être détectées grâce à une étude simultanée des vitesses de tranche et des temps de parcours des deux types d'ondes.ABSTRACT Seismic studies have been conducted in Tataria, USSR simultaneously with longitudinal and transverse waves in order to obtain lithological information. The transverse waves were of the SH type. They were generated by means of specially designed pairs of polarized shots. A al.processing step applied to each pair of shots made it possible to eliminate P waves and emphasize the SH content of the records. The P section was obtained by quite conventional means.As a rule the SH sectiotrexhibits greater detail in the description of the geological layers because of the smaller S wave length. By a combination of the P and SH sections an accurate and detailed mode1 of the local geological section was obtained. Facies variations along layers and velocity anisotropy were detected by means of a simultaneous study of layer velocities and transit times of both wave types.
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.
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