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Abstract: An Application of 2D Acoustic Datuming: Seeing through Outcropping Volcanics

OMNES, GILDAS

When Previous HitgeophonesNext Hit are planted on outcropping high velocity heterogeneous formations, the recorded seismograms present discontinuous events associated with surface waves, repeated refractions, shallow multiples and diffractions. Shallow multiples are particularly strong where volcanics are located above the water table because very strong acoustic impedance contrasts exist between interbedded lava flows and layers of dry unconsolidated sand, gravel, volcanic clastics,.....These are not appropriate conditions for the application of conventional 2D Previous HitfilteringNext Hit methods. This paper describes an approach based on 2D acoustic datuming used as a data conditioning method applied before wave separation by 2D Previous HitfilteringNext Hit. The method is illustrated by vibroseis data acquired over a basalt flow in the Great Basin in Nevada.

A raw record was extrapolated to a horizontal surface corresponding to the top of a 5000ft/s medium, 3000 ft above the average ground level. Contrary to the raw record, the datumed seismogram shows events almost parallel and very continuous but no reflection can be seen. Subsequently a narrow f-k rejection filter and a predictive deconvolution were applied. Reflections with hyperbolic moveouts became clearly visible showing an angular unconformity possibly corresponding to the Paleozoic basement underlying the Tertiary sediments,a spectacular improvement over the results of f-k Previous HitfilteringTop applied directly to the raw seismogram.

AAPG Search and Discovery Article #90942©1997 AAPG International Conference and Exhibition, Vienna, Austria