CALCUL D'UN FILM SYNTHETIQUE A PARTIR D'UN TRES GRAND NOMBRE DE COUCHES*

It is considered it might be useful to test the validity of the theoretical study “Sur ľinfluence ?un empilement de couches minces en sismique” by Messrs. I. ?Erceville and G. Kunetz, Geophysical Prospecting, XI, No. 2, June 1963.

We chose a borehole of the Compagnie Française des Pétroles where the velocity logs frequently show fine interbedding. Two synthetic records were prepared:

a) For the first one, a large step (τ= 3 ms) was selected to preserve the general appearance of the field records (convolution by a Ricker wavelet with a pseudo‐period of 26.6 ms);

b) For the second one, a small step (τ= 0.5 ms) was taken, convoluted using the same Ricker wavelet.

We observed that the synthetic records without multiples are not dependent on the step taken for calculation, provided this step remains small in relation to the period of the convolution signal (τ, one‐way time, smaller than 1/8 of the period of the signal). Nevertheless, when a small step is used, the high frequencies may be better reflected on the synthetic record.

We also noticed that the direct wave attenuates rapidly with depth and is replaced by an accumulation of multiples that restore it exactly, but with a slight time‐lag. We termed this the “pseudo‐wave”. The above‐mentioned delay between the two synthetic records is in good agreement with the results published by Messrs. 1. ?Erceville and G. Kunetz.