Using sonic log data to investigate a multiple problem in the Hassi Rʹmel field, Algeria

The conventional seismic reflection processing methods for detecting and suppressing multiples tend to fail in some circumstances. Vertical seismic profile (VSP) corridor stacks are now widely used to differentiate between primaries and multiples. However, they sometimes prove ineffective and the classical approach, using synthetics, becomes unoutlined. In the Hassi Rʹmel field, a strong event on the surface seismic sections, between clayey Dogger and lagoonal Dogger tops, is suspected to be a multiple because it is practically missing on the outside VSP corridor stacks. In this study, we show that the inside and outside VSP corridor stacks cannot help to identify this event. Using synthetics derived from well logs, acquired in six wells regularly distributed on the field, we conclude that the event is a primary. VSPs and sonic logs were recorded in the same well in every case. Synthetics, with multiples only, show an energetic long-path multiple with inverse polarity, at the same time as the event on the surface seismic sections. This explains: 1—the vanishing of the primary on the VSP corridor stacks, due to vertical recording in zero offset VSP acquisition, and 2—the efficiency of the standard common-midpoint (CMP) stacking to suppress the multiple on the surface seismic sections because of normal moveout differences between primaries and long-path multiples within the far-offset regions of data gathers.