The processed multichannel reflection seismic profile Lusigal 12 discussed here was acquired during the Lusigal campaign in 1990 under the leadership of the GEMCO working group in Villefranche (chief scientist G. Boillot).
The source used for the LG series was an array of eight waterguns (shot interval 50 m). A 2.4 km, 96-channel streamer (group interval 25 m) was used, resulting in a common midpoint (CMP) interval of 12.5 m. Profile LG12 was recorded from west to east normal to the Iberia passive margin and to the main structures, and so has been successfully time- and depth-migrated.
In processing the data, we took particular care to keep steeply dipping reflections. For instance, in the time migrated section (Fig. 3), we have applied neither trace mixing nor trace summation, and used fk-filter (frequency-wavenumber) only locally. We have also applied dip-moveout correction prior to final velocity analysis to improve the image of steep events on the stack section. As will be seen below, imaging of steep events greatly facilitates interpretation of the data.
Another aim of our processing was the depth-migration of the data, to better reveal the true geometry and geometrical relationships of key structures. As stacking velocity is not suitable for depth-migrating seismic data, we constructed a detailed velocity function by the technique of depth-focusing error analysis (Denelle et al., 1986). This analysis is a by-product of performing depth migration before stack and results from the importance of velocity both in converting a time section to depth, and in the migration of diffraction and reflection hyperbolae at different offsets to produce a final section (Fig. 4). As this technique is very expensive in terms of computer time, we have applied it only to the most important parts of the seismic profile covering the drill locations from Leg 149 at the outer and central basement highs of this section (Fig. 2). We then used the resultant velocity information to construct a velocity model across the whole transect for poststack depth-migration (Fig. 5).
Prestack depth migration has two main advantages compared to standard migration: as a migration before stack, it avoids the smearing effects of CMP-stacking (Sherwood, 1989), important in regions of complex geology (Peddy et al., 1986), and additionally as a depth migration it counts for effects like raypath bending and velocity pull-up/push-down (e.g., Yilmaz, 1987), in particular in areas with rapid lateral velocity changes as given by the complex geological structures on passive rifted margins. Thus, applying the depth migration before stack one can provide a much more constrained and accurate image of the structures with their true geometry in depth.
Using the Migpack® software (Denelle et al., 1986) we created the velocity model iteratively down from the ocean bottom to deeper levels through depth-focusing error analysis (Fig. 4), which compares apparent and focusing depth of reflected and diffracted energy with a spacing of 10 shotpoints (analysis interval of 500 m) along the profile. The model is created from top to bottom of the section, determining velocity in one layer at a time because the velocity structure of the upper layer affects the accuracy of the velocity determination from the deeper levels.
Therefore, we first determine the correct water velocity (and in the process check on the geometry of the data), then determine the velocity in the uppermost sedimentary layer in the next iteration. These steps are repeated until the whole section and all specific layers have been analyzed through depth-focusing error analyses. As this method needs clear and strong reflections for reliable focusing of reflected and diffracted energy, it is difficult to determine different velocity layers within the basement where reflections are limited. Basement is migrated with a half-space velocity revealed by the bright reflections present within the basement (5.5-6 km/s). In contrast, the sedimentary layers above provide a clear image as long as lithological boundaries or high reflectivity interfaces are picked.
This procedure yields a depth migrated section, which images the geological structures with their true geometry, and results in a detailed velocity model. This model not only allows the correct migration of the data, but strongly constrains the lithologies present.