The seismic survey performed during this campaign is shown in fig. 4. Orientation of the lines was chosen in order to satisfy firstly, parallelism with respect to present day compressive field in this region, secondly, perpendicularity with respect to the orientation of the major deformation structures located in the area. Some lines were extended outside the target area to achieve a more general picture of the regional deformation pattem. Lines BS07 - BS16 are approximately parallel to the trajectories of the compressive stress in this area. Line BS17 is a strike line across the Gorringe Bank; it was shot in order to achieve a better understanding of the three-dimensional structure of this outcrop of mantle and eventually to reveal part of its rift stage history. Lines BS18 and BS19 were shot approximately perpendicularly to the eastern boundary of the Tagus Abyssal Plain and to an important geo-morphological scarp of the continental margin. This scarp might be a sort of northward continuation of the active thrust structure located west of Cabo de Sao Vicente, possibly the source structure of the lst November 1755 Lisbon earthquake. Lines BS20 - BS24 were shot in order to study in detail the orientation, geometry and kinematics of the above mentioned active thrust. Line BS25 were shot parallel to the axis of the thrust structure, to image the lateral variability of the faults. Line BS26 was designed for land recording of the refracted waves only, with shooting interval of 150 meter. The linees started 50 miles offshore Cabo S.Vicente with direction N50E. Because of the intense ship traffic around Cabo san Vicente the line was performed without the multi-channel streamer towed to allow to shoot very close to Cabo S.Vicente itself
During the survey some lines were shot to collect both reflection data and refraction data (lines
BS06 and BS07A and line BS26 for refraction data only). In order to obtain good results for the
acquisition of refraction data, these lines were shot with a shot interval of 100m, and using a GI
Gun extra chambers 150 in
with 200 bar working pressure.
The refraction signals were recorded by five mobile stations located onshore in the following
sites:
Each station was equipped with a seismograph and a DGPS both for positioning and reference clock. These data will be used to make a velocity model for the upper crust in this region, using backward ray-tracing technique with the purpose of constraining the geometrical and structural interpretation of the seismic reflection profiles. The results of this study will be used, at a later stage, to reprocess the reflection profiles and to refine structural models for the crust in this region. Refraction data acquired during the survey will be processed by Spanish team.