NEAR-VERTICAL VIBROSEIS VERSUS LARGE-OFFSET DYNAMITE SEISMIC-REFLECTION PROFILING - EXAMPLE OF THE ECORS NORTHERN FRANCE PROFILE

In the Paris basin (France), the ECORS project has recorded a deep Seismic profile using two different sources i.e. vibrators and explosive charges, both using the same in line receiver spread. The explosive shots implemented for refraction surveying have been recorded with offsets up to 45 km to obtain deep reflections. The results show these two techniques to be complementary. The VibroSeis 96-fold coverage shows a layered lower crust with a high Seismic quality for two thirds of the profile. The explosive Seismic survey, with a theoretical 3-fold coverage, has been displayed in three single-fold sections. More reflections are visible on these explosive source sections, especially where the quality of the VibroSeis section is poor. We can explain this difference, in this last part, by a distorted overthrust unit that exists near the surface. When crossing this unit, the Seismic rays are distorted and the conventional CMP stack is not optimal for imaging the lower crust. Another difference between the interpretations made from the two sections is the extension through the lower crust of the Bray fault that is well mapped at the surface. The VibroSeis image suggests that this fault extends throughout the crust, whereas the image of the single explosive section does not show the fault. Long-offset recording of powerful explosive shots using the same spread of geophones as those for the usual vertical Seismic reflection survey can provide invaluable complementary data, especially in tectonically disturbed areas. © 1990.