3-DIMENSIONAL IMAGING OF THE CRUST USING A SPARSE LAND ACQUISITION GRID - THE ECORS 2.5-D EXPERIMENT

In order to test the feasibility of 3-D imaging of the deep crust at a reasonable cost, ECORS has designed and conducted an experiment withi n the western North Pyrenean thrust zone, using a sparse 3-D land acqu isition grid. Each shot was recorded on 6 parallel geophone lines samp ling an 8 x 12 km2 area, and 14 shot columns covered a total surface o f 8 x 27 km 2 in a single swath. Despite the complex structure of the zone, standard processing using a bin size of 50 x 200 m2 provided low -fold 3-D stacks on which coherent reflection surfaces could be picked interactively down to 5 s. These surfaces, which originate from the N orth Pyrenean frontal thrust, associated thrust sheets, and an uplifte d basement wedge, have a clearly 3-D geometry. The internal structure of the thrust sheets appears to be particularly complex. A two-pass fr equency-wavenumber 3-D migration, using a dip limitation of 45 and 15- degrees for in- and crosslines, respectively, effectively focused the main reflections and attenuated 2-D migration smiles. Whereas processi ng provides a general outline of the complex 3-D structure of the thru st system, a detailed interpretation will require an interactive integ ration of the information present on both original and processed data.