Three-dimensional imaging of the crust using a sparse land acquisition grid: The Eecors 2.5-D experiment

Abstract

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 within the western North Pyrenean thrust zone, using a sparse 3-D land acquisition grid. Each shot was recorded on 6 parallel geophone lines sampling an 8 × 12 km 2 area, and 14 shot columns covered a total surface of 8 × 27 km 2 in a single swath. Despite the complex structure of the zone, standard processing using a bin size of 50 × 200 m 2 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 North Pyrenean frontal thrust, associated thrust sheets, and an uplifted basement wedge, have a clearly 3-D geometry. The internal structure of the thrust sheets appears to be particularly complex. A two-pass frequency-wavenumber 3-D migration, using a dip limitation of 45 and 15° for in- and crosslines, respectively, effectively focused the main reflections and attenuated 2-D migration smiles. Whereas processing provides a general outline of the complex 3-D structure of the thrust system, a detailed interpretation will require an interactive integration of the information present on both original and processed data.