Simple analytical consideration show that for the velocities and travel
times relevant in deep seismic reflection data, migration displacements
easily exceed 5 km vertically and 10 km laterally. This implies that
virtually every deep seismic reflection profile needs to be migrated
and that a minimum profile length of 30 km is required to allow structural
interpretation at lower crustal depths. Estimates of the influence
of uncertainties in velocity upon migration show that the error in
the average velocity must not exceed 0.2 km/s at Moho depth, in order
to allow a meaningful comparison of the reflectivity imaged by normal-incidence
profiling and the crustal velocity structure inferred from seismic
wide-angle data. Conventional migration schemes based on the solution
of the scalar wave equation rarely produce satisfying results when
applied to deeper crustal data. A review of the corresponding algorithms
shows why these methods are highly sensitive to lateral velocity
variations, to the short, laterally discontinuous reflection segments
and to characteristics of deep seismic reflection data such as high
noise levels in conjunction with the high velocities and long travel
times. These problems can be largely overcome by ray theoretical
depth migration of digitized line drawings. As a case study the individual
deep seismic reflection profiles of the eastern and southern traverses
across the Swiss Alps, which were acquired as part of the National
Research Program 20 (NFP20), have been combined along the course
of the European Geotraverse (EGT). The resulting reflectivity distribution
was simultaneously depth migrated with the contours of the smoothed,
laterally consistent velocity field obtained by reinterpretation
of the seismic wide-angle profiles running parallel to the strike
of the Alpine arc. This led to an overall excellent agreement between
the most prominent reflectivity patterns and the strongest wide-angle
reflections, which is considered to be an important criterion for
successful migration. Assuming that the result of this migration
represents an unbiased acoustic image of the present-day tectonic
configuration of the crust below the central Alps, low angle subduction
of the lower crust and upper mantle of the European plate below the
Adriatic promontory of the African plate is clearly depicted. Orogenic
crustal thickening is interpreted to arise from the stacking of nappes
onto the European upper crust and from wedging of the European and
Adriatic middle crusts in accordance with existing geological models.
At least part of the southvergent upper crustal thrusting in the
Southern Alps can be accounted for by the inferred northward downbending
of Moho and lower crust of the Adriatic plate.