The complexity of the crust and Moho under the southeastern Superior and Grenville provinces of the Canadian Shield from seismic refraction - wide-angle reflection data

The major features of the individual velocity models, Poisson's ratio value s, and crustal complexity derived from the interpretation of seismic data s ets from four long-range seismic refraction - wide-angle reflection experim ents are summarized. The experiments were conducted from 1982-92 in the sou theastern portion of the Canadian Shield. In the conventional analysis of s eismic refraction - wide-angle reflection data, only the onset times and am plitudes of the major arrival phases are used to derive seismic velocity mo dels of the region under study. These models are over smoothed, have a numb er of intermediate discontinuities, are unable to explain the Pg coda, and bear very little resemblance to the models derived from the analysis of nea r-vertical seismic reflection data. In this paper some of the differences b etween seismic models derived from near-vertical reflection analysis and th ose from refraction analysis are reconciled from an analysis of the wide-an gle reflection fields of the crustal coda waves that follow the first arriv als. This was done using a migration technique that to a first approximatio n maps the amplitudes of the record sections into a two-dimensional (2-D) c omplexity section. These new sections show significant lateral variations i n crustal and Moho reflectivity and may be used to complement the 2-D veloc ity anomaly sections and near-vertical reflection sections. The method was based on a numerical study that showed that the coda can be explained with a class of complex heterogeneous models in which sets of small-scale, high- contrast sloping seismic reflectors are "embedded" in a uniform seismic vel ocity gradient field.