Deep seismic reflection data typically sample the entire Earth's crust
and usually have reflection patterns different from those of sediments.
Continuous reflections over distances longer than a few kilometers
are rare, and their amplitudes and dips may vary considerably within
a small portion of the seismic section. This makes it difficult to
observe consistent trends in the reflections. Such observations are
important because the interpretation of deep seismic data relies
on consistent reflection patterns that identify tectonic units. For
example, Marillier et al. [1989a, b] identified three lower crustal
blocks underlying the northern Appalachians in eastern Canada, on
the basis of consistent seismic characteristics over several hundreds
of kilometers. Another important feature, the crust-mantle boundary,
is sometimes difficult to detect because the reflectivity of the
lower crust diminishes gradually with depth.