Since Conrad [1925] first identified a midcrustal refraction event
in central Europe seismologists have developed layered models of
the continental crust with velocity steps at various levels. Modern
analysis of crustal refraction data, including automated inversion,
relies on ray theoretical techniques or wave theory for plane layered
media, which generally parameterize the Earth as a series of grossly
horizontal layers. The bias toward layered models is in part based
on physical and geologic intuition, but it is to a large measure
a result of available interpretation methods, for example layer-based
reflectivity and ray tracing methods. In contrast, reflection seismology
has produced a picture of a highly heterogeneous crust, with wavelength-scale
variations in velocity existing at different levels of the crust
in different tectonic regimes. Finite difference modeling shows that
zones of small amplitude random velocity fluctuations not only produce
short discontinuous reflection segments as observed on deep seismic
reflection data but also produce strong wide-angle reflections. Some
of the events identified as crustal wide-angle reflections in field
data may in fact be the result of scattering from wavelength-scale
heterogeneities and need not result from reflection from a first-
or second-order velocity discontinuity. Significant changes in the
large-scale velocity structure associated with zones of random velocity
fluctuations produce relatively small changes in the dynamic properties
of the backscattered wave field. This observation is in disagreement
with the common credo that the seismic refraction method is primarily
sensitive to changes in the large-scale velocity structure. As a
consequence, the ray theoretical interpretation of such wide-angle
reflections as first-order discontinuities leads to crustal velocity
structures that are nonunique and may be erroneous in detail, while
correctly predicting average crustal velocity and thickness. We are
not suggesting that crustal velocity does not change with depth on
the large-scale; rather, we are proposing that crustal seismic events
observed at intermediate and large offsets may be the result of scattering
from wavelength-scale velocity fabric rather than specular reflection
or refraction from first-order discontinuities.