Seismic sections from COCORP's 1982 survey from the eastern Basin and Range to the Colorado Plateau of central Utah exhibit distinct cutoff times after which reflections are rare to nonexistent. In the eastern Basin and Range, this cutoff time is approximately 11 s (33 km), but beneath the central Colorado Plateau it exceeds 15 s (45 km). These depths appear to correspond to the base of the crust (Moho), with the lack of reflections from greater depths indicating mantle homogeneity. In general, absence of deeper reflections may be due either to geologic homogeneity or to lack of signal penetration. COCORP line 3 in the Colorado Plateau-Basin and Range transition zone shows that variations in penetration are significant. On line 3 few reflections are evident below the structurally complex sedimentary cover, which extends to only 4 s (8 km), and virtually none are identifiable later than 7 s (21 km). Lateral variations in the temporal decay of source-generated energy, together with estimates of corresponding ambient noise levels, infer that limited signal penetration, rather than geologic homogeneity, causes the lack of subsedimentary reflections within the transition zone. Deep reflections, if any, from beneath the westernmost Colorado Plateau appear to be masked by unusually high local environmental noise. In contrast, continued decay of source-generated energy at traveltimes significantly greater than Moho arrival times within the Basin and Range and Colorado Plateau suggests (though it cannot confirm) that the underlying mantle is seismically transparent. Variations in signal penetration, such as those documented here, severely constrain interpretations of nonreflective zones in deep reflection data and should be a standard estimation in any interpretational procedure.