That the output spectrum of a PE filter is white is also useful geophysically. Imagine the reverberation of the soil layer, highly variable from place to place, as the resonance between the surface and deeper consolidated rocks varies rapidly with surface location because of geologically recent fluvial activity. The spectral color of this erratic variation on surface-recorded seismograms is compensated for by a PE filter. Of course, we do not want PE-filtered seismograms to be white, but once they all have the same spectrum, it is easy to postfilter them to any desired spectrum.