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Weathered layer resonance

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 shallow more-consolidated soil layers 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. Usually 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.