Spectral Decomposition of Time- vs. Depth-Migrated Data

Summary Spectral decomposition is a powerful analysis tool that has been significant success in delineating channels, fans, overbank deposits and other relative thin architectural elements of clastic and carbonate depositional environments. Because of its success in fluvial-deltaic and basin floor turbidite-fan systems, most publications of spectral decomposition have used time-migrated data. Interpreting spectral components and spectral attributes such as peak frequency on depth migrated data requires a slightly different perspective. First, the results are computed as cycles/km (or alternatively as cycles/1000 ft) rather than as cycles/s or Hertz, with the dominant wavenumber decreasing with increasing velocities at depth. Second, interpreters resort to depth migration when there are significant lateral velocity changes in the overburden and/or steep dips. All present-day implementations compute spectral components vertical trace by vertical trace rather than perpendicular to the bedding plane, giving rise to tuning and other anomalies at an apparent rather than at a true frequency or wavenumber. We illustrate the interpretational differences of spectral decomposition between time- and depth-migrated data through the use of a simple synthetic model and a modern 3D data volume. We show how one can approximately compensate for reflector dip by normalizing each spectral magnitude component by 1/cosθ, where θ is the volumetric dip magnitude commonly computed in seismic attribute analysis