Seismic reflections from an interface are relatively short in time; they are transient. Because of this fact, estimation of the covariance matrix is accomplished by the use of either time-slice vectors (across traces) or Fourier transform value vectors. Although we might have a dozen or so time-slice vectors, this is generally not adequate, (a few hundred would be nice, and appropriate for temporally stationary signals). In the frequency domain, the short reflection only affords one Fourier transform. Thus, the multiplicity of vector samples must be found another way if possible.