It is shown that principal component analysis (eigenvector-eigenvalue decomposition of array spectral matrices) can be used for the approximate separation of superposed plane-wave signals only if at least one of the following conditions is satisfied:

  1. The signals are grossly unequal in amplitude.

  2. The directions and the velocities of the various signals are such that if the array is beamed at a given signal the other signals do not fall on prominent sidelobes of the array response.

Application of the method for such cases is demonstrated on composited long-period seismograms using recordings from the D ring and center element of ASA.

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