AN EVALUATION OF THE USE OF HIGH-RESOLUTION WAVENUMBER SPECTRA FOR AMBIENT-NOISE ANALYSIS
Advanced array research special rept. no. 8
TEXAS INSTRUMENTS INC DALLAS SCIENCE SERVICE DIV
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A principal objective of the network studies is to characterize the worldwide ambient-noise field. An excellent tool for such studies is the frequency-wavenumber spectrum of the ambient noise recorded by an array station. Since all network array stations were of small aperture, conventional techniques for computing wavenumber spectra could not achieve the wavenumber resolution required to resolve and accurately locate multiple noise sources, especially bodywave noise sources. To establish the procedure to use for computing the spectra, the experiment described in this report was performed prior to the computation of ambient-noise wavenumber spectra for the network studies. Analysis of ambient seismic noise using the high-resolution technique is shown to be valid. Wavenumber spectral estimates made from unsmoothed crosspower spectra appear more stable, less sensitive to SNR, and computationally much simpler than those made from smoothed crosspower spectra. The summing of several MCF wavenumber power responses, each designed with respect to a different reference sensor, stabilizes the wavenumber estimate. The summing of individual spectral estimates, i.e., the reciprocals of the MCF wavenumber responses, yields better resolution of coherent peaks relative to background noise levels.