Bispectral analysis of underwater acoustic data

Estimates of the power spectral density have been found to be very useful in a variety of signal processing applications over the last several decades. Higher-order spectra contain information not present in the power spectrum and recently, estimates of higher-order spectra have been shown to be useful in certain signal processing problems. In particular, estimates of the bispectrum and bicoherence have been found useful in detecting non-Gaussianity and nonlinearity in system identification, and in detecting transient signals. This paper is concerned with the estimation of the bispectrum and bicoherence of underwater acoustic signals. Some of the properties of the bispectrum and bicoherence are introduced and described. Estimates of bispectra of actual data taken from a freely drifting Swallow float and also from an element of a moored acoustic array are presented and discussed. In the results portion of the paper, three particular properties of bispectrum estimation are focused on. It is demonstrated how the bispectrum estimate can be used to detect non-Gaussianity, nonlinearity, and harmonic coupling. The detection of harmonic coupling is shown to be a useful property of the particular bispectrum estimation algorithm employed here.