Comparison of the Waveguide and Phase–Energy Invariants in the Near-Field Zone of Acoustic Illumination

In many problems of practical importance the interference structure of broadband-signal intensity field in shallow water is determined by the close-to-unity value of the waveguide invariant (WI) β (which is often referred to as the Chuprov invariant). This circumstance is used in ranging, when estimating the distance to the source or its relative velocity. However, the characteristics of the β invariant in deep sea have been studied insufficiently. The WI properties in the near-field acoustic-illumination zone (NFAIZ) of deep sea are investigated below. Its value is shown to be unstable: WI changes in a wide range and actually is not an invariant. Another value—phase–energy invariant (PEI) β_ef—proved to be more promising in deep sea. In the NFAIZ of deep sea, at real depths of sources and PEI detectors, it is equal to unity with a high accuracy (except for the interference minima zones). It is also found that coherent addition of Fourier components in the complex plane can be implemented in the NFAIZ, provided that a correction to phase variation is introduced when summing spectral densities along ridges. To this end, one must take into account that the Fourier-component phase on a ridge changes almost linearly with an increase or decrease in frequency. In principle, consideration of these characteristics of signals makes it possible to solve more efficiently various applied problems of acoustics. However, to implement this possibility, it is necessary to develop a fairly complex algorithm of signal power accumulation in the frequency–space domain.