The use of invariants for detecting weak signals in the near acoustic illumination zone

In solving many practically important problems of hydroacoustics, the properties of the fan interference structure of the signal intensity field are used, which in the shallow sea in the coordinates “distance – frequency” are largely determined by the value of the waveguide invariant β (S.D. Chuprov invariant) close to one. Below, the properties of the waveguide invariant are studied in the near acoustic illumination zone (NAIZ) of the deep sea, and it is found that its values are unstable – when the propagation conditions change, the waveguide invariant varies widely and is not an invariant. It is shown that in the NAIZ the use of the phase-energy invariant βef is more promising, since in the NAIZ it is equal to one with high accuracy and stable. It is also discovered for the first time that, under certain conditions, coherent addition of Fourier components on the complex plane is possible in the NAIZ if, when summing the spectral components of complex spectra along the ridges, an adjustment for phase variation is introduced. With such processing, in the case of stationary noise, the probability of detecting weak signals can significantly increase.