Hydroacoustic antenna complexes and methods for acoustic field exploration

To experimentally explore a hydroacoustic field in shallow seas, a Marine autonomous measuring complex (MAMC) was designed under the supervision of B.M. Salin at the IAP RAS in the 1990s. The complex is intended for studying the acoustic field and noise characteristics of surface and underwater sources in a frequency range of up to 500 Hz. The MAMC is based on cable hydroacoustic antennas 200 m long with a self-contained system for data record.

Reconstruction of the acoustic irradiance distribution over the ship hull length from the measurements made in the near field (the irradiance is plotted in the "frequency — alongside coordinate" plane) using the MAMC	Complex "Neva-IAP" on the deck of the measuring vessel: a reel with a 30-m antenna, a pressurized container with data transmission equipment, and a cable hoist with an 1100 m communication cable

Ship processing station of the "Neva-IAP" complex

The unique measuring complexes "Neva-IAP" and "Broker-IAP" were designed on the basis of cable-type digital hydroacoustic antennas (chief designer P. I. Korotin) for measuring the underwater noise of ships in the presence of near maritime traffic and interference noise typical of a shallow sea and then were delivered to the customers.
The technology and methods for the antenna use were significantly improved in the recent decade. The measurement metrology was provided by developing and certifying (A. S. Chashchin and B. V. Kiyashko) the digital hydroacoustic receivers (DHAR) with an integrated microcontroller. The inherent hydrophone noise does not exceed 35 dB rel. 20 μPa in the third-octave bands of an operating range of up to 10 kHz with a dynamic range no worse than 110 dB for an output digital signal according to the Ethernet protocol.

The methods associated with the synthesis and adaptation of the directivity characteristics of multi-element distributed receiving systems to acoustic, hydrodynamic, and vibratory noise, which do not have any analogs, were developed and certified (V. I. Turchin and I. Sh. Fix). The noise suppression level was achieved at the antenna output, which was sufficient for solving almost any sort of the measuring problems.

Variants of installing the antennas of the "Neva-IAP" complex": horizontal (left) and vertical providing a circle of equal sensitivity on the surface of a water area (right)	Adaptive synthesis of the equal-sensitivity area for the vertical antenna in a circle of radius 40 m — the lower curve; the upper curve — noise in the absence of adaptation

Antenna arrays can be used in various scientific and applied problems: for studying the spatial anisotropy of noise and the dynamics and fluctuations of signals on stationary paths, for selecting the signal modes in shallow water, for illuminating the underwater environment, for the near-field holography, and for the acoustic imaging of the underwater sources. Methods for synthesis of receiving systems with regard to both passive and active location were developed for these purposes at the IAP RAS. A theory for synthesis with the adaptation of the receiving system to the current noise characteristics has been elaborated (A. A. Rodionov). This approach, in particular, allows the antenna to be installed on board the ship, as the noise caused by the ship machinery is effectively suppressed.
For the conventional problem of noise direction finding, the possibility is found to increase the accuracy of determining the line of bearing and the distance using aperture synthesis when the vehicle carrying the antenna is in motion.

Examples of the effective use of the aperture synthesis method for solving the problems of the source localization in the horizontal plane of a channel for a tone signal of an unknown frequency (left) and for a broad-band signal (right)

The possibilities of linear antenna arrays of vertical and horizontal arrangements in the forward-scattering (transmissive) and bistatic sonar modes were theoretically and experimentally studied for active location.
A weak scattered signal was detected by special methods for the suppression of direct signal fluctuations that mask the scattered signal.

	Result of operation of the program-detector for the forward-scattering location under tone illumination in the plane "target velocity — time". Crosses show target blips
	Detection of a moving object under tone illumination in bistatics: oblique blip — target is in the plane "fluctuation frequency — bearing line"

A time reversal method was implemented by employing receiving antennas in a marine experiment (A. A. Stromkov). A method for scanning the sources by hydroacoustic systems in a shallow sea using time reversal and a numerical model of the waveguide for range and depth focusing was proposed and experimentally tested (V. A. Zverev). To match the reception with the medium, the method uses the source signal inversion according to the well-known dispersion relations based on a numerical model of the waveguide.
In a marine experiment in the signal frequency band from 100 to 300 Hz, the cross section of the formed focal region corresponded in depth to the measured sizes of the focal region of the test source and was only three times larger in range. The relations for an ideal waveguide are used as the dispersion model. An increase in the receiving system capacity by 10—12 dB due to the use of this method is shown experimentally.

Example of time-reversal focusing in a shallow sea of a depth of 125 m at a distance of 17.5 km, obtained using a test source (TS) (left), and example of a focal spot shift by 100 m in range and by 12 m in depth (right)