A. I. Khil’ko

Low-mode tomography of inhomogeneities in a shallow sea

Potentialities of observing spatially localized inhomogeneities by the low-frequency low-mode pulsed diffraction tomography in a shallow sea are analyzed. A numerically simulated model is used to study the procedure of reconstructing the parameters of inhomogeneities by coprocessing the signals of the tomographic projections formed at the multistatic excitation and reception of low-frequency low-mode acoustic pulses.

Space-time coherence of low-frequency acoustic waves in a shallow-water area with fluctuating parameters

The space-time correlations of narrowband acoustic signals propagating in shallow water are theoretically investigated. It is assumed that acoustic field fluctuations are caused by the combined effect of wind waves and random internal waves. The acoustic field is represented as a superposition of two components: one of them is characterized by relatively small space-time scales and is formed by wind waves, whereas the other is characterized by large scales and is caused by both random internal waves and wind waves. A numerical simulation is performed for space and time correlation functions of tone signals under typical conditions of the Barents Sea. It is demonstrated that the results of numerical calculations are in good agreement with experimental data.

Experimental investigation of potentialities of seismoacoustic sea-bottom sounding using coherent pulse signals

We describe the results of experimental investigations of the seismoacoustical sounding of the bottom structure of the Caspian Sea. They were obtained using a ship towed hydroacoustic emitter of LFM pulse signals in several frequency ranges of frequency band from 100 to 1000 Hz. Based on the high coherence and relatively high frequencies of emitted signals, the results point to feasibility of substantial improvement in noise immunity and resolution of sounding the bottom rocks’ structure at depths of up to 1000 m thanks to combined application of a series of procedures of coherent processing of incoming signals. The processing involves matched filtering of individual pulses, coherent accumulation of pulse trains within the horizontally uniform bottom area, and adaptive path accumulation of pulses accounted for inclination of individual reflecting layers. The resulting gain in noise immunity came to about 30 dB, which points to possibility of efficient use of relatively low-power (up to 100 W) coherent sources for seismoacoustic sounding of sea bottom at minimal damage to local ecology.

Experimental study of mode selection in shallow-water sea

The possibilities of matching low-frequency underwater sound pulses to the parameters of an oceanic waveguide are considered. The objective is to optimize the system of few-mode tomographic observation in a shallow-water sea. Experimental data are analyzed for two methods of selecting the low-frequency fewmode pulses propagating in a shallow-water sea. The first method excites probe pulses by vertically elongated arrays, with spatial filtering after vertical or horizontal arrays receive the pulses. The second method is based on exciting broadband signals with linear frequency modulation by a single transmitter. The selection of the few-mode signal is performed by time strobing the signals at the output of the matched filter after a horizontal array receives the pulses. The distance between the sound sources and receiving systems varied from 10 to 300 km.

Optimization of Mode Composition of the Acoustic Field Excited by a Vertical Antenna Array in a Shallow Sea

The problem statement is described, and the solution to the problem of optical tuning of the radiating antenna array providing for the optimal mode composition of the field radiated to a waveguide is determined. Matlab software is developed based on the proposed algorithms, and the optimal fields for shallow sea waveguides are calculated.

Optimization of ocean acoustic monitoring systems

We formulate and solve certain problems of the optimal amplitude-phase tuning of antenna complexes that emit and receive acoustic signals in inhomogeneous ocean-type waveguides.

Conditions for the existence of weakly divergent bundles in plane-layered waveguides

We found conditions for the existence of weakly divergent bundles (WDB) of rays of different types in waveguides with a power-law dependence of the square of the refractive index.

Reconstruction of the Sea Bottom Parameters for Coherent Seismoacoustic Sounding: I. Decision Rules

We have developed an algorithm for reconstructing the geoacoustic parameters of bottom layers using parametric models of the formation of signals reflected from a layered halfspace during coherent sounding of the bottom of a sea shelf. We propose a method for layer-by-layer reconstruction that makes it possible to efficiently search for a solution in the multiparameter space with limited a priori data.

Target strength in oceanic waveguides

The structure of the target strength in oceanic waveguides is analyzed. The role of the target strength is discussed as one of the key parameters for designing and estimating the efficiency of promising underwater observation systems.

Coherent seismic sea-bottom profiling based on broadband signals

Experimental results of the seismic profiling with bottom penetration up to 1000 m based on broadband signals and conducted in the Caspian Sea sites are presented. Use has been made of synchronized sequences of probing pulses with linear frequency modulation at a frequency deviation of 50 to100 Hz. The pulses were emitted by a towed sound source of an original design (acoustic power up to 300 W, frequency ranged from 100 to 1000 Hz) and received by a standard digital seismic streamer. The processing of the signals involved the matched filtering of the individual pulses and the trajectory accumulation of a long sequence of pulses lengthwise the horizontal-homogeneous reflecting layers of the bottom structure. The adaptive stacking procedure taking into account the linear inclinations of the individual layers allowed us to enlarge the stacking interval by up to 100 pulses and to increase the effective depth and the spatial resolution of the seismic profiling, which gave us a total increase of more than 30 dB in the S/N ratio. In our view, the seismic profiling using low-power (about 100 W) and broadband (up to several hundred Hz) coherent sound sources represents a promising technology for decreasing the hazardous impact on aquatic ecosystems. The approach developed is an alternative to the conventional technology of marine seismic prospecting based on powerful pulse sources of the shock type (air guns, sparkers) in the low frequency range (less than ∼200 Hz).