V. V. Goncharov

Acoustic tomography at shelf of the Black Sea

Using self-contained acoustic buoys developed at the Institute of Oceanology, Russian Academy of Sciences, experiments are performed to verify the possibility of acoustic tomography in a local region of a shallow-water sea. In the experiments, compound phase-manipulated signals are transmitted and the time responses of the medium are measured using three bottom-moored receiving and transmitting transducers separated by a distance of about 1 km. Identification of different groups of rays using simple models of the medium is considered. The results of reconstructing the sound and current velocities are compared with the data of independent measurements. The experiment is performed in complex weather conditions as an anticyclone passed to the experimentation region. It is shown that this acoustic tomography method is somewhat advantageous in the case of the absence of volumetric scatterers in the water bulk when the Doppler method of measuring currents fails.

Passive ocean acoustic tomography

The possibility to apply natural acoustic ocean noise in the ocean and noise of distant vessels as sounding signals in order to determine the physical parameters of a water layer is considered in this paper. We developed the methods making it possible to suppress the non-diffuse components of noise produced, e.g., by local vessels and to account for hydrophone motion. These methods are applied to the noise records obtained in the course of a year-long experiment on long-range sound propagation in the Pacific Ocean. We confirmed experimentally our theoretical predictions as to the possibility of retrieving deterministic acoustic ray travel times in a nonuniform environment from a mutual correlation function of imperfectly diffuse (gradually anisotropic and spatially nonuniform) noise without invoking any data on its source. We performed passive measurements of sound velocity in the ocean with a relative error of about 0.1% by correlation of noise fields recorded with vertical aerials. This accuracy approaches that needed for oceanological applications. Further investigations are necessary to study the feasibility of passive acoustic tomography and thermometry in the ocean at distances of tens and hundreds of kilometers and the possibility to use simpler arrays not equipped with hydrophone positioning systems.

Separation of acoustic modes in the Florida Straits using noise interferometry

We consider separation of acoustic modes in an experiment carried out in the Florida Straits. The features of the approach are separation of modes using data from single hydrophones, not vertical mode arrays, and a passive scheme of noise interferometry in which the source consists of ocean noise. Processing made it possible to reliably separate the first four modes of the acoustic field. The results allow a conclusion on the possible use of this method for shallow-water monitoring under complex hydrological conditions.

Tomographic inversion of measured cross-correlation functions of ocean noise in shallow water using ray theory

Based on experimental data obtained in 2012 in the Florida Strait, we study the feasibility of employing ray tomography to retrieve sound speed and flow velocity profiles from measured noise cross-correlation functions. We describe the results of numerical experiments that characterize the inversion errors resulting from peculiarities of the ray structure in shallow water, difficulties in unambiguous identification of ray arrivals, and a decrease in accuracy of ray theory at low frequencies. We show that under conditions of low-mode sound propagation, the use of the classical ray tomography scheme can yield only a rough estimate of the sound speed profile, but it allows approximate reconstruction of the current velocity profile. Application of passive ray tomography to the experimental data yields the current velocity profile in the Straits of Florida, which agrees with independent measurements within the inversion error limit.

Ocean acoustic thermometry with ambient noise.

Diffuse acoustic illumination, provided by ambient and shipping noise in the ocean, can be used as a probing signal to characterize the environment in a cost‐effective and non‐invasive manner. Valuable quantitative information about sound speed and, hence, temperature fields in water is contained in acoustic travel times between spatially separated receivers. The travel times can be retrieved from cross‐correlations of noise recorded by the receivers. Practical applications of noise interferometry to ocean thermometry face a number of obstacles, the main challenge being the need to achieve very high accuracy of passive sound‐speed measurements in a dynamic environment with a complex noise field. This paper reviews the theoretical background of wave interferometry in inhomogeneous media with non‐perfectly diffuse noise from the viewpoint of ocean remote sensing. Feasibility of ocean acoustic thermometry is illustrated using noise recordings of opportunity, which were obtained in the North Pacific Ocean as ...

Noise cross‐correlation and passive measurements of the sound speed profile in deep water.

Ambient acoustic noise in the ocean contains extensive information about its sources and the propagation environment. Successful application of the noise observations to acoustic characterization of the environment depends on identifying the noise properties which can be reliably measured and are sensitive to variations in temperature and other physical parameters of the ocean. This paper focuses on two‐point noise cross‐correlation and the environmental information that can be retrieved from it without any a priori knowledge about properties and locations of the noise sources. A technique to retrieve deterministic acoustic travel times from cross‐correlations of noise recorded on two vertical line arrays is described. Feasibility of environmental monitoring with ambient noise is illustrated by results of passive tomography of the water column using noise recordings of opportunity, which were obtained in the North Pacific Ocean as a by‐product of a long‐range sound propagation experiment. Conflicting requ...

Ocean tomography with acoustic daylight: A case study.

Ambient and shipping noise in the ocean provides acoustic illumination, which can be used, akin to daylight in the atmosphere, to visualize objects and characterize the environment [Buckingham et al., Nature (London) 356, 327–329 (1992)]. It has been shown theoretically [O. A. Godin, Phys. Rev. Lett. 97, 054301 (2006)] that, under rather general conditions, deterministic travel times between any two points in an inhomogeneous, moving or motionless, time‐independent medium can be retrieved from the cross‐correlation function of non‐diffused acoustic noise recorded at the two points, without a detailed knowledge of the noise field’s sources or properties. Using the data obtained during the 1998–1999 Billboard Array Experiment [Worcester et al., J. Acoust. Soc. Am. 117, 1499–1510 (2005)], this paper demonstrates the feasibility of a tomographic reconstruction of the sound speed field from cross‐correlation of acoustic noise recorded on a pair of vertical line arrays (VLAs) in deep water. Limitations of the n...

Acoustic tomography of currents in the ocean by the linearized method of matched nonreciprocity

The matched nonreciprocity method recently developed for acoustic diagnostics of ocean currents is based on matching the nonreciprocity of a sound field propagating along the current and against it. In this paper, the possibilities for linearizing this method are studied on the basis of numerical experiments. In contrast to the strict limitations of the linear approach in the tomography of the field of sound velocity by the matched field methods, in the method of matched nonreciprocity, the linearity is provided by the smallness of the Mach number M of currents and by the fact that the terms quadratic in M are partially suppressed in the course of the nonreciprocity calculation. The linear approach makes it possible to speed up the procedure of calculation (including calculations in higher dimensional parametric spaces) and to efficiently use nonlinear procedures for the localization of the main minimum.

Modal Analysis of Sound Field in Deep Sea

This paper describes the experiment on measuring modal spectrum of monochromatic sound field of the point source in open sea.