Study of seasonal acoustic properties of sea water in selected waters of the southern Baltic

Climate Research of the globe is a current task, in particular to confirm the general hypothesis of global warming associated with an increase in average temperature. The sea acoustic climate is derived from the concept of climate and refers to the conditions of propagation of acoustic waves in a particular basin. In this paper, the results of systematic measurements of temperature, salinity and velocity distributions of sound in the area of the southern Baltic will be present. The focus will be on the results of extensive measurements carried out „in situ” in particular

Conditions for propagation wave of finite amplitude in the Southern Baltic

In order to improve the buried objects search capability, there is a need to investigate new techniques that will enable the detection, localization and classification of buried mines. Indeed, mines buried in marine sediments represent a severe threat, especially in coastal waters. More-over, an inventory of objects buried in harbor areas is necessary to assure the security of ships. Parametric sonars are the new class of devices that allow to penetrate the sea floor. However their efficiency depends strongly on natural conditions in the sea. The dependence is of great importance in the shallow sea with considerable changes of acoustical conditions during the year. The paper contains the results of research of effectiveness of nonlinear wave generation in the Southern Baltic conditions basing on in situ measurements of the hydrological parameters

Multi-element sources focusing energy

In underwater imaging the beam-forming technique plays one of the most important roles. Focusing acoustic energy is one of the methods used to improve lateral resolution. The paper contains the results of the experimental research in a piezoelectric four- and six-element annular arrays of a resonant frequency equal to 1.5 MHz. The third examined source is built using one PZT bowl element. The sources are constructed with a layer adjusting the PZT transducers to water. In that way we obtained the sources of broadened band radiating in the frequency range 1.2 MHz - 1.8 MHz. The space distribution of the pressure field has been determined for various frequencies and various intensity of the phenomenon of the non-linear distortion of the wave. The experimental data have been compared with the results of the numerical research carried out with the use of a model constructed on the basis of the KZK equation

Pressure field produced by some finite amplitude sources

Pressure field of finite amplitude sources is examined in the paper. Detailed measurements of nonlinear distortion within beams radiated by plane and focused circular pistons are presented. Comparison of experimental results are made with numerical calculations based on the nonlinear parabolic wave equation (KZK) with the equivalent boundary condition adequate to the actual boundary condition. Special attention is paid to impact that the pressure distribution at the radiating surface has on the phenomena occurring in the vicinity of the source. Focused beam is examined over a distance extending up to post local region. Investigation is carried out with various pressure amplitude at the source in pseudo continuous wave conditions

Underwater Acoustic Imaging of the Sea

Acoustic waves are a carrier of information mainly in environments where the use of other types of waves, for example electromagnetic waves, is limited. The term acoustical imaging is widely used in the ultrasonic engineering to imaging areas in which the acoustic waves propagate. In particular, ultrasound is widely used in the visualization of human organs – ultrasonography (Nowicki, 2010). Expanding the concept, acoustical imaging can also be used to presentation (monitoring) the current state of sound intensity distribution leading to characterization of sources in observed underwater region. This can be represented in the form of an acoustic characteristic of the area, for example as a spectrogram. Knowledge of the underwater world which is built by analogy to the perception of the space on the Earth’s surface is to be systematize in the form of images. Those images arise as a result of graphical representation of processed acoustic signals. In this paper, it is explained why acoustic waves are used in underwater imaging. Furthermore, the passive and active systems for underwater observation are presented. The paper is illustrated by acoustic images, most of them originated from our own investigation

Investigation of bottom sediment stratification

The main goal of this paper is to find a method to assess remotely the type of sediments on the basis of the signal received using the parametric echosounder. The images of the sea bottom sediments taken by a parametric echosounder show the fine structure of the upper layer of the bottom sediments. The depth of penetration depends on the type of sediment, in fact on the attenuation of acoustic wave. In the paper the method of distinguishing the sediment layer basing on interpretation the received bottom signal is proposed. The knowledge on acoustic parameters of different type of sediments obtained in laboratory condition or using the literature data, allows to predict the composition of upper layer of the bottom. The considerations are illustrated by the results recorded from the bottom of the Gulf of Gdansk

Comparison of characteristics of focused and non-focused beam

The paper presents results of comparison of characteristic features of wave of finite amplitude propagating inform of focused or non-focused beam. Some resemblances in both types of beams are pointed out. It occurs mainly due to nonlinear wave distortion during the propagation. In addition, certain differences are indicated, this is the consequence of impact of diffraction in the beam forming process. Findings are illustrated with results of experimental and numerical investigations

Theoretical model of acoustic wave propagation in shallow water

The work is devoted to the propagation of low frequency waves in a shallow sea. As a source of acoustic waves, underwater disturbances generated by ships were adopted. A specific feature of the propagation of acoustic waves in shallow water is the proximity of boundaries of the limiting media characterised by different impedance properties, which affects the acoustic field coming from a source situated in the water layer “deformed” by different phenomena. The acoustic field distribution in the real shallow sea is affected not only by multiple reflections, but also by stochastic changes in the free surface shape, and statistical changes in the seabed shape and impedance. The paper discusses fundamental problems of modal sound propagation in the water layer over different types of bottom sediments. The basic task in this case was to determine the acoustic pressure level as a function of distance and depth. The results of the conducted investigation can be useful in indirect determination of the type of bottom

Tracking of the broadband source of the underwater noise in the very shallow water conditions

The paper contains the results, both theoretical and experimental, connected with the tracking of the underwater noise source as small ships, pontoon, diver and so on. The problem of security in the shallow water area is the challenge for underwater acousticians. In this paper there is taken into account the detection of the sources that move on the surface of the sea or underwater in shallow and very shallow water. The main goal deals with characterization of the sound propagation at the shallow water channel. The next problem is formulation of the sound propagation inside of this acoustic duct including normal mode creations and dispersion of the acoustic energy. When we take into account the small ships noise the two main sources are analyzed. The first one is the vibration energy produced by the mechanisms located inside of the ship hull. The acoustical energy is transformed through structural elements of the hull to surrounding water. This energy propagates as broadband underwater noise. The next source is the ship propeller. The sources of the underwater noise give the some deposit to the total acoustical energy that means the ambient noise. The problem that should be solved is to discover these acoustical disturbances along with their classification, identification and tracking

Acoustic streaming in focal and post focal region

The paper presents concise theoretical description of mechanism of acoustic streaming in the field of finite amplitude wave. Special attention was concentrated on acoustic streaming in focal area into a beam produced by a bowl shaped source. A set up used in experimental investigation is also shown. The measurement was carried out at fixed conditions. Its result was compared to the result of calculation based on theory derived from the reasoning presented by W. Nyborg et al