In the underwater acoustic domain, there is a large interest for techniques able to achieve seafloor characterization. During some cruise campaigns - i.e. Calimero (2004) and Excapi (), it was clear that the frequency multiplicity could offer necessary information data about seabed. Classical acoustic surveying tools are sidescan sonars and multibeam echo-sounders which provide images of the backscattered acoustic intensity, and maps of the relief. However the bandwidth of each individual system is narrow and does not offer frequency diversities. Interesting investigation consisted of surveying the same seafloor areas [ ] with several systems offering various space-time characteristics. Therefore the problem is that data acquired by using different imaging systems are not perfectly collimated in time and space: a major challenge is to correlate consistently data obtained in different conditions. Thus the idea is to realize a new kind of source capable of generating simultaneously several frequencies taking advantage of the saturation effect: generated harmonic components of a shock wave become the exploited acoustic beams. The source is unique in time and space so that the multi-frequency responses are inherently perfectly matched. Such a system could offer frequency diversities useful for applications such as detection of sunken oil slicks or sediment characterization. In the underwater acoustic domain, the nonlinearity of the propagation is always perceived as a penalizing effect - with the exception of parametric antennas application, because it leads to losses of efficiency in transmitting antennas [ ]. When increasing the transmitted power, part of the acoustical energy is transferred along the propagation to higher harmonic components. It leads to a saturation effect on the fundamental frequency which limits the source level that can be reached. At the present time, there is no significant study on the harmonic fields generated by nonlinear propagation within the context of the devised application in underwater acoustics. Our present concern is the feasibility of such a multi-frequency tool with the suitable characteristics needed to handle surveying tasks. The main challenge is to obtain sufficient source levels, while keeping large enough angular apertures for the generated harmonic beams. First experimental results about obtainable on-axes level and beam patterns are presented. Since now three different antennas have been tested. All of them have been realized in Jean le Rond d'Alembert Institute. First two sources, a directive disk and an elongated rectangle, are mono-element piston like emitter, both tested in the institute pool. Third has been conceived with three independent elongated rectangle elements with the intention to use it as side scan sonar: it has been tested at IFREMER basin in Brest. Simplistic theoretical model based on generalized Burgers equation and its development help to have an idea about obtainable levels with different configurations of antennas. However its implementation is at his first stage.
Underwater multi-frequency transmitter for seabed characterization
L Di Marcoberardino;
2011
Abstract
In the underwater acoustic domain, there is a large interest for techniques able to achieve seafloor characterization. During some cruise campaigns - i.e. Calimero (2004) and Excapi (), it was clear that the frequency multiplicity could offer necessary information data about seabed. Classical acoustic surveying tools are sidescan sonars and multibeam echo-sounders which provide images of the backscattered acoustic intensity, and maps of the relief. However the bandwidth of each individual system is narrow and does not offer frequency diversities. Interesting investigation consisted of surveying the same seafloor areas [ ] with several systems offering various space-time characteristics. Therefore the problem is that data acquired by using different imaging systems are not perfectly collimated in time and space: a major challenge is to correlate consistently data obtained in different conditions. Thus the idea is to realize a new kind of source capable of generating simultaneously several frequencies taking advantage of the saturation effect: generated harmonic components of a shock wave become the exploited acoustic beams. The source is unique in time and space so that the multi-frequency responses are inherently perfectly matched. Such a system could offer frequency diversities useful for applications such as detection of sunken oil slicks or sediment characterization. In the underwater acoustic domain, the nonlinearity of the propagation is always perceived as a penalizing effect - with the exception of parametric antennas application, because it leads to losses of efficiency in transmitting antennas [ ]. When increasing the transmitted power, part of the acoustical energy is transferred along the propagation to higher harmonic components. It leads to a saturation effect on the fundamental frequency which limits the source level that can be reached. At the present time, there is no significant study on the harmonic fields generated by nonlinear propagation within the context of the devised application in underwater acoustics. Our present concern is the feasibility of such a multi-frequency tool with the suitable characteristics needed to handle surveying tasks. The main challenge is to obtain sufficient source levels, while keeping large enough angular apertures for the generated harmonic beams. First experimental results about obtainable on-axes level and beam patterns are presented. Since now three different antennas have been tested. All of them have been realized in Jean le Rond d'Alembert Institute. First two sources, a directive disk and an elongated rectangle, are mono-element piston like emitter, both tested in the institute pool. Third has been conceived with three independent elongated rectangle elements with the intention to use it as side scan sonar: it has been tested at IFREMER basin in Brest. Simplistic theoretical model based on generalized Burgers equation and its development help to have an idea about obtainable levels with different configurations of antennas. However its implementation is at his first stage.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
