Measurements and modelling of acoustic scattering by a spherical elastic shell near an interface.
Résumé
When an object is placed near an interface, the interface will modify both the incident and scattered field by the object. There is, in fact, an infinite sequence of interactions between the scatterer and interface which leads to a very complex structure of the scattered signal. In this paper we have studied the backscattered field of a thin spherical elastic shell filled with air. The shell was supposed to be close to the sea bottom or to the air-water interface. We compared the full multiple scattering model with the single-scatter approximation. In the case of a thin air-filled shell and in the range of frequency we considered, the main contribution to scattering is due to the lowest-order compressional and flexural waves of the shell. For calculating the full multiple scattering solutions we have followed the Hackman and Sammelmann's general approach. The arising wave number integrals for the fluid bottom were evaluated with the use of the method of complex images. The obtained results have been compared with the data from a well-controlled experiment in a water tank for different angles of illumination and different distances of the shell from the interface. To make a comparison with experiments, we used a shell of thickness 4% of the outer radius. The bottom was assumed to be fluid and the attenuation in this medium was taken into account assigning a complex wave number. In order to insonify the shell, two wide-band Panametrics transducers working at respectively 250 kHz and 500 kHz were used. Free field resonances of the shell were identified in the measured spectrum as well as in numerical modeling. It was shown that when the object is close to the interface in addition to geometrical reflections between the shell and the interface, strong interactions due to these resonances can be observed.