Broadband Backscattering from fluid spheres and finite cylinders

As a side project to our thin layers work, we investigated the use of multi-static scattering to extract data from scatterers. The goal was to come up with an alternative to, or an adjunct to, multi-frequency backscattering for assessing properties of zooplankton. Some research into the literature suggested that this should be possible. The ultimate goal was to be able to estimate the relative density and compressibility (or sound speed) of zooplankters in situ.

For a beginning, I fabricated a series of spheres and finite cylinders from RTV (actually, Tom Kleinwaks labored long and hard to make a series of usable targets). We put together a rig in our small test tank to hold these targets centered on a circle about which we could rotate a broandband transducer. A fixed column could be installed to ensonify the target. With considerable futzing about to get things centered, we could obtain over 160° of multi-static, broadband scattering from these targets -- from nearly backscattering to nearly forward scattering.


The data from the spheres was found to be completely in accordance with the fluid sphere model of Anderson (1950). This was expected, since some rtv sphere data have been reported before (ref?). However, the agreement at angles other than backscattering is believed to be new.

The data from the cylinders was unique. The cylinders were capped with hemispheres, which complicates the theoretical modelling enormously. However, we used a finite cylinder model from Zhen Ye to obtain satisfactory estimates. The nature of the ensonifying signal -- a 2 1/2 cycle burst -- permitted the examination of the echo process inside the scatterer as echoes arrived at distinct times.


In the plot below, the cylinder was rotated while the transducer remained fixed; thus, all the data are "backscattering" but at varying aspect angles for the cylinder. In this figure, 90° represents the cylinder axis at right angles to the line from the transducer to the center of rotation. Each vertical line is the broadband echo from the cylinder at that aspect angle. The incident pulse looks very much like the first echo seen at 90° aspect. You can see that many of the echoes consist of basically the incident pulse, delayed by a varying amount with aspect angle. Towards end-on aspect, these various echoes tend to merge together, except for the specular echo from the endcap itself.


It is relatively easy to separate specular echoes (from the closest part of the cylinder to the transducer) from echoes that appear to arise from a circumferential path around -- or a direct path through -- the cylinder. Knowing the speed of sound in the water and in RTV, one can draw pictures and trace the probable paths of sound propagation in and around the target.