Acoustic Backscattering-Based Radius and Thickness Estimation for Thin to Thick Cylindrical Shells Submerged in Water

Abstract By transmitting acoustic waves and analyzing the wave reflections from underwater objects, one can extract object information. A common method for estimating the radius of thin shells involves the use of the antisymmetric Lamb wave mode (a0−), vibrations in which result in a series of high-amplitude peaks over a limited frequency range in the reflection spectrum (also referred to as mid-frequency enhancement). However, in thick shells, this enhancement is found to be weak and overlaps with low-frequency features, making the estimation more challenging and less accurate. To address this, here, the use of the symmetric Lamb wave mode (s0) has been investigated. Although this mode lacks enhancement, which is likely a reason it has not been explored for size estimation previously, there are aspects of this mode that make it attractive for use. By using a symmetric mode-based method, the radius estimation accuracy is found to be improved significantly for thin to thick shells. On the other hand, for thickness estimation, the conventional approach has been to use the frequency of greatest enhancement (FGEN) in the mid-frequency range. However, this method is also adversely affected by overlapping resonances in thick shells. To overcome this limitation, the frequency of the last enhancement method (FLEN) is proposed here, thereby improving both the accuracy and range of thickness estimation. By using the numerical approach employed here, the limitations of existing estimation methods are overcome, and a basis is developed for a general feature estimation method.