Analysis of Acoustic Scattering from Large Fish Schools Using Bloch Wave Formalism

date September 18, 2015
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Friday, September 18, 2015 4:00 p.m. in ETC 4.150

Dr. Jason A. Kulpe
Applied Research Laboratories
The University of Texas at Austin

In the open ocean, scattering of sonar signals in the 1-10 kHz frequency range is dominated by large fish schools, where multiple scattering effects between the air-filled swim bladders of the fishes within the school are strong. These schools are typically large in comparison to the acoustic wavelength, and the fish typically swim in nearly-periodic arrangements with a separation distance of approximately one body length. Analysis of the periodic school is performed using the Bloch theorem, which reduces the study of the entire school to the study of a unit cell containing a single fish’s swim bladder. Acoustic reflection from the school is considered, using a finite element discretization of the unit cell, via an expansion of Bloch waves for the transmitted wave field. Next, acoustic scattering from a large finite school is studied using the Helmholtz-Kirchhoff integral theorem, with the Bloch solution used as inputs to the integral. A general model using the Bloch formalism that encompasses the internal structure, fish biologic properties, and realistic school effects such as disorder and geometry, will be explored. Transient analysis of the frequency dependent scattering, using the proposed model, may assist sonar operators to better classify large fish schools.