Theobald Peter D.
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ArticleAcousto-optic effect compensation for optical determination of the normal velocity distribution associated with acoustic transducer radiation(Acoustical Society of America, 2015-09-28) Foote, Kenneth G. ; Theobald, Peter D.The acousto-optic effect, in which an acoustic wave causes variations in the optical index of refraction, imposes a fundamental limitation on the determination of the normal velocity, or normal displacement, distribution on the surface of an acoustic transducer or optically reflecting pellicle by a scanning heterodyne, or homodyne, laser interferometer. A general method of compensation is developed for a pulsed harmonic pressure field, transmitted by an acoustic transducer, in which the laser beam can transit the transducer nearfield. By representing the pressure field by the Rayleigh integral, the basic equation for the unknown normal velocity on the surface of the transducer or pellicle is transformed into a Fredholm equation of the second kind. A numerical solution is immediate when the scanned points on the surface correspond to those of the surface area discretization. Compensation is also made for oblique angles of incidence by the scanning laser beam. The present compensation method neglects edge waves, or those due to boundary diffraction, as well as effects due to baffles, if present. By allowing measurement in the nearfield of the radiating transducer, the method can enable quantification of edge-wave and baffle effects on transducer radiation. A verification experiment has been designed.
ArticleQuantitative imaging of calibrated acoustic backscatter data from the seafloor(Institute of Acoustics, 2015-09-07) Foote, Kenneth G. ; Robinson, Stephen P. ; Theobald, Peter D.Two factors have often led to the neglect of instrument calibration and the loss of information in the imaging process: the power of the image and the convenient signal processing expedient of disregarding the physical nature of data. Seafloor imaging by sonar is a case in point. Notwithstanding ambitions and needs to remotely detect and identify bottom objects, determine seafloor properties, and quantify benthos, among other things, images of acoustic backscattering data are often used, misleadingly, as proxies for physical data. Since image processing is inherently nonlinear, the loss of physical data is immediate. Three processes that are essential to the attainment and maintenance of the physical nature of backscattering data are elaborated: sonar calibration to determine the transfer characteristics of the sonar, range compensation that addresses both geometric and radiometric factors, and beam pattern measurement or estimation.