@article {Williams:2018:0736-2935:919,
title = "Predicting the bistatic scattering from subsea buried targets using in-air laboratory measurements",
journal = "INTER-NOISE and NOISE-CON Congress and Conference Proceedings",
parent_itemid = "infobike://ince/incecp",
publishercode ="ince",
year = "2018",
volume = "257",
number = "1",
publication date ="2018-12-01T00:00:00",
pages = "919-930",
itemtype = "ARTICLE",
issn = "0736-2935",
url = "https://ince.publisher.ingentaconnect.com/content/ince/incecp/2018/00000257/00000001/art00089",
keyword = "equivalent source method, nearfield acoustical holography, structural impedance, scattering",
author = "Williams, Earl and Rakotonarivo, Sandrine and Sarkar, Jit and Kuperman, William",
abstract = "Identification of unexploded ordinance buried in the sediment in the littoral waters throughout the world is a problem of great concern. When illuminated by low-frequency sonar most of these targets exhibit an elastic response that can be used to identify them. This elastic behavior
is embodied and identified by a quantity called the in vacuo structural admittance matrix Ys, a relationship between the sonar-induced forces f and resulting vibration on its surface v, v= Ys f. Nearly impossible to measure in situ, Ys can be measured in air in a simple, acoustically unaltered
laboratory space using an array of loudspeakers and the cross-correlations of velocity and pressure measurements made on the target's surface. We demonstrate success, when compared with theory, with a simple target - a thick spherical air-filled shell outfitted with a set of eight collocated
accelerometers and microphones placed on its surface. Yuri Bobrovnitskii showed in a cornerstone 2006 paper on the impedance theory of sound scattering that a measurement of Ys combined with two surface impedances, can predict the complete 3D bistatic scattering for any given incident field.
These impedances are easy to compute using the equivalent source method (ESM), and furthermore, modification of these impedances provides a scattering prediction in any fluid-like media and for any burial state (depth and orientation) making unnecessary an otherwise extremely difficult in
situ measurement. This theory accurately predicts the lowering of the modal resonance frequencies for a fluid (water) loaded shell, and predicts how these flexural modes disappear to give way to the important fast extensional modes that dominate the far-field scattering signature. However,
the measurement of the surface fields needed for Ys could be prohibitive as the temporal frequency of interest increases. Instead of a surface measurement, one can use dual conformal surfaces of microphones placed near to the object, together with near-field acoustical holography (NAH) to
determine the fields on the surface via the back projection capability of NAH. This is combined with Field Separation Techniques, based on ESM or on the Helmholtz integral equation - techniques that have been popular in the past decade. These are needed to predict the total pressure and velocity
on the surface of the target since inverse NAH propagators do not exist. Although a large number of sensors may be required, the advent of inexpensive MEMS microphones combined with a 3D printing capability to construct the support structure makes this measurement very plausible. Presently
we are constructing two conformal 256 element MEMS arrays. Simulations demonstrate the accuracy of resulting 3D bistatic predictions using this measurement system. Work supported by the Office of Naval Research.",
}