Broadband, Multi-Aspect Scattering from Proud and Buried UXO

摘要

Sonars which form high resolution images operate at high frequencies where the acoustic wavelengths are short compared to the target dimensions and the waves are scattered predominately from the geometric target boundary. In the structural acoustic regime where acoustic wavelengths are comparable to the target dimensions, sound penetrates the target and the scattered echoes are related to the vibrational dynamics of the object. The time-frequency features in the echoes can then be used to 'fingerprint' the target in addition to examining an image. We have been carrying out studies to evaluate the potential for the detection and identification of unexploded ordnance (UXO) in coastal and inland waters by exploiting their structural acoustic response. We will present and discuss underwater broadband multi-aspect scattering measurements and numerical simulations made on a number of UXO objects and several false targets in the proud and buried condition. The measurements were, for the most part, conducted in the NRL sediment laboratory pool facility. The simulation studies were carried out using two structural acoustic codes--one time based and the other frequency based--for treating target, sediment, and acoustic propagation. The UXO scattering data bases include both mono-static and bi- static source/receiver configurations, with both large look-down and near- grazing angle geometries, with the special case of forward scattering included in the latter. These broadband data bases, which can support both standard imaging and structural acoustic feature identification, are being used to train and test relevance vector machine (RVM) and imaging algorithms for identifying the UXO targets and to evaluate how the sediment and associated burial effects impact the identification process.