Broadband acoustic scattering measurements of underwater unexploded ordnance (UXO)

Abstract

In order to evaluate the potential for detection and identification of underwater unexploded ordnance (UXO) by exploiting their structural acoustic response, we carried out broadband monostatic scattering measurements over a full 360 degrees on UXO's (two mortar rounds, an artillery shell, and a rocket warhead) and false targets (a cinder block and a large rock). The measurement band, 1-140 kHz, includes a low frequency structural acoustics region in which the wavelengths are comparable to or larger than the target characteristic dimensions. In general, there are aspects that provide relatively high target strength levels ( approximately -10 to -15 dB), and from our experience the targets should be detectable in this structural acoustics band in most acoustic environments. The rigid body scattering was also calculated for one UXO in order to highlight the measured scattering features involving elastic responses. The broadband scattering data should be able to support feature-based separation of UXO versus false targets and identification of various classes of UXO as well.

Wide Area Detection and Identification of Underwater UXO Using Structural Acoustic Senors: 4th Annual Report to SERDP MM-1513

“…In Fig. 7a we show the comparison between the EFIT simulation and the laboratory measurement 8 for the free-field target case. Overall, the agreement is reasonably good given the lack of a direct inclusion of scattering and elastic wave interactions that can take place by virtue of the presence of the physical ends of the target…”

“…These measurements were completed on target set 1 in 2006, and in 2007 the broadband measurements were extended to the 105 mm sabot solid projectile, the 25 mm solid dummy round, the two overlap targets, and the large rock and the common cinder block false targets. These measurements 8 made in the free-field were then augmented by those made on some of the targets proud of the bottom using a companion sediment scattering facility and an at-sea rail-based system in St. Andrew's Bay, Florida. The Laboratory for Structural Acoustics (LSA) 5 at NRL (see Fig.…”

“…al. 8 versus frequency (2kHz to 25 kHz) and the source angle minus receiver angle. 0° and 90° source angles are on the left and right, respectively.…”

“…Generally, experimental studies of scattering from proud or buried targets 8,21,22,23 have focused on monostatic scattering wherein the source and receiver are co-located. Here we present results 24,25 related to bistatic scattering and in particular its forward scattered component.…”

“…al. 8 for the monostatic case which used a nearfield cylindrical source array and a small "point" receiver rotating about the target. The data was collected and processed to recover complex scattering cross-section expressible as TS.…”