Toward a robust target scattering measurement sonar for low frequencies

Abstract

In order to develop sonar systems that are non-image-based and that take advantage of target structural acoustics features, broadband capabilities have to be created to operate in shallow water and in the nontraditional frequency band below 30 kHz. Here, source wavelength–aperture ratios are high, resulting in significant source divergence and multipath. At the lowest frequencies, evanescent wave penetration is important in the detection of structures buried below the sediment–fluid interface. Further, the recovery of scattered energy requires nontraditional vertical apertures and synthetic array techniques that have both similarities and differences when compared to high-frequency imaging-based systems. The differences include relatively large resolution cell sizes, robustness to vehicle motion, and the clear potential for higher coverage rates. Towards this end, we discuss the precise measurement of low-frequency scattering cross section in shallow water using an UUV-based synthetic array approach. Broadband environmental acoustic propagation data taken in a bay environment will be presented, together with synthetic aperture data taken with a 20-m horizontal rail. The results of these measurements and the implications for UUV-based broadband sonars will be discussed.