Near-Surface Environmental Limitations to High-Frequency Sonar Performance: A Review
Abstract
This work reviews relevant environmental constraints on the performance of high-frequency forward-looking obstacle or mine avoidance sonars. These high-resolution. narrow-beam systems typically operate at frequencies 100-500kHz, focused on detection and classification of near-surface targets up to 500 m range. Relative signal-to-reverberation performance vs. range for a -15 dB surface target is modeled using a generic sonar model approach. Reverberation from and absorption by near-surface micro-bubble layers is shown to create very strong interference. Breaking-wave induced bubbles typically begin to appear above 6 - 10 knots wind speed, with typical bubble density spectra strongly decreasing from 10 to 400 J.Ul1 radius. At high frequencies these micro-bubble layers exhibit strong volumetric backscatter, with maximum volumetric backscattering strengths near -10 dB (re 1 m-1), and strong extinction losses of order 1 dBm-'. Delectability of surface targets is limited to less than 20m at 100kHz, but the detection range increases with frequency to roughly 250m at 400kHz and above. Another significant performance limitation is acoustic refraction by near-surface sound speed gradients commonly found in coastal regions, especially near river-mouths and estuaries and during summer months. The most serious problems arise under downward-refracting conditions with the creation of near-surface acoustic shadow zones. Ray-tracing predictions under typical summertime coastal conditions show surface shadow regions (i.e. zero detectability) beyond 80 m range. Reverberation from dense zooplankton layers and schooling fish is shown to create minor levels of interference. Reverberation from surface wave roughness scattering in the absence of bubbles is shown to have negligible impact.
Document Details
- Document Type
- Technical Report
- Publication Date
- Jan 01, 2001
- Accession Number
- AD1004995
Entities
People
- Mark V. Trevorrow