Vertical array passive geo-acoustic inversion in range-dependent environments

Antisubmarine Warfare (ASW) is often conducted in littoral, shallow-water areas, where hostile subsurface enemies pose a constant threat and where seabed geophysical properties are complicated, and to a great extent unknown to us. Accurate estimates of seabed interface roughness and sediment geophysical properties are critical for proper prediction of sensor and weapon system performance. In the absence of good seabed characterization, tactical mission planning is seldom optimal or efficient. Current data collection survey techniques for geo-acoustic bottom characteristics are expensive, time consuming, and they suffer from time latency (months to years) between collection, processing, analysis, and tactical use. In response to this problem, the Space and Naval Systems Command and the Naval Air Systems Command have investigated several new inversion techniques to characterize littoral seabed sediments. Most of these techniques use an active sonar approach that is not covert and is usually limited to seabed areas near the receiver. Purely passive techniques offer the potential to remain covert and greatly extend the coverage of the seabed characterization. The research described here concerns a new set of algorithms called Passive Geo-Acoustic Inversion Techniques (PGAIT) that act on passive acoustic signals and thus allow non-provocative operations in forward areas and geo-acoustic characterization at ranges well beyond a local footprint. PGAIT uses coherent and incoherent matched-field processing on signals from passing ships received on a vertical aperture in shallow water. There is no need to know the source spectrum. Broadband and temporal averaging techniques are used to reduce ambiguities and to increase the output Signal-to-Noise Ratio (SNR). The algorithms are robust to environmental mismatch and usually produce an output with at least 10 dB SNR, which is sufficient to identify sediment types. A key element of PGAIT is to consider a range-dependent environme- nt as a sequence of range-independent slabs. The performance of PGAIT is demonstrated at frequencies between 30 and 50 Hz in several sediment conditions, ranging from very soft to very hard. The results show that: 1) the vertical aperture should contain at least 3 hydrophones per wavelength to ensure high quality inversions; 2) coherent (phase-only) matched-field processing outperforms standard intensity processing by about 2 dB in good input SNR conditions; 3) incorrect assumptions about the sound-speed profile (e.g., a bias or incorrect mixed-layer-depth) do not significantly affect the inversion results; and 4) the new range-independent slab approach is computationally intensive, but it can resolve discrete sediment boundary discontinuities.