Horizontal array passive inversion in range-independent environments

Survey techniques for geo-acoustic bottom characteristics are expensive and time consuming. The U.S. Navy has investigated several inversion techniques to characterize seabed sediments, most of which use active sonar and a vertical array. Passive techniques offer the potential to remain covert and extend the area of seabed characterization. This paper describes a passive approach, called Passive Geo-Acoustic Inversion Techniques (PGAIT) that uses matched-field processing on signals from ships of opportunity for water sound speed and sediment characterization. The signals are received on a horizontal array and broadband and temporal averaging techniques are used to reduce ambiguities. The algorithms are robust to environmental model mismatch and usually produce an output that is sufficient to identify sediment types. PGAIT performance is demonstrated at frequencies between 110 and 190 Hz in several sediment conditions, ranging from very soft to very hard and with several water mixed-layer depths. The results show that: 1) inversion can be effective on a horizontal array; 2) sediment properties are much easier to estimate than mixed-layer depths; and 3) incorrect assumptions about the assumed sound-speed profile do not significantly affect the results.