Sonar Performance Enhancement in the Littoral Environment

A significant amount of effort has been applied to the extremely complex littoral environment, particularly in the area of acoustics. The Navy's increasing requirements to operate in shallow water necessitates examination of the Navy's models for prediction of shallow water phenomena. Ambient noise in shallow water is not well understood or well modeled. Surf noise is a major contributor to ambient noise in shallow water and offshore. Surf noise has been observed at distances well over 10 km seaward of the surf zone by operational Navy sonars (Stewart, 1994) and by a limited number of measurement exercises (O.B. Wilson, et al., 1985, Fabre and J.H. Wilson, 1995). Surf noise is a shallow water phenomena that is only recently becoming understood. It has been shown that surf noise is broadband noise that propagates from unique surf zones several kilometers to sea and can be dominate measured broadband ambient noise fields (Stewart, et al., 1994). To ensure success in today's shallow water missions, the Navy needs a clear understanding of the propagation characteristics and noise fields. A surf noise model (Wilson, et al., 1997; Fabre and Wilson, 1997) developed by Neptune Sciences, Inc. (NSI) and the Naval Postgraduate school (NPS)is based on an inverse technique in which the surf noise source level density (SLD) in the surf zone is deduced from ambient noise measurements taken many kilometers seaward of the surf zone. The critical factor in applying this inverse technique is to be able to model the transmission loss (TL) accurately from points along the surf zone to the surf noise measurement location seaward. Ambient noise due to shipping and local wind/wave-breaking are estimated and subtracted from the total noise measured. The reason that the site dependent TL modeling is so critical is that, if done accurately, the SLDs estimated for surf should be site independent. The SLD should only depend on frequency, wave height and type, and surf zone width and not on the propagation conditions from the surf zone, seaward. Thus, once derived, the SLD can be used to estimate surf noise in other areas where surf noise measurements do not exist. That is, the surf noise model (the SLD and TL model with inputs) can be used to accurately estimate surf noise at any location in the world. These concepts will be illustrated with past surf noise data analyses performed by NSI/NPS (Wilson et al., 1997; Fabre and Wilson, 1997).