Deep ocean survey and search using synthetic aperture sonar

Deep ocean towed surveying is an ideal application for synthetic aperture sonar (SAS). The towed platforms move slowly to maintain operational depth. Operational frequencies are necessarily low to provide adequate coverage range, yet the desire for improved resolution at range demands longer arrays. A new 60kHz SAS system employing a three stave interferometric swath bathymetry component is presented. The bathymetric data are analyzed for performance and compared with International Hydrographic Organization (IHO) standards. Explanations of an optional Synthetic Aperture Interferometric Bathymetry processing package are undertaken showing increased data density which by filtering to produce a decimated set provides lower apparent noise from multiple scatterers. Low frequency, high resolution SAS systems will have area coverage rates comparable to conventional sidescan sonars but will provide resolution that is hard to match, even with the highest frequency survey grade conventional sidescans. A brief discussion examines some benefits of SAS in this regard. Recent surveys for buried cables and wreck searches have revealed the benefits of using lower frequency sidescan sonars. The low frequencies penetrate bottom surface layers of thin sediment to identify plow hazards and search objects while higher frequency systems reflect off of the water-sediment acoustic interface to present imagery devoid of harder material actually present.