A wide swath parametric 3.5 kHz sub-bottom profiling system

Although various types of sub-bottom profiling systems, both direct frequency and parametric, are commercially available, none have wide swath capability. The system described in this paper is based on an adaptation of an existing US Navy 12 kHz multi-beam bathymetric sonar system, SASS, but may also be combined with other existing commercially available multi-beam array sonars. Parametric theory indicates that the difference frequency beam pattern produced by the non-linear interaction in the water column of two higher frequencies output from a projector is the same as that of the original higher frequencies but without the side lobes. The idea then is to utilize the existing projector line array to take advantage of its narrow beam in the fore/aft direction. The hydrophone array, and part of the transmitting electronics will also be utilized, and the necessary receiving and processing equipment added to form the complete 3.5 kHz sub-bottom profiler. Additionally, the 12 kHz and 3.5 kHz sonars can operate either simultaneously, or on a time share basis with each system pinging alternately, to maintain the 12 kHz bathymetry function. Modifications to the hardware and firmware of the US Navy 12 kHz system, and the theory leading up to them, are described. Shipboard experiments performed on the modified sonar successfully demonstrated the feasibility of such a system by operating the projector array in a multi-frequency mode to generate sum and difference frequencies. Acoustic measurements were made with a test hydrophone and spectrum analyzer. Additional shipboard tests were performed on the recently acquired target receiver section to determine its capability. System testing, which had been planned for Spring 1995, and follow on work has been delayed indefinitely due to DOD down-sizing and base re-alignment and closure issues.