The design of sidelobe suppressed PVDF beamforming elements for high performance underwater multi-beam swath transducers

In recent years, multi-beam systems have gained widespread acceptance as the hydrographic surveyor´s tool of choice for many reconnaissance as well as survey applications. In this discussion, we present the design and performance of a multi-beam receiver and broad beam projector for a bathymetry system which can collect data 30 times faster than a typical single beam system. Performance requirements of the receiver at 200 kHz realized with PVDF elements which produce a 3 degree beamwidth and sidelobe level of -30 dB by shading the element electrode pattern. These light weight receiving element are mounted in a precision-molded housing to create 30 contiguous beams separated 3 degrees in elevation. Data from all 30 beams are collected simultaneously, and the well-controlled beam shape and low sidelobe minimize interference between the beams. The projector, required to insonify a fan beam 2.5 degrees by 127 degrees, was constructed from 330 PZT elements to achieve a source level of 228 dB re 1 μPa at 1 meter at 5 kW pulse power. With this design approach, artifacts common to electronically formed beams are eliminated. This simplifies signal processing, improves system accuracy, and lowers the overall system cost. Both theoretical simulations and experimental data for the Mills´ Cross configuration of the projecting and receiving elements, the system design principles, theoretical prediction, laboratory test and field measurements on various underwater topologies are presented