Acoustic near field effects for the Simrad–Mesotech SM 2000 multibeam sonar

Abstract

Targets quantitatively observed with a multibeam sonar based on a truncated circular arc transducer geometry, focused at infinity, display anomalous apparent target strengths at short measurement ranges. Three near field mechanisms, affecting both the sonar transmit and receive responses, are identified: (1) Anomalous inter-elemental differential phase shifts, (2) inter-elemental differential spreading losses, (3) shifts in elemental viewing aspect. With reference to a commercial 200 kHz Simrad–Mesotech SM 2000 multibeam sonar, the relative importances of the three mechanisms are numerically examined at a 1-m target range. Combined effects in both transmit and in receive are further evaluated for target ranges between 0.5 and 20 m. For the instrument considered, near field effects are significant at ranges of 5 m and less and are characterized by systematically reduced target amplitudes after the application of ‘‘normal’’ sonar time variable gain. Systematic broadening of combined transmit–receive response patterns at short target ranges result in target defocusing in sonar imaging applications. Experimentally measured responses are presented for comparison with theory. [Work supported by the DFO National Hydroacoustic Program and generous access to the Defence Research & Development Canada—Atlantic acoustic calibration facilities.]