Acoustic Robustness of Two Standard Spheres for Calibrating a Broadband Multibeam Sonar

The Simrad MS70 is a broadband scientific multibeam sonar with nominal operating frequency band 70-120 kHz. Two spheres have been specified for use in calibrating this sonar, with piecewise coverage of the band and target strength values in the range from -35 to -30 dB. The sphere material is tungsten carbide with 6% cobalt binder; the diameters are 75 and 84 mm. The same spheres can also be used as standard targets for calibrating the Simrad ME70 scientific multibeam echo sounder. The target strengths of such spheres at ultrasonic frequencies depend on the sphere material properties and the physical properties of the immersion medium. In this study, the frequency dependences of the corresponding target strengths are given for ±2% variations in mass density, longitudinal-wave sound speed, and transverse-wave sound speed of the sphere, and for variations in mass density and sound speed of seawater that span the ordinary total range expected between winter and summer. In practice, there need be no uncertainty in knowledge of the sphere material properties; this sensitivity study quantifies potential errors incurred by using nominal rather than measured values of the several parameters. Similarly, for the physical properties of seawater, there need be no uncertainty in knowledge of these, hence in target strength; measurement of the seawater properties in situ at the time of calibration will enable compensation to be made to high accuracy. It should be possible to determine the target strength of the two calibration targets to within ±0.1 dB.