Scheme for parametric sonar calibration by standard target

Quantitative backscattering applications of parametric sonar require calibration. A scheme for this based on the standard-target method is described. Several candidate targets are considered for use over the frequency bands spanned by the primary and secondary difference frequencies. For the Simrad TOPAS PS 18 Parametric Sub-bottom Profiler, which is being applied in fisheries research, the respective bands are 15-21 and 0.5-6 kHz. The suitability of the several targets is theoretically investigated. These are homogeneous solid elastic spheres made of aluminum, copper, or tungsten carbide with 6% cobalt binder. The initial criterion on sphere mass is that this should be less than 10 kg. The frequency dependence of the target strength is computed, and a sensitivity analysis is performed to quantify the range of variability in target strength that might be encountered due to uncertainty in knowledge of the material properties of the spheres or variations in sound speed of the immersion medium. Numerical results are presented for the largest aluminum sphere initially considered, with diameter 160 mm. Because of the weakness in target strength of the spheres at the lowest difference frequencies, the constraint on sphere mass is subsequently relaxed, and larger spheres are considered for use specifically over the difference-frequency band. Target strengths for aluminum spheres with diameters 240 and 280 mm are much larger and may be sufficient for calibration at the lowest frequencies. The importance of the transmit signal spectrum and system frequency response function for determination of operational measures of target strength is noted. The proposed targets may be used to calibrate other low-frequency sonars, both parametric and conventional