Standard-target calibration of a parametric sonar over the difference-frequency band, 1–6 kilohertz.

The Kongsberg TOPAS PS18 parametric sub-bottom profiling sonar operates over the frequency band 15–21 kHz, with nonlinearly generated difference-frequency radiation in the band 0.5–6 kHz. The TOPAS transducer mounted on R/V G. O. Sars is flush with the hull in the near-horizontal plane. The sonar has been calibrated by the standard-target method using a 280-mm diam sphere of aluminum alloy 6082 T6 [K. G. Foote et al., J. Acoust. Soc. Am., 121, 1482–1490 (2007)]. The target was suspended beneath the vessel at each of three ranges, successively 100, 200, and 300 m. Because of conditions in Soerfolla fjord on Dec. 10, 2008, the target sphere was moving slowly relative to the vessel. Its instantaneous position was determined by geometrical considerations through synchronous observation with the Simrad EK60/38-kHz scientific echo sounder, with split-beam transducer mounted approximate to the TOPAS transducer. Data were collected for a number of parameter settings for each of three signal types: continuous wave, chirp, and Ricker pulse. Measurements are compared with predictions based on laboratory measurements of the frequency-dependent sensitivities of the parametric transmitter and conventional linear receiver, using a range-compensation function based on theoretical nearfield modeling. [Work partly supported by Norwegian Research Council Grant No. 184705.]