Calibration sphere for low-frequency parametric sonars

Abstract

The problem of calibrating parametric sonar systems at low difference frequencies used in backscattering applications is addressed. A particular parametric sonar is considered: the Simrad TOPAS PS18 Parametric Sub-bottom Profiler. This generates difference-frequency signals in the band 0.5-6 kHz. A standard target is specified according to optimization conditions based on maximizing the target strength consistent with the target strength being independent of orientation and the target being physically manageable. The second condition is expressed as the target having an immersion weight less than 200 N. The result is a 280-mm-diam sphere of aluminum. Its target strength varies from −43.4 dB at 0.5 kHz to −20.2 dB at 6 kHz. Maximum excursions in target strength over the frequency band due to uncertainty in material properties of the sphere are of order ±0.1 dB. Maximum excursions in target strength due to variations in mass density and sound speed of the immersion medium are larger, but can be eliminated by attention to the hydrographic conditions. The results are also applicable to the standard-target calibration of conventional sonars operating at low-kilohertz frequencies.

Broadband Ultrasonic Target Strengths of Hollow Ceramic Flotation Spheres

“…While the method was first developed and applied to narrowband but finite-bandwidth systems [6], the method is also being used for broadband systems [7]. Recently, a standard target has been designed for use in calibrating the difference-frequency band of a parametric sonar, 0.5-6 kHz, but with applicability to other mid-frequency sonars [8].…”

Discriminating between the nearfield and the farfield of acoustic transducers

“…Admittedly, the array was virtual, existing in space away from the transducer, as the formation of an exceptionally directional beam at very low frequencies depends on the cumulative effect of the nonlinearity inherent in the medium on the propagating and interacting primary, high-frequency waves. 34 A scheme for quantification of parametric sonar echoes has been developed, 35,36 drawing on a computational model for the difference-frequency nearfield of the parametric sonar, i.e., the nearfield. 37,38 In some underwater geophysical investigations, measurements are inevitably made in the nearfield, for example, with high-frequency sidescan sonars operating at short ranges.…”

Range compensation for backscattering measurements in the difference-frequency nearfield of a parametric sonar

“…When available, results for the standard-target calibration 42,43 performed during the first cruise 9 will be used in frequency-dependent scaling factors so that the rangecompensated echograms can be expressed in absolute physical units of volume backscattering strength. This will enable measurements made in the mid-frequency sonar band to be compared with echo measurements made at ordinary ultrasonic frequencies, e.g., 18, 38, 120, and 200 kHz, among others.…”