Sonar performance quantification: report on two DLs

Kenneth G. Foote, IEEE Fellow, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 USA      

Introduction

The author gave two seminars under the aegis of the OES Distinguished Lecturer Program in conjunction with the OCEANS Conference in Seattle, Washington, USA, which was held 27-30 October 2019. Each of these seminars was preceded by a short presentation on OES as an IEEE member society, with emphasis of these benefits of membership: networking, access to OCEANS Conference proceedings and JOE, and ready opportunity to pursue topics such as sonar performance quantification through OES, noting its joint hosting of OCEANS Conferences; conduct of other meetings, symposia, and workshops; and the OES Standards Initiative, with Internet presence.

Information about the DLs is given in the following. The preambles were prepared specifically for the Beacon readership.  They would have been understood implicitly by members of the audience with professional interest in the respective subject.

First Distinguished Lecture

Title: Optical measurement of transducer vibration, including acousto-optic effect compensation

Time: 24 October 2019, 1500 PST

Place: Applied Physics Laboratory (APL), University of Washington, Seattle

Audience: The University of Washington, including especially APL scientists and engineers, and the public.

Preamble: A transducer is a device that converts energy from one form to another. An acoustic transducer converts a mechanical vibration, e.g., pressure fluctuation, to an electrical signal, e.g., voltage, and vice versa. The effectiveness of this conversion process is critical to many applications of transducers. Examples of applications include the biological: finding and quantifying fish in the water column; the hydrographic: bathymetric mapping; the industrial: chemical processes stimulated by sound; the medical: diagnostic imaging and diverse therapies using ultrasound; the military: detecting and locating mines on the seafloor; the offshore oil and gas industry: surveying applications to floating, standing, and bottom structures, including pipelines and pipeline routes. All of these applications are addressed by acoustic instruments, which presume a particular standard of transducer operation. How do we know that the transducer is operating as it should? What happens when things go wrong? Sometimes the answer involves detailed measurement of transducer vibration. This is casually believed to be straightforward, but a figurative wrinkle – namely the acousto-optic effect – has upset, precluded, or complicated measurement results since the beginning of optical interferometric measurement of vibration, at least as early as 1974. The DL addresses this matter.

Abstract: The connection of transducer vibrations and acoustic radiation is well known, but are the transducer vibrations themselves known? Optical measurement methods based on laser interferometry are described. These include homodyne and heterodyne laser interferometers, and laser Doppler velocimeters, whose operating principles are reviewed. Use of a pellicle, defined as a very thin, optically reflecting and acoustically transmitting membrane, which is suspended in the radiation field, is presumed. This enables convenient measurement configurations, sometimes allowing direct measurement of acoustic vibrations. In general, however, the acoustic field is unknown, especially in the transducer nearfield. This impacts the optical measurement, for the acoustic wave changes the local index of refraction along the path of the transiting laser beam. This acousto-optic effect is treated generally by solving the governing integral equation. In this way, acoustic vibrations can be measured without regard to the proximity of pellicle and transducer.

Follow-up reading: K. G. Foote and P. D. Theobald, “Acousto-optic effect compensation for optical determination of the normal velocity distribution associated with acoustic transducer radiation,” J. Acoust. Soc. Am., 138, 1627-1636 (2015); doi 10.1121/1.4929372

Second Distinguished Lecture

Title: Acoustic quantification of water-column scatterers

Time: 1 November 2019, 1400 PST

Place: School of Aquatic and Fishery Science, University of Washington, Seattle

Audience: Scientists and engineers at the University of Washington, NOAA Northwest Fisheries Science Center including an Oregon laboratory by video link, NOAA Alaska Fisheries Science Center, sonar manufacturers, other institutions, and the public.

Preamble: Acoustic quantification of fish has been practiced for a half-century. Improvements have been pursued vigorously throughout this period. An operation that is essential to the quantification process is sonar calibration. The DL addresses this subject through the simplest, most accurate and rapid, hence cost-effective, method: that of the standard target.

Abstract: The two traditional methods of acoustic quantification of water-column scatterers, e.g., fish, namely echo counting and echo integration, are briefly reviewed. Both involve the acoustic sampling volume. This stochastic quantity is defined operationally. Echo integration additionally involves the scattering cross section. This is also defined operationally. The standard-target sonar-calibration method is then described and applied to measurement of the volume scattering coefficient in both the energy and spectral domains. The method is indeed robust, treating the sonar and its transducers, whether monostatic or bistatic, as a black box. The claim is supported by reference both to obsolete sonars whose output once consisted of markings on a strip-chart recorder, and some current sonars whose output is essentially an image. It is further bolstered by reference to a method that enables a calibration performed under a particular set of environmental conditions to be extended to the generally different environmental conditions of application.

Follow-up reading: K. G. Foote, “Standard-target calibration of active sonars used to measure scattering: Principles and illustrative protocols,” IEEE J. Oceanic Eng. 43, 749-763 (2018); doi 10.1109/JOE.2017.2713538 [Date of Publication: 14 July 2017]

Concluding remarks

Process: As a matter of process, these steps were followed in seeking support under the OES DL Program: preliminary inquiries to potential hosts, preliminary inquiry with attached abstracts to the VPTA on the possibility of qualifying for DL support, preparation and submission of a budget to the VPTA. After VPTA approval, the DLs were confirmed with the identified hosts. It can be imagined that there is a lead time to be observed. This is necessary both for OES processing and to find mutually agreeable dates close to the those of the principal event.

Acknowledgement: The VPTA, Dr. Malcolm Heron, is thanked for supporting the lectures described here.