Tutorial: Underwater Acoustic Imaging

Underwater Acoustic Imaging

Tuesday, October 13

Presenters: Dr. Cheng Chi & Dr. Peng Wang
Institute of Acoustics, Chinese Academy of Sciences (China)

Background and Motivation:

Due to the poor visibility of seawater, underwater acoustic imaging is becoming increasingly important for ocean investigation and exploration. Underwater acoustic imaging systems are an indispensable payload of underwater vehicles. Advances in underwater acoustic imaging have enabled different types of practical imaging sonars such as side scan, multi-beam, synthetic aperture and 3D. Among these imaging sonar systems, side scan and multi-beam sonars are invented much earlier than synthetic aperture and 3D sonars. In terms of system implementation, synthetic aperture and 3D sonars are more complex. Consequently, synthetic aperture and 3D sonars are able to obtain more information than side scan and multi-beam sonars. However, there still exist some questions, which have not been addressed in underwater acoustic imaging. Three important questions are listed as follows:

  • How does the current state-of-the-art acoustic imaging technology give the capabilities to users of imaging sonars as well as integrators of underwater vehicle systems to create optimal imaging solutions?
  • What are the advantages and shortcomings of these imaging sonars, when employing them in different applications?
  • What are the existing challenges and potential methods for improving underwater acoustic imaging systems?

Topic Overview:

This is a comprehensive tutorial on the use, design and development of underwater acoustic imaging systems, namely imaging sonars. This tutorial aims at expanding the audience’s knowledge in the domain of underwater acoustic imaging. The current state-of-the-art of different types of imaging sonars, such as side scan, multi-beam, synthetic aperture and three-dimensional (3D), is updated in this tutorial. The existing challenges and future directions of underwater acoustic imaging are discussed. This tutorial is able to equip the audience with the cutting-edge of underwater acoustic imaging. Additionally, how to use different imaging sonars in different applications when integrating underwater vehicle systems is also presented. The tutorial is not only useful for the fresh researchers in underwater acoustic imaging, but also for the wide users of imaging sonars as well as the integrators of underwater vehicle systems.

The subject matter is covered through oral presentation. The audience will learn a lot about the imaging theory, algorithms of image reconstruction, system design, existing challenges, future directions and others. The content outline section presents in detail the topics which are included.

Target Audience:

We anticipate the number of participants is around 50. The target audiences are:

Engineers interested in designing, developing or deploying underwater acoustic imaging systems.

  • Researchers interested in developing imaging sonars, improving algorithms of sonar imaging, or sonar image processing.
  • Early-stage PhD students focusing on underwater acoustic imaging.
  • Integrators of underwater vehicle systems

Content Outline:

The presentation consists of three parts, as follows:

Introduction to underwater acoustic imaging (60min)

The basic theory of side scan, multi-beam, synthetic aperture and 3-D imaging sonars is introduced first. Side scan and multi-beam sonars have been developed and widely applied for more than 50 years. The main problem in these two sonars is the poor imaging quality. Some images obtained by these sonars are also given. The complexity of synthetic aperture and 3-D sonars is much higher than that of side scan and multi-beam sonars. Images obtained by synthetic aperture or 3-D sonars provide more information than images obtained by side scan and multi-beam sonars. The advantages and shortcomings of different imaging sonars are discussed and summarized. This part will conclude that more focuses should be given to synthetic aperture and 3-D sonars.

Synthetic aperture sonar (60min)

The algorithms and sensors used in synthetic aperture sonars are much different from traditional sides can sonars. synthetic aperture sonars need to employ extra sensors such as Doppler velocity log and inertial navigation system to locate each transmitter and receiver accurately. Due to the motion of transmitters and receivers in synthetic aperture sonars, the motion compensation is one of the most important considerations in the image reconstruction algorithms. In recent years, there arises some new trends in the research of synthetic aperture sonars, such as high-speed, downward-looking 3D, multiple-input and multiple-output (MIMO), and bistatic. We describe a fast imaging algorithm developed in our lab for downward-looking 3D synthetic aperture sonars. Some images obtained by the MIMO synthetic aperture sonar developed in our team are shown. Bistatic synthetic aperture sonars are able to obtain more information than monostatic ones. We analyze the advantages of bistatic synthetic aperture sonars and summarize the existing challenges in developing bistatic synthetic aperture sonars.

(Break – 30min)

Real-time 3-D imaging sonar (60min)

Real-time 3D imaging sonars are able to obtain 3D underwater images in real time. First, this part introduces the fast beamforming methods: chirp-zeta transform and nonuniform fast Fourier transform. Then, ultrawideband ultrasparse 3D imaging is presented, which helps decrease the hardware cost dramatically. Finally, the efforts on how to improve the spatial resolution are shown. There exist two directions: hardware and software for improving the resolution. In this presentation, the hardware method is using ultra large ultrasparse UWB 2-D arrays. The software method is the deconvolved conventional beamforming [14]. Some promising simulation and experiment results are shown in this part.

Cheng Chi (S’14-M’16) was born in Anhui Province, China. He got the Ph.D. degree in signal and information processing at Peking University, Beijing, in 2016, and received the B.Eng. degree in electronic engineering from University of Electronic Science and Technology of China, Chengdu, in 2011. Now, he is working as a Research Fellow of Acoustic Research Laboratory, National University of Singapore. He will shift to Institute of Acoustics, Chinese Academy of Sciences at the end of 2019, and act as an assistant professor.

His research interests include signal processing for underwater acoustic imaging, wideband array design, noise cancellation, underwater acoustical communication, ultrasound medical imaging, and broadband acoustic current measurement. Dr. Chi has served as reviewer for several reputed international journals. He is currently the Secretary of the IEEE Oceanic Engineering Society, Singapore Chapter, also serves as a committee member of IEEE OCEANS2020 Singapore and OES Singapore AUV challenge.

Peng Wang was born in Heilongjiang Province, China. He got the Ph.D degree in signal and information processing at University of Chinese Academy of Sciences, Beijing, in 2015, and received the B.Eng. degree in electronic engineering from Northwestern Polytechnical University, Xi’an, in 2010. Now, he is working as an assistant professor of Key Laboratory of Science and Technology on Advanced Underwater Acoustic Signal Processing, Chinese Academy of Science. His research interests include 3-D acoustical imaging, synthetic aperture sonar, passive sonar signal processing.