Underwater Acoustics

News

nderwater Acoustic Imaging: Exact Geometrical-Acoustics Treatment of the Image Due to a Specular Reflector (Abstract) (Full report) David G. Blair. 2009.

Dr John Dunlop and Dr Andrew Madry are conducting research into acoustics. The OTG has recently issued a report entitled "Acoustic reflectivity at megahertz frequencies" by Andrew Madry.

Dr Dunlop

Dr Dunlop examining a tile from the Underwater Acoustic Imaging System (see abstract below).

HIGH RESOLUTION UNDERWATER ACOUSTIC IMAGING

Ian S F Jones

Ocean Technology Group

University of Sydney,

Australia

ABSTRACT

A high resolution underwater imaging system using sparse array technology has been developed and tested in turbid waters around Australia. High resolution three dimensional imaging implies many voxels. The present system can image 103x103 x4x103 voxels although faster images can be obtained by zooming to a smaller volume of interest. With three dimensional images, precise dimensions of complex shapes such as pipe joints can be obtained. Potential applications in offshore platform inspection are discussed.

The longer wavelengths of Megahertz acoustic signals produces less scattering than light while array technology allows the imager to take advantage of pulsed ensonification to further reduce backscatter fogging of the image. The advantage of acoustics is most pronounced in very turbid waters.

The megahertz frequencies allow millimeter resolution with array sizes compatible with ROVs. Digitization takes place on the acoustics tile that make up the array and the signals are then passed by fiber optical cable to the above surface processor to reduce the volume and underwater weight of the system

There are a number of obstacles to producing an innovation in underwater imaging. Worries about adequate coherence lengths to support arrays capable of adequate resolution were found to be groundless. One impediment to success was the computing loads implied by multi-element arrays and large voxel numbers. To attack this problem international groups were funded to provide potential solutions and a decreasingly smaller number of groups supported at each review point. This technique has produced a lower cost approach for the underwater acoustic camera design.

INTRODUCTION

Finding objects in turbid waters with underwater video is a problem due to the lack of visibility. High frequency acoustic signals suffer less scattering in turbid waters, so an attempt has been made to produce an innovation in high resolution underwater acoustic imaging. The innovation team needed to solve three problems before the "underwater camera" could produce resolutions of millimeteres at ranges of meters. It needed to model sparse arrays to determine the number of sensing elements (thousands), to produce an economical acoustic sensor tile that could be used to form such large element arrays and finally to simplify the data processing to be within the capabilities of today's computers.

These problems have been solved and the successful testing of the prototype has cleared the way for commercialisation and possibly wide diffusion of the technology to produce an innovation. Uses for the technology range from counting fish in aquaculture farms to finding bodies in dams and rivers. The production of dimensioned engineering drawings of damaged underwater structures by exploiting the three dimensional nature of the information is a novel use of the technology.

For the full paper please read Oceans '99 ISBN 0-933957-25-4.

CURRENT PROGRESS

The Australian Government is expected to award Thompson Marconi Sonar a further contract to develop a production prototype of an acoustic mine imaging system able to be fitted to ROV for use in mine clearing operations. It is based on Underwater Acoustic Imaging technology.

ACOUSTICS FOR DETECTION OF ARTEFACTS IN THE SEA BED (MARINE ACOUSTIC ARCHEOLOGY)

Researchers

  • Phil Mulhearn
  • Shuying Zhang
  • David Blair
  • Final Year thesis students

Project Description

Optical sensors have difficulty seeing underwater because of suspended sediments which scatter and attenuate light. The amount of scattering depends on wavelength so that long waves, like ultrasonic sound waves, are scattered much less than short waves, such as light. This was the physical basis for motivating an Australian Defence R & D program to develop a new technology for high resolution underwater acoustic imaging (UAI) primarily for use in very turbid waters.

A possibly more important application of the new UAI and other ultrasonic devices lies with the opportunity to detect objects in muds and lightly consolidated sediments. Generally low frequency acoustics are more suitable for use in sediments because the attenuation of sound is high in such materials. However low frequency implies poor resolution for sonar devices of practical size. We expect to show there is a range of acoustic frequencies in the low megahertz range which can penetrate 10 cm or so of mud.

We see the important uses for high frequency acoustics devices in muds as:

  1. Searching for evidence (guns, bodies) disposed of as a result or criminal or terrorist activities.
  2. Marine archaeology prospecting and identification
  3. Mapping of benthic fauna burrows in soft sediments for biological and ecological studies.

Our initial investigations will focus on the extent to which ultrasonic devices can detect sub-surface objects in soft sediments, in the laboratory. The next stage will be to carry out field programs with the prototype UAI and other ultrasonic equipment to study benthic fauna burrows and the detection of buried artefacts.

prototype UAU

The new prototype UAU, circled, on a Remotely Operated Vehicle

Ultrasonic images of a G-clamp

Ultrasonic images of a G-clamp, compared with an optical view. The prototype UAI obtains three-dimensional data so it can digitally rotate an object and view it from any direction.

Professor Shuying Zhang

Professor Shuying Zhang of the Shanghai Acoustics Laboratory, Academic Sinica with the fluid instrument developed under a joint programme with the Ocean Technology Group.