Three European research projects - ACID, SAMI and REBECCA - are throwing new light on underwater geography and geology. Sonars with remarkable visual acuity now make it possible to map seabeds and analyse their nature.
Making use of the reflecting characteristics
of ultrasound, sonars are the major "remote vision" technological
instrument at our disposal to explore the world beneath the sea.
For a number of years, European researchers have been developing
new methods to enable this essential tool to draw up increasingly
precise maps of seabeds and to incorporate qualitative information
on the nature of the sea floor. They have been applying a technique
known as "synthetic aperture" in order to improve resolution.
This leading-edge technology in information processing was devised
when scientists observed the earth by satellite, in order to obtain
high-resolution images with small antennas such as can be provided
by a large parabolic antenna.
The ACID (Acoustical Imaging Development) project, completed in September 1993, had shown that it was feasible to apply this synthetic aperture technique to underwater imaging. Compared with traditional sonars, which could only give still shots, the project developed a synthetic sonar which memorises a collection of echoes from successive emissions in order to simulate a large-scale antenna and to provide high-resolution images.
Trials carried out in the Mediterranean have made it possible to obtain series of acoustic images of the sea bottom of a constant longitudinal resolution of 1 metre and a horizontal resolution of 15cm. In this way, ACID validated the correctness of the approach and the technical solutions proposed, both as regards the sonar and the entire information processing chain on board ship which carries out its processes in real time.
This research has been followed up in practice by two other projects, SAMI and REBECCA, the one focusing on the water-sediment interface and the other on the seabed. The scientists involved in these projects are French (CPE Lyons, CNRS Marseilles, GEODIA and IFREMER), Danish (TUD, DMI and RESON) and British (University of Newcastle, University of Loughborough).
SAMI: underwater reliefs
The SAMI (Synthetic Aperture Mapping Imaging) project
was designed to be able to draw up underwater maps containing topographical
elements, or reliefs. To obtain this result, researchers use stereoscopic
vision with the aid of two sonars working in parallel. By processing
the two images, it is possible to calculate the elevation (equivalent
of altitude) for each point of the image.
The scientists began by carrying out a large number
of particularly detailed tests in a tank to determine the acuity
of "sonar vision" in less favourable circumstances. Trials
at sea then took place off Toulon and Nice in 1995 and 1996. Fifteen
microcomputers, three specialised processing racks, some 6m³
of electronic equipment in all were installed on an ocean research
vessel. Of a higher performance level than ACID, SAMI is also better
in terms of image resolution (less than 50cm) and, what is more,
provides underwater maps. This will provide geologists with more
details for analysing underwater reliefs.
"We would not have been able to carry out this project without international cooperation," observes Manell Zakharia, the project coordinator. "No Member State would have been able to afford an experiment of this kind, involving the creation of such an advanced prototype. The European Commission has not only helped us financially but has also closely followed all our work."
It will take some time to draw up a detailed map
of European seabeds. Mapping the Mediterranean on the basis of the
ultra-fine resolutions provided by SAMI would be equivalent to drawing
a 25m² office with a detail equivalent to a scale of one hundredth
of a millimetre.
REBECCA: characterising bottoms
The task of the REBECCA project (Reflection from Bottom, Echo Classification and Characterisation of Acoustical propagation) was to characterise the geological and geotechnical properties of sea sediments by describing with precision (at least for the first few metres) the nature and properties of the seabed. Such indications are extremely valuable for many subsea projects, such as laying or burying cables or pipelines, planting pylons, etc. They are of particular interest to those working in the offshore oil and gas industries, as a knowledge of sedimentary bottoms is vital when installing rigs.
The results of these research projects are therefore essential for Europe, given the extent of its coastline, not to mention its substantial sea-linked economy. Better knowledge of the topography and the sediments of the coastal strip can only benefit the various sectors whose livelihood depends on them.