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RESEARCH
2. Simulation of sonar signals and scene
interpretation
Simulating signals is a crucial tool in remote
detection, because it is very complicated and
costly to carry out experiments in situ. For years,
an evolving sonar signal simulator has been
under construction. After attempts with a
simulator based on acoustic ray launching, the
current energy tube engine makes it possible to
calculate precise energy balance. Until now,
simulations were carried out with tubes that only
deal with signal amplitude, in addition to the
propagation history.
Simulated sidescan sonar images using real bathymetric data (French
coast near Paimpol area).
Consequently, the current work on the sonar
simulator mainly concerns the generation of the
full signal using both convolutions with the scene
and acoustic tube propagation history. A first step
was taken in the testing of the notion of discrete
backscatterers: this concept allows a coherent
signal to be exactly computed. This work was the
subject of a publication at the Oceans 2008
conference. Currently, the integration of this
functionality is underway in the energy-based
engine of the simulator. Additionally, producing
speckle noise based on this backscatterers
approach is producing first results.
3. Tridimensional reconstruction
of underwater "scenes" from sonar image
sequences
The perception of underwater scenes from
optical sensors (video, for example) is a task too
difficult, even impossible, as the turbidity of the
water increases. This is why the use of new
DIDSON-type high-resolution acoustic cameras
appears to be so interesting. In effect, these
sensors, due to their ease of use and the high
level of resolution of the images they produce,
make them powerful tools in areas such as semiautomatic
control of port infrastructures
(detecting, identifying and automatically mapping
signs of use, for example) or the observation of
underwater fauna. Given the small size of these
acoustic cameras, these inspection approaches
can be carried out by autonomous vehicles (AUV)
with a front-looking camera. Thus, depending on
the vehicle trajectory and attitude, the
underwater scene is observed several times,
with different incidence angle. Sequences of
sonar images are thus collected where the
entities involved in the scene appear several
times, under a variety of angles. The aim of this
study is therefore to use this redundancy of
observation to reconstruct the scene observed in
3D, from a series of 2D images.
Geometrical acquisition model for DIDSON imaging sensor.
Phase simulation resulting from phase delay using several extended
scatterers
Several scenarios can be imagined, according to
the nature of either the camera movements or
the behaviours of the entities involved in the
scene. With a static scene observed by a moving
camera, or a scene observed by a static camera,
but whose entities are moving, it is possible to
draw on stereovision techniques to deduce from
a couple of sonar images the topography of the
underlying scene. Several major differences
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