EurOCEAN 2000 - Vlaams Instituut voor de Zee

hydrophone array (Multiple Outputs). To compensate for the motion of the tow fish
(acoustic transmitter) a real time electronically stabilised system will be developed, while
for the compensation of the movement of the towed array a special purpose acoustic
device will be developed as well.
4. Theoretical and numerical accurate acoustic wave propagation models as well as inverse
procedures will be investigated. The models and procedures will be generalised to include
realistic cases like oblique interfaces, hybrid velocity profiles (continuously layered and
stepped variations), inhomogeneities and inclusions. Direct inversion computational
methods and parameter estimators, such as an MLE (Maximum Likelihood Estimator),
will be developed and analysed taking into account instrumental factors, such as noise and
the transmitter and receiver characteristics, which will be obtained by extensive calibration
measurement procedures.
5. The theoretical models and inverse procedures will be validated based on the design of
measurement procedures adapted to the wave propagation models. The validation will first
be performed based on tank experiments (laboratory scale), before being applied to data
from sea experiments (real-life). Ground truth will be obtained using conventional
equipment and improved geophysical and geotechnical instruments like high resolution
seismics and a geotechnical module.
6. Data analysis will be performed in order to investigate, in detail, the functional relations
between the estimated acoustical parameters and sedimentological parameters obtained by
ground truth in the sea experiments or from tank experiments, where the simulated sea bed
can be made composed of calibrated sediments.
The data obtained, during the sea experiments, will be compiled in a GIS (Geographical
Information System). The surveys, conducted in selected representative test areas, will,
therefore, lead to the constitution of a well controlled data base on bottom characteristics. This
data base will be available on a CD-ROM and the small test areas may be used as benchmark
for future instrument calibration.
PROJECT METHODOLOGY
In this project, the estimation problem is formulated in the framework of system identification.
The noise on input as well as output data will be taken into account, since, beside direct
inversion schemes, a Maximum Likelihood approach will be used. This approach allows also
the incorporation of the calibration procedure, which is a necessary step, but which has been
omitted in various previous studies, as well as the model validation, since model errors can be
detected and corrected easily as well.
A key issue of this project requires the use of a parametric array. Although the principles of
operation of this type of sonar have been known for many years, the implementation of a
system for sea surveys during rough surface conditions, for example sea-state 5-6. In particular,
automatic beam stabilisation remains an important problem that will be addressed in detail in
this project, so that the array can be programmed to transmit vertically downwards, through a
repeated range of angles or at some preferential angle irrespective of the towed fish pitch.
In this project, it is aimed to demonstrate that the identification of sediment parameters can be
performed using underwater acoustics, because the identification of sediment parameters fits
into a global system identification approach. First acoustic parameters are estimated using
robust estimators and then, using tank experiments and extensive ground truthing, functional
relations between sedimentological and acoustic are inferred.
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