ACOUSTICAL IMAGING OF BURIED SEAFLOOR WASTE: CHALLENGES FOR AUTONOMOUS UNDERWATER VEHICLES

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ACOUSTICAL IMAGING OF BURIED SEAFLOOR
WASTECHALLENGES FOR AUTONOMOUS UNDERWATER
VEHICLES

• A. Caiti
• ISME Interuniv. Ctr. of Integrated Systems for
  the Marine Environment,
• DSEA Dept. Electrical Systems Automation,
  Univ. of Pisa, Italy

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Overview

• Motivation the SITAR project
• Inspection of buried waste by multiple-view
  measurement of the acoustic scattered field
• Experimental configuration within SITAR
• Beyond SITAR use of (semi?)autonomous vehicles
  for scattering measurements
• Lyapunov-like control techniques

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SITAR Seafloor Imaging and Toxicity Assessment
of Risk caused by buried waste

• Acoustical imaging, biotoxicology, decision
  support systems
• EU funded project, partners
• - Universities of Trondheim, Stockholm (2), Bath
• - Swedish Defence Res. Est., Ecole Navale (Brest)
• - Swedish Environmental Prot. Ag., ECAT Lithuania
• - Kongsberg Defence Aerospace
• - ISME

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SITAR project motivations

• Toxic dumping in shallow and close seas
• forbidden by the London Convention (1975)
• covert practice after 1975
• partial or complete burial of pre-London dumpings
• even for known sites, lack of information for a
  rational risk assessment

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Toxic waste dumping a case study

• Chemical munition waste dumped in the Baltic Sea
  after WW-II
• 65.000 Tons of munition and warfare agents,
  including mustard gas and other arsenic compounds
• Containers state preservation from perfectly
  preserved to totally corroded
• Quantity of buried containers unknown

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Risk assessment of dumping sites needs

• Maps of containers distribution at the site
  (localization)
• State of preservation, exact location,
  orientation of each container (inspection)
• Characterization of biological effects
  (bioassessment)

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Risk assessment of dumping sites available tools

• localization side-scan sonar
• inspection cameras (from ROVs)
• bioassessment concentration measurements and
  acute toxicity analysis
• Lack of tools for localization and inspection of
  buried waste
• Lack of tools for bioaccumulated toxicity
  evaluation

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SITAR developments

• localization a parametric side-scan sonar
  (bottom penetration, 3-D imaging capabilities,
  development of associated visualization tools
  needed)
• inspection multiple view measurements of the
  scattered 3-D acoustic field
• bioassessment relative measurements of in-situ
  bioaccumulated toxicity

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Multiple view measurement of the scattered field

• reconstruction of 3-D object characteristics from
  2-D slices of the scattered field
• scattering strength as a function of grazing
  angle and scattering angle (figures from Hovem
  Karasalo, 2000 tank experiment, acoustic source
  500 kHz)

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Acoustic eigenrays
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Model prediction capabilities arrival times
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Model prediction capabilitiesscattering strenght
thick line experimental data thin line model
predictions
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Multiple view scattering measurement minimal
requirements

• 2-D scattering angle sampling ? 20 at each
  transmitted grazing angle
• Directional source/receivers, transmission at
  20-40 kHz (wavelenghts 4-8 cm)
• Acoustic pingers (?100 kHz) to assess
  source/receiver relative position (? max
  source/receiver distance ?40 m)
• Azimuthal sampling? 30

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SITAR experimental configuration
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SITAR experimental configuration

• Useful for test-of-concept experiment
• Evident drawbacks for repeated inspections of a
  large number of containers
• Beyond SITAR explore the possibility of multiple
  view scattering measurements with (semi?)
  autonomous vehicles in cooperation

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Beyond SITAR
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Requirements

• directional acoustic pingers on both
  source/receivers vehicles for relative
  positioning control (attitude and distance)
• bi-directional acoustic communication
• station keeping capabilities
• movement from one position to another as a task
  accomplished in three subtasks

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Subtask 1 align with desired relative angle

• From current position and attitude, move upward
  until detection of the transmitted signal, at
  fixed attitude
• Choose maximization of the received acoustic
  energy as stopping criterion

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Subtask 2 attitude correction

• From reached position, the receiving vehicle
  changes attitude to align with the transmitted
  signal
• Choose maximization of the received acoustic
  energy as stopping criterion

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Subtask 3 distance correction

• Keeping the attitude fixed, move to the desired
  distance x
• Use time-of-flight measurements to estimate the
  distance
• Requires clock synchronization between the
  vehicles

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Control Lyapunov functions
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A Control Lyapunov Function (CLF) approach to
subtasks execution

• Easy case subtask 3
• Let e x - x be the measured distance error
• Pure kinematic model (but plenty of space for
  robust design, backstepping, change of
  coordintaes ...)

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The more difficult cases subtasks 12

• Basic idea apply the same CLF approach
• However, in subtasks 12, the error cannot be
  measured
• Define a tentative CLF V in terms of the measured
  acoustic pressure level
• Move in steps in the directions minimizing V
  (somehow similar to other approaches proposed in
  visual feed-back applications)

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Example subtask 2
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Subtask 2 conditions and requirements

• What does it mean as ???? It depends on
  source/receiver beam pattern and signal to noise
  ratio
• Step-by-step exploration of the admissible
  configuration space
• Communication and synchronization among
  source/receiver vehicles

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Conclusions

• Motivations and goals of the SITAR project
  development of tools for inspection of buried
  toxic waste
• Multiple view scattering measurements with
  semiautonomous vehicles in cooperation
• Use of CLF advantages and drawbacks

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References

• I. Karasalo, J.M. Hovem, Transient bistatic
  scattering from buried objects, in Experimental
  Acoustic Inversion Methods for exploration of the
  shallow water environment, Caiti, Hermand, Jesus
  and Porter (Eds.), Kluwer, 2000
• M. Aicardi, G. Casalino, G. Indiveri, New
  techniques for the guidance of underactuated
  marine vehicles, IARP Workshop Underwater
  robotics for sea exploration and environmental
  monitoring, Rio de Janeiro (Brazil), October
  2001.
• A. Caiti (coordinator), SITAR Description of
  Work, available on request contacting
  caiti_at_dsea.unipi.it