acoustical imaging of buried seafloor waste: challenges for autonomous underwater vehicles

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ACOUSTICAL IMAGING OF BURIED SEAFLOOR WASTE: CHALLENGES 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. Overview. Motivation : the SITAR project - PowerPoint PPT Presentation

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  • ACOUSTICAL IMAGING OF BURIED SEAFLOOR WASTE:CHALLENGES FOR AUTONOMOUS UNDERWATER VEHICLESA. Caiti

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

  • OverviewMotivation: the SITAR projectInspection of buried waste by multiple-view measurement of the acoustic scattered fieldExperimental configuration within SITARBeyond SITAR: use of (semi?)autonomous vehicles for scattering measurementsLyapunov-like control techniques

  • SITAR Seafloor Imaging and Toxicity: Assessment of Risk caused by buried wasteAcoustical imaging, biotoxicology, decision support systemsEU 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

  • SITAR project: motivationsToxic dumping in shallow and close seasforbidden by the London Convention (1975)covert practice after 1975partial or complete burial of pre-London dumpingseven for known sites, lack of information for a rational risk assessment

  • Toxic waste dumping: a case studyChemical munition waste dumped in the Baltic Sea after WW-II65.000 Tons of munition and warfare agents, including mustard gas and other arsenic compoundsContainers state preservation: from perfectly preserved to totally corrodedQuantity of buried containers: unknown

  • Risk assessment of dumping sites: needsMaps of containers distribution at the site (localization)State of preservation, exact location, orientation of each container (inspection)Characterization of biological effects (bioassessment)

  • Risk assessment of dumping sites: available toolslocalization: side-scan sonarinspection: cameras (from ROVs)bioassessment: concentration measurements and acute toxicity analysisLack of tools for localization and inspection of buried wasteLack of tools for bioaccumulated toxicity evaluation

  • SITAR developmentslocalization: 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 fieldbioassessment: relative measurements of in-situ bioaccumulated toxicity

  • Multiple view measurement of the scattered fieldreconstruction 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)

  • Acoustic eigenrays

  • Model prediction capabilities: arrival times

  • Model prediction capabilities:scattering strenghtthick line: experimental data

    thin line: model predictions

  • Multiple view scattering measurement: minimal requirements2-D scattering angle sampling: 20 at each transmitted grazing angleDirectional 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

  • SITAR experimental configuration

    vertical hydrophone array

    acoustic source (ROV)

    (20-40 kHz)

    relative distance: acoustic pingers (100 kHz)

    distance:

    max 40 m

  • SITAR experimental configurationUseful for test-of-concept experimentEvident drawbacks for repeated inspections of a large number of containersBeyond SITAR: explore the possibility of multiple view scattering measurements with (semi?) autonomous vehicles in cooperation

  • Beyond SITAR

    EMBED Word.Picture.8

    scattering direction

    acoustic receiver (ROV/AUV)

    acoustic source (ROV/AUV)

    nominal grazing angle

    d

    x

    ,x: desired distance and relative angle

    (known from d,,)

    _1062419727.doc

    acoustic source (ROV)

    (20-40 kHz)

    _1062420021.doc

    acoustic source (ROV/AUV)

    acoustic receiver (ROV/AUV)

    (20-40 kHz)

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

  • Subtask 1: align with desired relative angleFrom current position and attitude, move upward until detection of the transmitted signal, at fixed attitudeChoose maximization of the received acoustic energy as stopping criterion

  • Subtask 2: attitude correctionFrom reached position, the receiving vehicle changes attitude to align with the transmitted signalChoose maximization of the received acoustic energy as stopping criterion

  • Subtask 3: distance correction Keeping the attitude fixed, move to the desired distance xUse time-of-flight measurements to estimate the distanceRequires clock synchronization between the vehicles

  • Control Lyapunov functions

  • A Control Lyapunov Function (CLF) approach to subtasks executionEasy case: subtask 3 Let e = x* - x be the measured distance errorPure kinematic model (but plenty of space for robust design, backstepping, change of coordintaes ...)

  • The more difficult cases: subtasks 1&2Basic idea: apply the same CLF approachHowever, in subtasks 1&2, the error cannot be measuredDefine 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)

  • Example: subtask 2

    absolute orientation (

  • Subtask 2: conditions and requirementsWhat does it mean: as *? It depends on source/receiver beam pattern and signal to noise ratioStep-by-step exploration of the admissible configuration spaceCommunication and synchronization among source/receiver vehicles

  • ConclusionsMotivations and goals of the SITAR project: development of tools for inspection of buried toxic wasteMultiple view scattering measurements with semiautonomous vehicles in cooperationUse of CLF: advantages and drawbacks

  • ReferencesI. 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, 2000M. 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@dsea.unipi.it