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

1. 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

2. 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

3. SITARSeafloor 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

4. 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

5. 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

6. 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)

7. 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

8. 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

9. 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)

10. Acoustic eigenrays

11. Model prediction capabilities: arrival times

12. Model prediction capabilities:scattering strenght thick line: experimental data thin line: model predictions

13. 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°

14. SITAR experimental configuration

15. 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

16. Beyond SITAR

17. 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

18. 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

19. 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

20. 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

21. Control Lyapunov functions

22. 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 ...)

23. The more difficult cases: subtasks 1&2 • Basic idea: apply the same CLF approach • However, in subtasks 1&2, 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)

24. Example: subtask 2

25. 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

26. 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

27. 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@dsea.unipi.it