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
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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: Seafloor Imaging and Toxicity:
Assessment of Risk caused by buried wasteAssessment 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:localization: a parametric side-scan sonar (bottom penetration, 3-D imaging capabilities, development of associated visualization tools needed)
• inspection:inspection: multiple view measurements of the scattered 3-D acoustic field
• bioassessment: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 capabilities:scattering 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
verticalhydrophonearray
acousticsource (ROV)(20-40 kHz)
relative distance:acoustic pingers(100 kHz)
distance:max 40 m
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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
θnominal
grazing angle
acousticsource(ROV/AUV)
acousticreceiver(ROV/AUV)
ρscatteringdirection
d
φ
x
φ,x: desired distanceand relative angle(known from d,θ,ρ)
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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
xxx
xxf
0xxxxfu
uxfx
xx21
uxfx
T
T
T
)(
)(
))((
)(
2
2
V
V
V
V
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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 ...)
0
21
2
2
eVeu
exV
eV
ux
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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)
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Example: subtask 2
field)-(far /)(*)(
litydirectionaeiver source/rec :,
level pressureeiver source/rec :,
2xSLDDP
DD
PSL
RS
RS
2//1
:CLF tentativeas choose
0,
,)*(/1
:* As
2
SLPV
kk
kkSLP
)/(
!measurable :,
00),*(
)*(
Vu
V
Vu
kV
u
absoluteorientation
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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 [email protected]