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

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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 caiti@dsea.unipi.it