Underwater Sensor Networks: The physical layer

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Underwater Sensor Networks:The physical layer

Short tutorial on the physics of sound in underwater environments.

Jens M. HovemScientific Advisor SINTEF-ICT

Professor emeritus NTNU [email protected]

Sensorcom2009 Athens

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An overview

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Applications of Marine Acoustics

• Military applications and surveying of coastal areas, vessels and mines.• Fishery acoustics and abundance estimation of fish and plankton.• Sea floor acoustics, hydrography and mapping of the topography and structure of the

sea floor and sub-bottom. • Underwater communication • Underwater navigation and position reference systems.• Acoustic remote sensing of the oceanic conditions.

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Issues in Acoustic Underwater Communication

Low data rates: Bandwidth limitations, absorption, noise.Channel characteristics: Range (time) and frequency shift (Doppler). Coherence degradationOceanic variability: Significant problem, but the seriousness dependent on positions of the nodes.Transmitters/receivers: No standards (jet) and limited availability.

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Applications of UAN

• Environmental data: Large area, very low datarates, long lifetimes.

• Protection of critical structures: Harbors, oil and gas pipes, offshore production faclities

• Event driven applications: Oil and gas leakage, pollution monitoring, equipment malfunctioning.

• Stationary sensor networks (often backup system for other systems)

• Ad-hoc sensor networks

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Sensor network

•

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Underwater navigation and positioning

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Autonomous Underwater Vehicles

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Future sensor network-concept

10 km

10 km

100 sensors

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Ray modeling of underwater propagation conditions

depth

range

sound speed

sourcereceiver line

Elastic half space

Fluid sediment layer

1.Computes the received field from a source to a number oft receivers located on a horizontal line. The bottom can be a fluid sedimentary layer over an elastic half space and both can be range dependent.

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Sound speed − m/s

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Angles=24 : −23° Sd. = 25 m

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I =12

Time − ms

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1.Coherently additions of all multi-path contributions to produce broad band time and frequency field descriptions.

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Seasonal variations in sound propagation

1460 1480

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Sound speed (m/s)

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Feb 1999

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Sd=5 m, Rd=255 m, Angles= -30° : 30°

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Sound speed (m/s)

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April 1998

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Sd=5 m, Rd=255 m, Angles= -30° : 30°

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Sound speed (m/s)

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June 1999

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Sd=5 m, Rd=255 m, Angles= -30° : 30°

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Sound speed (m/s)

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July 1999

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Sd=5 m, Rd=255 m, Angles= -30° : 30°

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Sound speed (m/s)

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th (m

)Oct 1999

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Sd=5 m, Rd=255 m, Angles= -30° : 30°

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Sound speed (m/s)

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th (m

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Nov 2006

0 0.5 1 1.5 2 2.5 3 3.5

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Range (km)

Sd=5 m, Rd=255 m, Angles= -30° : 30°

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Ambient noise

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Range limitationsSpherical spreading loss and salt water attenuation

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Range - m

Tran

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- dB

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Range (m)Frequency (kHz)

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Modeling the communication link from platform to a bottom production unit

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DIP2

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Sd=50 m, Rd=255 m, Angles= -30° : 30°

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th (m

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DIP2

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Sd=50 m, Rd=255 m, Angles= -8° : 3°

direct surface & bottom bottom only surface only

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DIP2: Sd=50 m, Rd=255 m

Freq = 1000 HzFreq = 12000 HzFreq = 25000 HzFreq = 50000 Hz

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Reduced time (s)

Ran

ge (k

m)

DIP2: Sd=50 m, Rd=255 m

(a) (b)

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Transmission losses as function of frequency and range

Source and receiver at 290 m, 10 m above the bottom.

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Freq = 10000 HzFreq = 30000 HzFreq = 50000 Hz

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Sensor network-concept

10 km

10 km

100 sensors

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Generic Marine Sensor Unit (GMSU)

Signal processing module•Data conditioning and storage •Data reduction •Storage for retrieval

BatteriesSensor module and interface:•Temperature, Depth, (pressure), Salinity•Optical•Hydrocarbon•Other sensors specific for the mission

Ballast tank

Acoustic module:•Echo sounder for fish and plankton •Wireless underwatercommunication

Acoustic transducerAcoustic modem

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Conclusion and recommendations

• We ( the underwater people) have a lot to learn from you(the earth people)

• If you want to be successful and useful in our field, youneed to understand our problem

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What to learn more….?

To be published by:

Peninsula Publishing Company, Los Altos, California, USA