Acoustic Monitoring with the Beaufort Shelf Cabled Observatory

Peter Mikhalevsky

Acoustic and Marine Systems Operation

SAIC, Arlington, VA

Cabled Observatory Workshop

Barrow, Alaska Feb. 7-8, 2005

Acoustics in Arctic Observatories• Acoustic Remote Sensing

– Acoustic Thermometry and Tomography

– Acoustic Halinometry

– Acoustic Digital Current Profilers

– Upward looking Ice Profiling Sonar

– Multi-beam and side scan bathymetry and bottom profiling from AUV’s

– Acoustic backscatter for biologics, effluents

• Ambient Noise Monitoring– Marine mammals (bowhead whales)

– Ice noise (correlated with thickness)

– Seismics

– Manmade noise, seismic surveys, drilling

• Acoustic Communications – Data Telemetry and Network Control

• Subsurface Navigation

Acoustic Remote SensingSpeed of sound is a function of water

temperature and velocity and these can be derived from travel time measurements

• Acoustic Thermometry– Average temperature and integral or

average heat content along the propagation path between source and receiver, scale is function of source-receiver separation.

– Arctic Ocean uniquely suited for acoustic thermometry due to good coupling of acoustic modes and major Arctic water masses

Acoustic Remote Sensing• Transport monitoring with acoustics

– Use reciprocal transmissions to obtain projection of average transport along the propagation path

– Two paths at different angles resolve current direction and speed

– Combination of transport and heat measurements can provide heat flux

• Acoustic Tomography– Use many intersecting paths to obtain

spatial resolution and circulation, scale function of path density

Acoustic Remote Sensing: Examples• Convection Studies

– Greenland Sea 1988-1989– Mediterranean Sea 1991-1992, THETIS I

• Transport Studies– Gibraltar 1996

• Basin Scale Studies – heat content– Mediterranean Sea 1994, THETIS II– North Pacific 1996-1999, 2002-?, ATOC– Arctic Ocean, 1994, 1998-1999, ACOUS

Dushaw, et al.,”Observing the Ocean in the 2000’s: A Strategy for theRole of Acoustic Tomography in Ocean Climate Observation” inObserving the Oceans in the 21st Century, GODAE Project Office

Transport Monitoring in the Straits of Gibraltar

Send, et al., Deep Sea Res., 2002

Acoustic measurements blackCurve, current meters red

Monitoring Temperature in the Western Mediterranean

Send, et al., Nature,1997

Acoustic measurement ofaverage temperature 0-2000m~30 m°C uncertainty

Arctic Climate Observations using Underwater Sound (ACOUS)

• US/Russia bilateral program started in 1992• Use acoustic thermometry to measure Arctic

Ocean temperature and heat content• Feasibility exp. in 1994 showed strong coupling

between travel times and AIW temp., observed basin scale AIW warming (~.4 °C avg. max)

• Source installed in Oct. 1998 with transmissions every 4 days to receive array in Lincoln Sea (array recovered in Spring 2001)

• Reception of source signals at APLIS ice camp in April 1999 showed continued warming in AIW consistent with SCICEX CTD’s (~.5°C avg. max)

MAJOR ARCTIC OCEAN WATER MASSES ARE WELL SAMPLED BYACOUSTIC MODES/RAYS

Acoustic Remote Sensing in the Arctic

Mikhalevsky, “Arctic Acoustics”, in Encyclopedia of Ocean Sciences, 2001

MODAL ARRIVALPATTERN 1962BEAUFORT SEA

MODAL ARRIVALPATTERN 1994TRANS-ARCTICPROPAGATIONEXPERIMENT

Mode 2 arrival ~ 2 secs earlier than modelbased on climatology

20 Hz

STABLE ACOUSTIC PROPAGATION in the ARCTIC OCEAN

ACOUS SOURCE – Franz Victoria StraitACOUS RECEIVE ARRAY - Lincoln Sea

106, 20 min transmissions every four days Oct. 10, 1998 to Dec. 8, 1999

ACOUS - LINCOLN SEA EXPERIMENT

ACOUS SOURCE Deployed OCT. 1998

ACOUS VLA Deployed OCT. 1998 Recovered MAR. 2001

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DIFFERENCE IN ARRIVAL TIME OF MODE 1 and MODE 2

SCICEX 99

Temperature sectionusing profiles fromearly 1990’s perturbedto fit “cooler” part ofacoustic record

Temperature sectionusing profiles fromlate 1990’s perturbedto fit late 1999 “warm”part of acoustic record

Modeled sections consistent with acoustic data

ACOUS-LINCOLN SEA

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A seasonal component can be clearly seen in the acoustic data

Modeled seasonal cycle*

Received acoustic energyon Lincoln Sea array

Synoptic long term monitoring of sea ice

*D.A. Rothrock et.al., GRL,v.26(23), 1999

oFiltered long period component

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Bathymetry along the pathLomonosov Ridge

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TEMPERATURE SECTIONS FROM SCICEX 1995, 1998, AND 1999 ACROSSTHE ARCTIC BASIN WITH CORRESPONDING BATHYMETRY. WARMING INTHE ATLANTIC LAYER IS EVIDENT IN TOPOGRAPHICALLY GUIDED EXTENSIONSOF THE ATLANTIC WATER CIRCULATION.

SCICEX CTD SECTIONS

AVERAGE TEMPERATUREOVER RANGE and DEPTHBETWEEN 0°C ISOTHERMS:

1995 - .421°C

1998 - .478°C

1999 - .488°C

Section AverageTemp. vs Travel Timerms fit error ~ 9 m°C

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5 yr increase of ~1012 kJ/m over 2269 km path

is 2.8 W/m2 heat flux

Linear dependence ontravel time of mode 2 (*) and Mode 3 (o)

ACOUSTIC REMOTE SENSING Technical Requirements

• Temperature and Heat content– Good clock for absolute travel time

measurements, rubidium standard 1-8 ms/yr– Acoustic pulse travel time measurement

precision of .5 ms demonstrated in TAP 1994– Signal in Arctic from 1994-1999 for mode 2

(AIW temperature) was ~.4 secs/yr– Acoustic source and receiver technology

very mature– Power requirements for basin scale

observations ~250 watts acoustic power, ACOUS source ~50% efficiency, 20 min transmission ~.17 kWh

ACOUSTIC REMOTE SENSING CAPABILITIES in the ARCTIC

• UNDER RESEARCH– Combine transport and temp. measurements

for heat flux thru Fram Straits etc.– Average sea ice thickness and roughness

over several hundreds of kms– Average thermocline layer depth over

several hundreds of kms– Acoustic halinometry: changes in salinity and

thickness of cold halocline layer

Acoustic Remote Sensing: Advantages

• Permits rapid and repeated measurements over large ocean areas (3000 kt ship!) ie. basin scales for climate oriented studies

• Measures the average properties between the moorings, “extends” coverage of field, the data grows as the product of the number of source and receiver moorings

• Makes possible monitoring in regions where mooring installation/maintenance/recovery is difficult, eg. Arctic, Gulf Stream, Gibralter, Fram Strait, Bering Strait, etc.

VISION FOR EULERIANARCTIC MOORING GRIDBASIN & STRAITSExact number, layout andmooring design determined bymultidisciplinary requirements

Build in stages,Barrow Cabled Observatory,SEARCH, and build on experience from MARS and NEPTUNE International participationwith cable terminations inSvalbard, Greenland, andRussia (as well as US andCanada) will greatly reduceundersea cable costs withcost sharing for systeminstallation and operation

ARCTIC REGIONAL UNDERSEA OBSERVATORY for RESEARCH and ANALYSISAURORA

Acoustic monitoring: bowhead whales• Well documented that acoustic listening provides more

accurate count than visual methods– Possibility to identify and track particular individuals

• Current methods involve establishing ice camps and deploying hydrophones through the ice– Temporary, must be re-established, spring migration only– Subject to weather and ice conditions– Difficult to establish and maintain long baselines– Can drift out of optimal locations

• Cabled observatory with field of distributed hydrophones or several distributed hydrophone arrays– Permanent presence, spring and fall migration– Available during all weather conditions year-round– Optimal sensor locations can be established and maintained

1/31/0228

Phone Locations at the Pacific Missile Range Facility

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A u t o m a t e d M o d e l - B a s e d L o c a l i z a t i o n

P a i r - w i s e s p e c t r a l p a t t e r n c o r r e l a t i o n o n t r a n s i e n t s m e a s u r e s t i m e l a g …

… C o m p a r i s o n o f m o d e l e d a n d m e a s u r e d t i m e l a g s d e t e r m i n e s s o u r c e l o c a t i o n .

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M a m m a l B e h a v i o r S t u d i e s

• L o c a l i z a t i o n s o v e r m a n y h o u r s g i v e c l u e s t o a n i m a l b e h a v i o r

• T r a c k i n g o f p e r s i s t e n t s i n g e r sP o s s i b l e

• C a n p o t e n t i a l l y r e l a t e s u r f a c e ( v i s u a l ) c o u n t s t o v o l u m e t r i c c o u n t s f o r m o r e a c c u r a t e p o p u l a t i o n s t u d i e s

Courtesy of Bruce Howe, Craig Lee and Jason Gobat, APL/UW

RECOMMENDATIONS• Integrated acoustic systems and sensors needed

for observatory planning and design (http://www.oce.uri.edu/ao/IASOO)– Manage frequency spectrum, passive and active– Exploit same signals and sources/receivers for

tomography, data telemetry and control, and navigation• Control noise environment (sources, pumps,

AUV’s, etc.)– Impact on marine habitat and acoustic systems

performance • Undertake detailed cost study for cable-based

mooring undersea Arctic Ocean observing system and develop integrated plan leveraging existing and proposed programs like SEARCH, Neptune and the NSF Ocean Observatories Initiative

BACKUP

Consensus StatementOCEANOBS 1999 Conference*

“In terms of the scientific approach to sampling the full depth of the ocean, globally, acoustic tomography… provides long-path, integral (low wavenumber) measurements of thermal variations to complement those from Argo and satellite altimetry.”

“The Conference concluded that acoustic tomography did represent a potentially valuable approach and that, initially, it should be implemented in the Arctic…”

*Koblinsky and Smith eds.,”Observing the Oceans in the 21st Century”

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EWG Climatology andSCICEX 95, 98, 99, 00

Acoustic Remote Sensing Implementation

• Addition of acoustic hydrophones to moorings during the design phase costs ~$6K/phone for cabled moorings, and ~$26K/phone for autonomous moorings (on board data storage and rubidium clock required for latter), 4-8 phones per mooring required

• Acoustic sources cost $300-500K depending on whether cabled or autonomous

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