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Technical Innovations to Domed Scanning Sonar Improve Performance in Deepwater OperationsDaryl MorseMark AthertonKongsberg Mesotech Ltd.Port Coquitlam, BC

Abstract

Scanning sonar has been used on ROVs since the early 1980s. Inthe 1990s, designers enclosed the transducer in an oil-filled dometo provide mechanical protection and eliminate flooding due to O-ring failure on the exposed transducer shaft. While the oil-filleddomed design solved the O-ring flooding failures, it introducedbeam defocusing in two conditions:

WARM & SHALLOWCOLD & DEEP

The beam defocusing effect becomes more extreme in cold deepwater as depth increases. This paper presents a new technologythat addresses the problem of beam defocusing.

27.01.2016 WORLD CLASS - through people, technology and dedication

Scanning Sonar BackgroundEarly 1980s through Late 1990s - Gradual Evolution

Circa 1983– Fixed-frequency, monolithic transducer, typically 330 kHz or 675 kHz– Transducer exposed to sea water– 1000 m depth rating– Analog telemetry– Hardware sonar processor

Circa 1995– Monolithic transducer, protected from sea water by oil-filled

polyurethane dome– 6000 m depth rating


Scanning Sonar – 1980s


Scanning Sonar – 1990s


Scanning Sonar BackgroundLate 1990s through Today – Revolutionary Changes

Circa 1998– Digital telemetry– Software / PC sonar processor

Late-2000s– Variable-frequency, narrow-band, monolithic transducer, typically 300-

400 kHz or 450-700 kHz

Circa 2010s– Variable-frequency, wide-band, composite transducer, typically 300-

525 kHz or 625-1200 kHz• Significant reduction in side-lobes and beam width• Significant improvement in resolution• Significant improvement in usable operating range (minimum range

to maximum range)


Sound Velocity in Sea Water

• Velocity of sound in sea water is well understood• Sound velocity (c) in sea water is a function of:

– Pressure (depth, in meters)– Temperature (T, in degrees Celsius)– Salinity (S, in parts per thousand)

• According to K. Mackenzie (“Discussion of sea water sound-speed determinations”, Journal of the Acoustical Society of America, Volume 70, Issue 3, 1981)– c = 1448.96 + 4.591T - 0.05304T2 + 0.0002374T3 + 0.01630z +

(1.340 - 0.01025T) (S - 35) + 1.675×10-7z2 - 7.139×10-13Tz3

• Increase pressure (depth) → Increase sound velocity• Increase temperature → Increase sound velocity• Increase salinity → Increase sound velocity

• Who’s the boss?– It depends…


Shallow Water ProfileGulf of Mexico (June 2015)


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Deep Water ProfileGulf of Mexico (June 2015)


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Sound Velocity in Oil

• Kongsberg Mesotech uses Shell Naturelle oil, chosen for properties including biodegradability, stable viscosity at low-temperatures and rust / oxidation inhibitors

• The acoustic properties of oil are very different from sea water!


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Sound Velocity in Real Conditions

• The sound velocities of oil and seawater only match at one depth!


Snell’s Law

• Snell’s Law says, sin θ1 / sin θ2 = v1 / v2, where– v1 and v2 are the velocity of sound in medium 1

and medium 2, respectively, and– θ1 and θ2 are the angle of incidence and the

angle of refraction, respectively• This tells us that refraction will occur when a sound

wave passes from one medium to another medium with a different sound velocity

• The sound wave will bend towards normal as it passes from the medium with the faster velocity of sound to the medium with slower velocity of sound (or vice versa)

• Example (4000 m depth): θ1 = 30 deg, v1 = 1670 m/s, v2 = 1530 m/s, θ2 = 27.3 deg


Beam Defocusing

Question: Why does Snell’s Law matter to sonar?Answer: Because oil and water have different sound velocities.

• As sound passes from oil to water (transmit) and water to oil (receive), the beam refracts, which causes it to be defocused.

• The amount of refraction depends on temperature and pressure (depth).• As the depth increases, refraction increases.• The greater the difference in sound velocity, the greater the defocusing.


Beam Defocusing

71m (232.9')

47m (154.2')

70.2m (230.3')

49m (160.8')

Domed oil-filled head at31.6°C (89°F)

Bare transducer 0.9 horizontal beam at 31.6°C (89°F)

Data collected by Kongsberg Mesotech Ltd. with the support of Cochrane Technologies Inc., Lafayette, LA


Acoustic Lens

• Our solution to the problem of beam defocusing is a corrective water-filled acoustic lens, fixed to the transducer, described in A. Barzegar, “Acoustic Lens”, Patent US 20130208570 A1, August 15, 2013.

• The acoustic lens behaves like an optical contact lens, correcting the refraction caused by the oil in the dome.

• Benefits:• Significant reduction in side-lobes and

beam width• Significant improvement in resolution• Significant improvement in usable

operating range (minimum range to maximum range)


Beam Patterns

• At 40 C, with lens• At 40 C, without lens


Data Comparison


Status

• Acoustic Lens technology will be introduced into 1171 4000 m domed sonar production in the June 2016 time frame

• There will be an upgrade path for existing 1171 4000 m domed sonar to the wide-band composite transducers and the Acoustic Lens


Questions?


Visit www.km.kongsberg.com/mesotech for more information.

Contact:Daryl MorseInternational Sales Manager – Underwater Vehicle InstrumentsOffice: +1 604 464 8144Direct: +1 604 468 5220Mobile: +1 604 802 2717E-mail: [email protected]

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