Introduction to Multibeam – NOAA Hydro Training 2009
Introduction to MultibeamIntroduction to Multibeam
Introduction to Multibeam – NOAA Hydro Training 2009
Introduction to Multibeam
Topics covered in Introduction to Sonars:
•Introduction to types of sonars and how they areused (MBES, SSS, Inteferometric).
•How do sonars work?
•Materials used to make transducers
•Elements of a sonar
•Sonar beam patterns and their elements.
•Sonar Specifications (frequency, beam width,resolution, accuracy)
Introduction to Multibeam – NOAA Hydro Training 2009
Learning Objectives for MultibeamLearning Objectives for Multibeam
•Beam forming (How can this work)
•Multibeam transducer anatomy (transmit vsreceive arrays – Mill’s Cross)
•Vessel Attitude & motion and its effects onMBES
•Offsets and biases
•Mounting option for MBES transducers
•Error identification (DTM artifacts)
•Coverage and accuracy (as per HSSD)
Introduction to Multibeam – NOAA Hydro Training 2009
What is Multibeam Sonar?
• Increased:
• Bottom Coverage
• Productivty
• Resolution
• Confidence
Introduction to Multibeam – NOAA Hydro Training 2009
•Vertical BeamEchosounding (VBES)
–Used from 1939 to thepresent
–Better coverage thanleadlines
•VBES are still effectivewhen properly used
–Inshore areas, fasterspeeds, generalbathymetry trending
–Faster processing
–Cost-effective
What is Multibeam Sonar?
Introduction to Multibeam – NOAA Hydro Training 2009
SWMB coverage is better
•Less prone tointerpretive error thanSBES
–Improved technologyprovides betterresolution
–Can be combined withSide Scan Sonar(SSS) coverage
–Also provides precisebackscattermeasurements insome systems
What is Multibeam Sonar?
Introduction to Multibeam – NOAA Hydro Training 2009
Single-Beam vs. Multibeam Coverage
Introduction to Multibeam – NOAA Hydro Training 2009
Single Beam Density Selected Soundings
Sounding Density
Introduction to Multibeam – NOAA Hydro Training 2009
Sounding Density
Multibeam - Navigation Surface Depth Model
Introduction to Multibeam – NOAA Hydro Training 2009
Multibeam transducer anatomy
•Earliest and Simplest Systems used a Mill’s Cross
•Transmit Ping, Receive Beams
Introduction to Multibeam – NOAA Hydro Training 2009
Beam Patterns
Transmit and Receive Beams From a Mills Cross Array
Introduction to Multibeam – NOAA Hydro Training 2009
Phased Array & Beam Steering
We could physically movethe array to steer the beam
Or we could adjust therelative phase of thetransducer elements
Introduction to Multibeam – NOAA Hydro Training 2009
Beam Patterns
Beam Forming – Discrete Summation
Introduction to Multibeam – NOAA Hydro Training 2009
Beam Patterns
Using arrays of elements, the direction in which an array issensitive to incoming energy can be tuned
SE 3353 Imaging andMapping II:Submarine AcousticMethods
© J.E. HughesClarke, OMG/UNB
Introduction to Multibeam – NOAA Hydro Training 2009
Beam Forming
•So now we have a steerable single beam
•But, we can add multiple receiver circuits ontothe same hydrophone array.
•We can simultaneously listen in different sectors
Beam 1Circuit
Beam 2Circuit
Introduction to Multibeam – NOAA Hydro Training 2009
What is a “Beam”?
•Transmit energy (“Ping”) is released across the entire swath
–Transmit shown in BLUE
–Receive shown in GREY
–Intersection of transmit and receive samples is what we call a “Beam”The area this covers on the seafloor is called a “footprint”
–This process is called beam forming
Introduction to Multibeam – NOAA Hydro Training 2009
Beam Forming
•The Reson 8101 sends out one pulse, and thenlistens in 101 different sectors. Depending uponthe range scale in use, it can do this up to 30times per second
Transmit beam:
Receive beams:
Resulting Multibeam Footprints
•Q: What does a SWMB system meausre ?
•A: Travel time, angle, and perhaps someother information such as intensity
Introduction to Multibeam – NOAA Hydro Training 2009
Beam Patterns
•Controlling dimensions of beam patterns:
–Array Dimensions (i.e. length or diameter)
–Acoustic Wavelength
–Element Spacing
–Element Shading
•Beam pattern goals:
–Focused main lobe (narrower is better)
–Reduced side lobes (fewer and smaller is better)
–Finding the happy medium
Introduction to Multibeam – NOAA Hydro Training 2009
•The angle of the beam along which the acousticpulse traveled, relative to the receive center
–Referred to as “Launch Angle” or “Beam Angle”
What data are made by SWMB systems?
Beam 1 is port-mostbeam in NOAAsystems
Beam 101 isstarboard-most beamin Reson 8101systems
Reson 8101 is 150-degree system
Introduction to Multibeam – NOAA Hydro Training 2009
•The two-way travel time of the acoustic pulse
Travel-path can be assumed tobe based on homogenousvelocity regime at 1500meters/second speed of sound
Note that most sound isreflected away in a “flat bottom”,and not received at thetransducer! If power isincreased to make returningsignal stronger, this can createan extremely NOISY mess!
What data are made by SWMB systems?
Introduction to Multibeam – NOAA Hydro Training 2009
SWMB Bottom-Detection
•Near-nadir angles have excellent specularreflection. Bottom detection easy
•Beams with a low grazing angle have lessbackscatter and longer acoustic signature
Introduction to Multibeam – NOAA Hydro Training 2009
SWMB Bottom Detection
• Incident Angle of 15 degrees (mostly specular or backscatter?)
• Top graph: amplitude
• Bottom graph: phase
•Amplitude Detection
Introduction to Multibeam – NOAA Hydro Training 2009
SWMB Bottom Detection
• Incident Angle of 75 degrees (mostly specular or backscatter?)
• Top graph: amplitude
• Bottom graph: phase
•Phase Detection (or “Split-Aperture” Detection)
Introduction to Multibeam – NOAA Hydro Training 2009
•An intensity time series of the bottom return
–Travel time is T0 to Centroid or Leading Edge of return
–SWMB sonars also can output the angle independent imagery
•Side Scan Imagery is the received intensity georeferenced across theentire swath - the entire time sampling period
Depth=Speed X Time
What data are made by SWMB systems?
Introduction to Multibeam – NOAA Hydro Training 2009
SWMB imagery is generally not as good astowed side scan imagery
•The high aspect of a hull mounted SWMB resultsin high grazing angles
•High grazing angles result in small shadows
–This means reduced target detection, because the eyesees differences better than objects
•Larger ranges mean bigger footprints, thus lowerspatial resolution
Introduction to Multibeam – NOAA Hydro Training 2009
Sound Velocity
•Sound Velocity is second-largest source of error fornearshore surveys (what is thefirst?)
•Time and effort required foradditional casts is ALWAYSless than re-surveying an area,OR cleaning the error-pronedata!
•Payoffs in uncertainty andquality of final surface
•YOU control how accurateyour data can be
Introduction to Multibeam – NOAA Hydro Training 2009
Limitations of SWMB Systems
•Resolution
–Objects smaller thanthe wavelength of thesystem
–Objects smaller thanthe pulse lengthtransmitted
–Objects smaller thanthe footprint of thebeam
Introduction to Multibeam – NOAA Hydro Training 2009
•Beam width /footprint resolution
–Very difficult to identifynarrow objects suchas masts and pilings!
–Multiple returns addconfidence in resolvingwhether soundings areon features or arenoise
Limitations of SWMB Systems
Introduction to Multibeam – NOAA Hydro Training 2009
Operational Limitations
•Down-slope signal loss
•Grazing angle on shoals
•Biological interference
•Mechanical Interference
•Instrumentation Cross-talk
•Launch Liveliness
Introduction to Multibeam – NOAA Hydro Training 2009
Multibeam Offsets & Errors
Multibeams are much more sensitive thansinglebeams to measurement offsets anderrors.
And, we are much more likely to notice.
Introduction to Multibeam – NOAA Hydro Training 2009
Offsets and biases
•All measurements are critical to the error budget calculation!
Introduction to Multibeam – NOAA Hydro Training 2009
Multibeam Systems
A look at some of the multibeamsystems in use with NOAA today.
Introduction to Multibeam – NOAA Hydro Training 2009
Array configuration
•Flat
–EM3000
–Reson 8125
–SeaBeam/Elac
•Curved
–EM1002
•Flat transmit/Arc receive
–Reson 8101
•Arc transmit/Flat receive
–Reson 7125
Introduction to Multibeam – NOAA Hydro Training 2009
Array configuration – Flat Face
Frequency
455 kHz
Swath Angle
120°
Coverage
3.5 x depth
Depth Range
120 m
Number of Beams
240
Along-TrackBeamwidth
1°
Across-TrackBeamwidth
0.5° (at nadir)
Accuracy
Special Order
Maximum UpdateRate
40 Hz
OperationalSpeed
Up to 12 kts
http://www.reson.com/sw245.asp
RESON 8125
Introduction to Multibeam – NOAA Hydro Training 2009
Array configuration – Flat Face
RESON 8125
Introduction to Multibeam – NOAA Hydro Training 2009
Simrad EM3000
•Navigation Response Teams &NOAA ship Nancy Foster
•300 kHz
•127 beams
•Flat Face Transducer!
Array configuration – Flat Face
Introduction to Multibeam – NOAA Hydro Training 2009
Simrad EM3000 BeamPattern
Array configuration – Flat Face
Introduction to Multibeam – NOAA Hydro Training 2009
•SeaBeam/Elac 1050D and 1180
–Flat-face transducer
–1180: 180 kHz (max effective range ~350m)
–1050D: 180 kHz and 50 kHz (max effective range ~3000m)
–System pings into 14 sectors -- focused transmit beampattern
–Receive beamformer forms 3 beams for each sector
–The system does this across three pings (“rotating”) to formthe complete swath: 14 x 3 x 3 = 126 beams
–Why? Focus more energy using less power
–1.5 by 2.5-degree beam width (remember how beam widthaffects resolution?)
–Roll-compensated through beam steering
Array configuration – Flat Face
Introduction to Multibeam – NOAA Hydro Training 2009
ELAC Bottomchart MkII
SE 3353 Imaging andMapping II:Submarine AcousticMethods
© J.E. HughesClarke, OMG/UNB
Array configuration – Flat Face
Introduction to Multibeam – NOAA Hydro Training 2009
Launch Elac 1180 installation
Rainier Elac 1050D installation
LF
HF
Array configuration – Flat Face
Introduction to Multibeam – NOAA Hydro Training 2009
Elac Beam Pattern
Array configuration – Flat Face
Introduction to Multibeam – NOAA Hydro Training 2009
Surface Sound Speed
Flat-facetransducer
Water
Incoming sound “ray”
•Transducer material sound speed ≠Water sound speed
•Acoustic ray path “kinks” attransducer-water interface (similar to“pencil in a glass of water”experiment)
•Must be corrected:
•Real-time Surface Sound Speedprobe
•Digibar or Thermo-Salinograph(best)
Introduction to Multibeam – NOAA Hydro Training 2009
Simrad EM1002
NOAA Ships Thomas Jefferson
and Nancy Foster
Mid-water system
•95 kHz
•111 beams, 2° x 2°
•Curved Array constant beamwidtharound the curve (broadsidesectors) , optional beam steeringbeyond
Array configuration – Curved Face
Introduction to Multibeam – NOAA Hydro Training 2009
•Reson 8101
•101 beams, 1.5-degree beam width
–150-degree swath width
–240 kHz (max effective range 100-150m)
–Round-Face Transducer
•Advantages:
–No need for real-time sound velocity
–Can always be corrected in post-processing
•Disadvantages:
–Cannot beam steer
–No motion compensation
Array configuration – Combination
Introduction to Multibeam – NOAA Hydro Training 2009
RESON Seabat 8101 / 8111
Array configuration – Combination
Introduction to Multibeam – NOAA Hydro Training 2009
Array configuration – Combination
Introduction to Multibeam – NOAA Hydro Training 2009
• 100 kHz
• NOAA Ship Fairweather
• Depths to 1000m undergood conditions
Array configuration – Combination
Introduction to Multibeam – NOAA Hydro Training 2009
Multi Transducer Arrays
RESON 7125
NOAA Ship Thomas Jefferson & NOAAShip Rainier new Launches
Introduction to Multibeam – NOAA Hydro Training 2009
Multi Transducer Arrays
NOAA Ship Hi’ialakai
•Simrad EM3002D
–High resolution in shallowwater
–300 kHz
–508 beams, up to 200°swath
Introduction to Multibeam – NOAA Hydro Training 2009
Reson 8160
• 50 Khz
• NOAA Ship Fairweather
• Depth range to 3000 meters
Introduction to Multibeam – NOAA Hydro Training 2009
NOAA SWMB Systems
• Reson 8101
• No roll-compensation
• Elac 1180 and 1050D
• Roll-compensated
Introduction to Multibeam – NOAA Hydro Training 2009
Seabeam 2112
NOAA Ship Ronald H. Brown
•Deep water, “full oceandepth”
•12 kHz, 151 beams (1.5° x1.5°)
•Up to 150° swath width
Introduction to Multibeam – NOAA Hydro Training 2009
New Systems…
•Reson 7101 Series
–Thomas Jefferson
–NRT-7
•Simrad 700 Series
–“Chirp” system improves range and resolution
–EM710 replaces EM1002 in product line
•Interferometry
–Benthos C3D
–GeoSwath
Introduction to Multibeam – NOAA Hydro Training 2009
Sonar Arrays
Multibeam Coverage Comparison
