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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 are used (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 vs receive arrays – Mill’s Cross)
• Vessel Attitude & motion and its effects on MBES
• 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 Beam Echosounding (VBES) – Used from 1939 to the
present
– Better coverage than
leadlines
• VBES are still effective
when properly used
– Inshore areas, faster
speeds, general
bathymetry trending
– Faster processing
– Cost-effective
What is Multibeam Sonar?
Introduction to Multibeam – NOAA Hydro Training 2009
SWMB coverage is better
• Less prone to
interpretive error than
SBES
– Improved technology
provides better
resolution
– Can be combined with
Side Scan Sonar (SSS)
coverage
– Also provides precise
backscatter
measurements in some
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 SteeringWe could physically move the array to steer the beam
Or we could adjust the relative phase of the transducer 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 is sensitive to incoming energy can be tuned
SE 3353 Imaging and Mapping II: Submarine Acoustic Methods
© J.E. Hughes Clarke, 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 onto
the same hydrophone array.• We can simultaneously listen in different sectors
Beam 1 Circuit
Beam 2 Circuit
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 then listens in 101 different sectors. Depending upon the range scale in use, it can do this up to 30 times per second
Transmit beam:
Receive beams:
Resulting Multibeam Footprints
• Q: What does a SWMB system meausre ?• A: Travel time, angle, and perhaps some
other 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 acoustic pulse 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-most beam in NOAA
systems
Beam 101 is starboard-most beam
in Reson 8101 systems
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 to be based on homogenous
velocity regime at 1500 meters/second speed of sound
Note that most sound is reflected away in a “flat bottom”,
and not received at the transducer! If power is
increased to make returning signal stronger, this can create
an 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 specular reflection. Bottom detection easy
• Beams with a low grazing angle have less backscatter and longer acoustic signature
Introduction to Multibeam – NOAA Hydro Training 2009
SWMB Bottom Detection
+180
+180
-180
-180
0
0
0 10 20 30range (m)
Amplitude
Phase
0 10 20 30range (m)
• 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
-180
+180
0
+180
-180
0
Amplitude
Phase
80 120 160 200range (m)
80 120 160 200range (m)
• 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 the
entire 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 as towed side scan imagery
• The high aspect of a hull mounted SWMB results in high grazing angles
• High grazing angles result in small shadows– This means reduced target detection, because the eye
sees differences better than objects
• Larger ranges mean bigger footprints, thus lower spatial resolution
Introduction to Multibeam – NOAA Hydro Training 2009
Sound Velocity
• Sound Velocity is second-largest source of error for nearshore surveys (what is the first?)
• Time and effort required for additional casts is ALWAYS less than re-surveying an area, OR cleaning the error-prone data!
• Payoffs in uncertainty and quality of final surface
• YOU control how accurate your data can be
Introduction to Multibeam – NOAA Hydro Training 2009
Limitations of SWMB Systems
• Resolution– Objects smaller than
the wavelength of the system
– Objects smaller than the pulse length transmitted
– Objects smaller than the footprint of the beam
Introduction to Multibeam – NOAA Hydro Training 2009
• Beam width / footprint resolution– Very difficult to identify
narrow objects such as masts and pilings!
– Multiple returns add confidence in resolving whether soundings are on features or are noise
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 than singlebeams to measurement offsets and errors.
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!
Positioning system antenna
VRU
Multibeam transducer x
z
y
Direction of vessel travel
Port Starboard Pitch angle (TSS)
Roll angle
Yaw
Gyro
Y
Bow
Stern
X LL
LL
Introduction to Multibeam – NOAA Hydro Training 2009
Multibeam Systems
A look at some of the multibeam systems 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-Track Beamwidth
1°
Across-Track Beamwidth
0.5° (at nadir)
Accuracy Special Order
Maximum Update Rate
40 Hz
Operational Speed
Up to 12 ktshttp://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 Beam Pattern
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 beam pattern
– Receive beamformer forms 3 beams for each sector
– The system does this across three pings (“rotating”) to form the 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 width affects resolution?)
– Roll-compensated through beam steering
Array configuration – Flat Face
Introduction to Multibeam – NOAA Hydro Training 2009
ELAC Bottomchart MkII
SE 3353 Imaging and Mapping II: Submarine Acoustic Methods
© J.E. Hughes Clarke, OMG/UNB
Array configuration – Flat Face
Introduction to Multibeam – NOAA Hydro Training 2009
Launch Elac 1180 installation
Rainier Elac 1050D installation LFHF
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-face transducerWater
Incoming sound “ray”
• Transducer material sound speed ≠ Water sound speed
• Acoustic ray path “kinks” at transducer-water interface (similar to “pencil in a glass of water” experiment)
• Must be corrected:• Real-time Surface Sound
Speed probe• Digibar or Thermo-
Salinograph (best)
Introduction to Multibeam – NOAA Hydro Training 2009
Simrad EM1002NOAA Ships Thomas Jefferson
and Nancy Foster
Mid-water system• 95 kHz• 111 beams, 2° x 2°• Curved Array constant beamwidth
around the curve (broadside sectors) , optional beam steering beyond
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 under good conditions
Array configuration – Combination
Introduction to Multibeam – NOAA Hydro Training 2009
Multi Transducer Arrays
RESON 7125
NOAA Ship Thomas Jefferson & NOAA Ship Rainier new Launches
Introduction to Multibeam – NOAA Hydro Training 2009
Multi Transducer Arrays
NOAA Ship Hi’ialakai
• Simrad EM3002D– High resolution in shallow
water– 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 ocean depth”
• 12 kHz, 151 beams (1.5° x 1.5°)
• 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