Introduction 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 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
What is Multibeam Sonar?
• 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
Introduction to Multibeam – NOAA Hydro Training 2009
What is Multibeam Sonar?
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
Introduction to Multibeam – NOAA Hydro Training 2009
Single-Beam vs. Multibeam Coverage
Introduction to Multibeam – NOAA Hydro Training 2009
Sounding Density
Single Beam Density Selected Soundings
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 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
What data are made by SWMB systems?
• The angle of the beam along which the acoustic
pulse traveled, relative to the receive center
– Referred to as “Launch Angle” or “Beam Angle”
Beam 101 is
starboard-most beam
in Reson 8101
systems
Beam 1 is port-most
beam in NOAA
systems
Reson 8101 is 150degree system
Introduction to Multibeam – NOAA Hydro Training 2009
What data are made by SWMB systems?
• 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!
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
Amplitude
+180
0
-180
0
10
20
range (m)
30
Phase
+180
0
-180
0
10
20
range (m)
30
• 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
Amplitude
0
80
120
160
-180
200
range (m)
Phase
+180
0
80
120
160
range (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
-180
200
What data are made by SWMB systems?
• An intensity time series of the bottom return
Amplitude
+180
Depth=Speed X Time
0
-180
0
10
20
range (m)
30
– Travel time is T0 to Centroid
or Leading Edge of return
Phase
+180
– SWMB sonars also can output the angle independent imagery
0
• Side Scan Imagery is the received intensity georeferenced across the
20
30
entire swath10 - the entire time range
sampling
period
(m)
Introduction to Multibeam – NOAA Hydro Training 2009
0
-180
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 secondlargest 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
Limitations of SWMB Systems
• 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
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
z
Positioning system antenna
Stern
Yaw
VRU
Pitch angle (TSS)
Starboard
y Port
X
Gyro
LL
YLL
Roll angle
Multibeam transducer
x
Bow
Direction of vessel travel
• All measurements are critical to the error budget calculation!
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
RESON 8125
http://www.reson.com/sw245.asp
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 kts
Introduction to Multibeam – NOAA Hydro Training 2009
Array configuration – Flat Face
RESON 8125
Introduction to Multibeam – NOAA Hydro Training 2009
Array configuration – Flat Face
Simrad EM3000
•
Navigation Response Teams &
NOAA ship Nancy Foster
•
•
•
300 kHz
127 beams
Flat Face Transducer!
Introduction to Multibeam – NOAA Hydro Training 2009
Array configuration – Flat Face
Simrad EM3000 Beam
Pattern
Introduction to Multibeam – NOAA Hydro Training 2009
Array configuration – Flat Face
• 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
Introduction to Multibeam – NOAA Hydro Training 2009
Array configuration – Flat Face
ELAC Bottomchart MkII
SE 3353 Imaging and
Mapping II:
Submarine Acoustic
Methods
© J.E. Hughes
Clarke, OMG/UNB
Introduction to Multibeam – NOAA Hydro Training 2009
Array configuration – Flat Face
Launch Elac 1180 installation
Rainier Elac 1050D installation
LF
HF
Introduction to Multibeam – NOAA Hydro Training 2009
Array configuration – Flat Face
Elac Beam Pattern
Introduction to Multibeam – NOAA Hydro Training 2009
Surface Sound Speed
•
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)
Flat-face
transducer
Water
Introduction to Multibeam – NOAA Hydro Training 2009
Array configuration – Curved Face
Simrad EM1002
NOAA 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
Introduction to Multibeam – NOAA Hydro Training 2009
Array configuration – Combination
• 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
Introduction to Multibeam – NOAA Hydro Training 2009
Array configuration – Combination
RESON Seabat 8101 / 8111
Introduction to Multibeam – NOAA Hydro Training 2009
Array configuration – Combination
Introduction to Multibeam – NOAA Hydro Training 2009
Array configuration – Combination
• 100 kHz
• NOAA Ship Fairweather
• Depths to 1000m under
good conditions
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
Introduction to Multibeam – NOAA Hydro Training 2009