1. Introduction to Multibeam

2. 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)

3. 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)

4. What is Multibeam Sonar?
Increased:
Bottom Coverage
Productivty
Resolution
Confidence

5. 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

6. 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

7. Single-Beam vs. Multibeam Coverage

8. Sounding Density
Single Beam Density Selected Soundings

9. Multibeam - Navigation Surface Depth Model
Sounding Density
Multibeam - Navigation Surface Depth Model

10. Multibeam transducer anatomy
Earliest and Simplest Systems used a Mill’s Cross
Transmit Ping, Receive Beams

11. Transmit and Receive Beams From a Mills Cross Array
Beam Patterns
Transmit and Receive Beams From a Mills Cross Array

12. Phased Array & Beam Steering
Or we could adjust the relative phase of the transducer elements
We could physically move the array to steer the beam
We could pysically move the array. We have seen examples of this in the previous lecture. This is how a typical shipboard radar works.
Use demo from phased array1.

13. Beam Forming – Discrete Summation
Beam Patterns
Beam Forming – Discrete Summation

14. 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

15. 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
We can listen to multiple sectors simultaniously.

16. 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

17. 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

18. 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

19. 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 150-degree system

20. What data are made by SWMB systems?
The two-way travel time of the acoustic pulse
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!
Travel-path can be assumed to be based on homogenous velocity regime at 1500 meters/second speed of sound

21. 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

22. SWMB Bottom Detection
Incident Angle of 15 degrees (mostly specular or backscatter?)
Top graph: amplitude
Bottom graph: phase
Amplitude Detection

23. SWMB Bottom Detection
Incident Angle of 75 degrees (mostly specular or backscatter?)
Top graph: amplitude
Bottom graph: phase
Phase Detection (or “Split-Aperture” Detection)

24. What data are made by SWMB systems?
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

25. 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

26. 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

27. 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

28. 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

29. Operational Limitations
Down-slope signal loss
Grazing angle on shoals
Biological interference
Mechanical Interference
Instrumentation Cross-talk
Launch Liveliness

30. Multibeam Offsets & Errors
Multibeams are much more sensitive than singlebeams to measurement offsets and errors.
And, we are much more likely to notice.

31. Offsets and biases
All measurements are critical to the error budget calculation!

32. Multibeam Systems
A look at some of the multibeam systems in use with NOAA today.

33. Array configuration Flat Curved Flat transmit/Arc receive
EM3000
Reson 8125
SeaBeam/Elac
Curved
EM1002
Flat transmit/Arc receive
Reson 8101
Arc transmit/Flat receive
Reson 7125

34. Array configuration – Flat Face
RESON 8125
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

35. Array configuration – Flat Face
RESON 8125

36. Array configuration – Flat Face
Simrad EM3000
Navigation Response Teams & NOAA ship Nancy Foster
300 kHz
127 beams
Flat Face Transducer!

37. Array configuration – Flat Face
Simrad EM3000 Beam Pattern

38. 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

39. Array configuration – Flat Face
ELAC Bottomchart MkII
ELAC Example
SE 3353 Imaging and Mapping II: Submarine Acoustic Methods
© J.E. Hughes Clarke, OMG/UNB

40. Array configuration – Flat Face
Launch Elac 1180 installation LF HF
Rainier Elac 1050D installation

41. Array configuration – Flat Face
Elac Beam Pattern

42. 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
Incoming sound “ray”
Digibar or TSG
Refraction at transducer face

43. 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

44. Array configuration – Combination
Reson 8101
101 beams, 1.5-degree beam width
150-degree swath width
240 kHz (max effective range m)
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

45. Array configuration – Combination
RESON Seabat 8101 / 8111

46. Array configuration – Combination

47. Array configuration – Combination
100 kHz
NOAA Ship Fairweather
Depths to 1000m under good conditions

48. Multi Transducer Arrays
RESON 7125
NOAA Ship Thomas Jefferson & NOAA Ship Rainier new Launches

49. Multi Transducer Arrays
NOAA Ship Hi’ialakai
Simrad EM3002D
High resolution in shallow water
300 kHz
508 beams, up to 200° swath

50. Reson 8160
50 Khz
NOAA Ship Fairweather
Depth range to 3000 meters

51. NOAA SWMB Systems Elac 1180 and 1050D Reson 8101 Roll-compensated
No roll-compensation

52. 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

53. New Systems… Reson 7101 Series Simrad 700 Series Interferometry
Thomas Jefferson
NRT-7
Simrad 700 Series
“Chirp” system improves range and resolution
EM710 replaces EM1002 in product line
Interferometry
Benthos C3D
GeoSwath

54. Multibeam Coverage Comparison
Sonar Arrays
Multibeam Coverage Comparison