Introduction to Multibeam – NOAA Hydro Training 2009 Introduction to Multibeam

Introduction to Multibeam – NOAA Hydro Training 2009

Introduction to MultibeamIntroduction to Multibeam


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)


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)


What is Multibeam Sonar?

• Increased:

• Bottom Coverage

• Productivty

• Resolution

• Confidence


• 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



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



Single-Beam vs. Multibeam Coverage


Single Beam Density Selected Soundings

Sounding Density


Sounding Density

Multibeam - Navigation Surface Depth Model


Multibeam transducer anatomy

• Earliest and Simplest Systems used a Mill’s Cross• Transmit Ping, Receive Beams


Beam Patterns

Transmit and Receive Beams From a Mills Cross Array


Phased Array & Beam SteeringWe could physically move the array to steer the beam

Or we could adjust the relative phase of the transducer elements


Beam Patterns

Beam Forming – Discrete Summation


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


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


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


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


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


• 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


• 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!



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


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


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)


• 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



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


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


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


• 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


Operational Limitations

• Down-slope signal loss

• Grazing angle on shoals

• Biological interference

• Mechanical Interference

• Instrumentation Cross-talk• Launch Liveliness


Multibeam Offsets & Errors

Multibeams are much more sensitive than singlebeams to measurement offsets and errors.

And, we are much more likely to notice.


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


Multibeam Systems

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


Array configuration

• Flat – EM3000– Reson 8125– SeaBeam/Elac

• Curved– EM1002

• Flat transmit/Arc receive– Reson 8101

• Arc transmit/Flat receive– Reson 7125


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



RESON 8125


Simrad EM3000• Navigation Response

Teams & NOAA ship Nancy Foster

• 300 kHz• 127 beams• Flat Face Transducer!



Simrad EM3000 Beam Pattern



• 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



ELAC Bottomchart MkII

SE 3353 Imaging and Mapping II: Submarine Acoustic Methods

© J.E. Hughes Clarke, OMG/UNB



Launch Elac 1180 installation

Rainier Elac 1050D installation LFHF



Elac Beam Pattern



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)


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


• 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


RESON Seabat 8101 / 8111





• 100 kHz

• NOAA Ship Fairweather

• Depths to 1000m under good conditions



Multi Transducer Arrays

RESON 7125

NOAA Ship Thomas Jefferson & NOAA Ship Rainier new Launches


Multi Transducer Arrays

NOAA Ship Hi’ialakai

• Simrad EM3002D– High resolution in shallow

water– 300 kHz– 508 beams, up to 200°

swath


Reson 8160

• 50 Khz

• NOAA Ship Fairweather

• Depth range to 3000 meters


NOAA SWMB Systems

• Reson 8101• No roll-compensation

• Elac 1180 and 1050D• Roll-compensated


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


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


Sonar Arrays

Multibeam Coverage Comparison

Documents

Introduction to Multibeam – NOAA Hydro Training 2009 Introduction to Multibeam