Introduction 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 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

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

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

Single-Beam vs. Multibeam Coverage

Sounding Density Single Beam Density Selected Soundings

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

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

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

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 http://www.falstad.com/ripple/ phased array1.

Beam Forming – Discrete Summation 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 We can listen to multiple sectors simultaniously.

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

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

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

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

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

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

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

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

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!

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

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

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 http://www.reson.com/sw245.asp

Array configuration – Flat Face RESON 8125

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

Array configuration – Flat Face Simrad EM3000 Beam Pattern

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

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

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

Array configuration – Flat Face Elac Beam Pattern

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

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

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

Array configuration – Combination RESON Seabat 8101 / 8111

Array configuration – Combination

Array configuration – Combination 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 Elac 1180 and 1050D Reson 8101 Roll-compensated No roll-compensation

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 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

Multibeam Coverage Comparison Sonar Arrays Multibeam Coverage Comparison