OVERVIEWOVERVIEWHYPACK MULTIBEAM COURSEHYPACK MULTIBEAM COURSE

04/18/23 HYPACK® 2009 Training Seminar 2

What is Multibeam?What is Multibeam?

Multibeam System: 100% Bottom Swath Multibeam System: 100% Bottom Swath Coverage from a Single TransducerCoverage from a Single Transducer

Multibeam transducer

Single beam Coverage

Single transducer head that generates Single transducer head that generates a swath array of bottoma swath array of bottom

• Many swaths per secondMany swaths per second

• High resolution footprintsHigh resolution footprints

• 4545°° to 210° to 210° swath width swath width

Where did Multibeam come fromWhere did Multibeam come from• Developed by US Navy in early-mid 1960s … deep water Developed by US Navy in early-mid 1960s … deep water

“Swath” array systems“Swath” array systems

• ““Shallow Water” applications developed in early 1990s Shallow Water” applications developed in early 1990s What is “Shallow Water”??What is “Shallow Water”??

• First major application was during Mississippi River Flood of First major application was during Mississippi River Flood of 1993 … St. Louis District & John Chance Inc.1993 … St. Louis District & John Chance Inc.

• Cost has dropped from $500K to $Cost has dropped from $500K to $35K35K ( (++) ) (Multibeam Only)(Multibeam Only)

• With peripherals, complete system around With peripherals, complete system around $ 70K$ 70K

• Many dredging firms & A-E contractors are acquiring systemsMany dredging firms & A-E contractors are acquiring systems

What is Multibeam being used forWhat is Multibeam being used for

• Dredge clearance Dredge clearance … full bottom coverage… full bottom coverage

• Dredge paymentDredge payment

• Underwater structural investigation Underwater structural investigation … jetties, … jetties, hydropower plants, river control structures, locks & hydropower plants, river control structures, locks & damsdams

• Routine Routine channel condition channel condition surveyssurveys

• River & Reservoir surveysRiver & Reservoir surveys

Multibeam allows coverage under Vessels along Riverbanks, Piers, and Wharfs

River bank or revetment

Barge

Provides some ability to “see under” vessels … Provides some ability to “see under” vessels … had to wait for vessels to move with single had to wait for vessels to move with single

beam systemsbeam systems

Multibeam allows coverage around Piers, Bulkheads, Locks, Dams and other Underwater Structures

Sheet pile wall

Provides some ability to detect scour or voids Provides some ability to detect scour or voids around underwater structuresaround underwater structures

Erosion scour/void

Is This Really Here??

Single Beam vs. Shallow Water MultibeamSingle Beam vs. Shallow Water Multibeam

• 7.5° typical single beam width.7.5° typical single beam width.• Many passes to resolve extent Many passes to resolve extent

and least depth of a feature.and least depth of a feature.• << 5% bottom coverage<< 5% bottom coverage• Data processing not complex.Data processing not complex.• Acquisition uses established Acquisition uses established

technology.technology.

• Multiple narrow beams in a Multiple narrow beams in a fan-shaped pattern, or swath.fan-shaped pattern, or swath.

• 100% bottom coverage100% bottom coverage• Up to 40 profiles/secondUp to 40 profiles/second

( ( 2000020000 soundings/second ) soundings/second )• Resolve features quicklyResolve features quickly• Acquisition and processing Acquisition and processing

complexcomplex

Ensonified Area or “Footprint” Size Differences

(1) 8.0 degree

(7) 3.0 degree7) 3.0 degree

(19) 1.5 degree(19) 1.5 degree

(121) 0.5x1.0 degree(121) 0.5x1.0 degree

Footprint Size of 8.0 degree is 6.99 feet in 50 foot deep water

Typical Beam Angles & Swath WidthsTypical Beam Angles & Swath Widths

Swath Width vs. Water Depth

Degrees

2 x90°2.5 x102.7°

4.29 x130°4 x126.8°

3.46 x120°3 x112.6°

Swath Width vs. Water Depth

Degrees

7 x148.1°6 x143.1°5 x136.4°

150°157.4°161.1°

7.4 x10 x12 x

100’

740’

Remember This Illustration !!

2*TAN (MB Angle/2) = Swath Width Multiplier

TECHNICALTECHNICALHYPACK MULTIBEAM COURSEHYPACK MULTIBEAM COURSE

Multibeam Transducers & Multibeam Transducers & Beam GeometryBeam Geometry

Top-down (plan) view of transmit main lobe and Top-down (plan) view of transmit main lobe and receive main lobe intersection as projected onto receive main lobe intersection as projected onto the seafloor…with highlighted cross product.the seafloor…with highlighted cross product.

• Multibeam transducers are Multibeam transducers are typically based on cross-fan typically based on cross-fan geometry…a transmit array and geometry…a transmit array and a receive array in an “L” or “T” a receive array in an “L” or “T” configuration.configuration.

• Each array consists of multiple Each array consists of multiple identical transducer elements, identical transducer elements, equally spaced, in a line. Arrays equally spaced, in a line. Arrays can be flat or curved.can be flat or curved.

• Each array produces a flattened Each array produces a flattened main lobe which is narrow in the main lobe which is narrow in the array’s long axis.array’s long axis.

• The intersection of the two The intersection of the two flattened main lobes results in a flattened main lobes results in a narrow beam.narrow beam.

• Every “firing” or transmission of Every “firing” or transmission of the array is called a “ping”. the array is called a “ping”. During one ping, all beams are During one ping, all beams are transmitted & received.transmitted & received.

View of 16 near-View of 16 near-nadir beams nadir beams formed by a formed by a multibeam multibeam transducer.transducer.

Multibeam DirectivityMultibeam Directivity

• Across-track Across-track directivity is directivity is provided by provided by receiverreceiver

• Along-track Along-track directivity is directivity is governed by governed by transmittertransmitter

Multibeam MethodsMultibeam Methods

Beam Forming and InterferometryBeam Forming and Interferometry

The differences between these two methods have to do with the way the Angle-Travel Time pairs are determined.

A Beam Forming System Beam Forming System determines the Travel Time as a function of Angle (thus excluding the possibility of different travel times corresponding to a single angle)

An Interferometric System Interferometric System determines the Angle as a function of Travel Time (thus excluding the possibility of different angles corresponding to a single travel time).

Method AdvantagesMethod Advantages

Beam FormingBeam Forming InterferometryInterferometry

• Acceptable Imagery (Side Scan)

• Bathymetry excellent for high resolution mapping of irregular seafloors

• Good for all types of bathymetry requirements

• Wide swath of data, both bathy and imagery

• Good Imagery (Side Scan)

• Good for general bathymetry

• Wider Swath of data

Multibeam BasicsMultibeam Basics

• All multibeam systems use the same All multibeam systems use the same basic approachbasic approach

• The bottom is ensonified by a transmit The bottom is ensonified by a transmit beam ... Up to 40beam ... Up to 40 times times per second per second

• The echo return is received and The echo return is received and processedprocessed

• Individual beam travel time estimates Individual beam travel time estimates are converted into are converted into slant rangesslant ranges

• True True depth is computed by applying depth is computed by applying beam angles, orientation, motion and beam angles, orientation, motion and position data, and sound velocity position data, and sound velocity profile data.profile data.

( Same as Land Surveying with a Total Station )( Same as Land Surveying with a Total Station )

Multibeam Depth/Slope Range Measurement TechniquesMultibeam Depth/Slope Range Measurement TechniquesAmplitude & Phase DetectionAmplitude & Phase Detection

Most multibeam systems use either and/or Most multibeam systems use either and/or both both amplitude amplitude and and phasephase detection methods. detection methods.

Various physical & electronic “Beamforming” Various physical & electronic “Beamforming” techniques are used to detect which directional techniques are used to detect which directional point on the received multibeam array is being point on the received multibeam array is being

observedobserved

transducer

Multibeam Depth/Slope Range Measurement TechniquesMultibeam Depth/Slope Range Measurement TechniquesAmplitude & Phase DetectionAmplitude & Phase Detection

Most multibeam systems use either and/or both Most multibeam systems use either and/or both amplitude amplitude and and phasephase detection methods. detection methods.

• Amplitude detectionAmplitude detection relies on finding the travel time of relies on finding the travel time of an echo’s interception with the bottoman echo’s interception with the bottom. . Typically Typically determined using center of mass method. (Similar to determined using center of mass method. (Similar to single beam depth measurement methods).single beam depth measurement methods).

• Phase detectionPhase detection (Interferometric technique)(Interferometric technique) relies on relies on finding the phase shift at two subsections of the finding the phase shift at two subsections of the transducer receive array. (Somewhat analogous to transducer receive array. (Somewhat analogous to EDM or EDM or GPS carrier phaseGPS carrier phase).).

Amplitude detectionAmplitude detection relies on finding the relies on finding the travel time of an echo’s interception with the travel time of an echo’s interception with the bottombottom. . Typically determined using center Typically determined using center of mass method. (Similar to single beam of mass method. (Similar to single beam depth measurement methods).depth measurement methods).

peak return signal amplitudepeak return signal amplitude

(travel time … slope distance)(travel time … slope distance)

AMPLITUDE

transducer

Phase detectionPhase detection (Interferometric technique)(Interferometric technique) relies on finding the phase shift at tworelies on finding the phase shift at two subsections of the transducer receive array. subsections of the transducer receive array. (Somewhat analogous to (Somewhat analogous to GPS carrier phaseGPS carrier phase).).

receivedreceivedsignal phasesignal phase

phase shiftphase shift

(function of slope distance)(function of slope distance)

PHASE or INTERFEROMETRIC

transducer

Amplitude & Phase DetectionAmplitude & Phase Detection

• Amplitude detectionAmplitude detection is typically used for the is typically used for the inner beams (0° to ~45° off-nadir)inner beams (0° to ~45° off-nadir)

• The The change-over point change-over point between between amplitude amplitude and phase detectionand phase detection varies by design; varies by design; methods include absolute cutoff, real-time methods include absolute cutoff, real-time analysis of each beam, and analysis of each beam, and combinationcombination amplitude & phase (~45° to ~60° off-nadir) amplitude & phase (~45° to ~60° off-nadir)

• Phase detectionPhase detection is typically used for the outer is typically used for the outer beams (~60° out to ~100° off-nadir)beams (~60° out to ~100° off-nadir)

• Caution:Caution: depth accuracy can change at depth accuracy can change at bottom detection transition pointsbottom detection transition points

Ping Cycle OperationPing Cycle Operation

Ping GenerationPing Generation

Transmit PulseTransmit Pulse

Bottom DetectBottom Detect

Reflection ReturnReflection Return

Receive SignalReceive Signal

Amplify SignalAmplify Signal

Process SignalProcess Signal

Display Processed SignalDisplay Processed Signal

Multibeam Frequencies & Resolution Multibeam Frequencies & Resolution

• As Frequency and Water Temperature change, so does attenuation As Frequency and Water Temperature change, so does attenuation due to acoustic energy due to acoustic energy Absorption, Absorption, transmission loss due to transmission loss due to conversion of acoustic energy into heat . Increasing Absorption , will conversion of acoustic energy into heat . Increasing Absorption , will cause the Gain to increase more rapidly as the Range increases.cause the Gain to increase more rapidly as the Range increases.

• As Range increases, so does attenuation due to spherical spreading of As Range increases, so does attenuation due to spherical spreading of acoustic energy called acoustic energy called Spreading LossSpreading Loss (Beam Footprint). No true (Beam Footprint). No true loss of energy, the energy is simply spread over a larger surface area, loss of energy, the energy is simply spread over a larger surface area, thus reducing its density.thus reducing its density.

• Compensating Compensating for Absorption and Spreading Loss is best done by the for Absorption and Spreading Loss is best done by the use of use of TTime ime VVariable ariable GGain (ain (TVGTVG))..

• When When TVGTVG is used, the Gain values in the receivers at Nadir, will start is used, the Gain values in the receivers at Nadir, will start at a level and then increase, as a function of time, throughout the at a level and then increase, as a function of time, throughout the receive cycle.receive cycle.

Beam Width & ResolutionBeam Width & Resolution

• Vertical resolutionVertical resolution is the minimum differential in depth determined is the minimum differential in depth determined between adjacent acoustic returnsbetween adjacent acoustic returns

– The shorter the pulse length, greater the resolutionThe shorter the pulse length, greater the resolution– Pulse length corresponds to the bandwidth of the sonar Pulse length corresponds to the bandwidth of the sonar

frequencyfrequency– The higher the sonar frequency, the broader the bandwidthThe higher the sonar frequency, the broader the bandwidth– The higher the sonar frequency, the more accurately you can The higher the sonar frequency, the more accurately you can

resolve the distance measurement.resolve the distance measurement.

• Vertical resolution Vertical resolution is also influenced by beam width. A is also influenced by beam width. A pulse from a wider beam (e.g. 3.0°) will require more time pulse from a wider beam (e.g. 3.0°) will require more time to be reflected than a pulse from a narrower beam to be reflected than a pulse from a narrower beam

• (e.g. 0.5° or 1.5°).(e.g. 0.5° or 1.5°).

Beam Width & ResolutionBeam Width & Resolution

• The wider beam will cause an The wider beam will cause an “echo smear”“echo smear” which can which can lead to lead to reducedreduced vertical depth resolutionvertical depth resolution and and horizontal horizontal positioningpositioning… i.e., … i.e., Accuracy Accuracy

• The echo smear is a function of frequency, bottom The echo smear is a function of frequency, bottom topography, depth, and beam width.topography, depth, and beam width.

• In brief, the narrower the beam width, the better the In brief, the narrower the beam width, the better the vertical and horizontal resolution.vertical and horizontal resolution.

.

.

..

.

. ..

.

““Echo Smear”Echo Smear”

Beamforming TechniquesBeamforming Techniques

Equal Beam Spacing

Equal Footprint Spacing

Constant angle per beam

Constant alongtrack distance per beam

Advantages of Real-Time Beamforming with a Combined Amplitude Advantages of Real-Time Beamforming with a Combined Amplitude and Interferometric Phase Detection Multibeam Systemand Interferometric Phase Detection Multibeam System

• Roll corrected beam steerageRoll corrected beam steerage

• Variable beam angle control Variable beam angle control (equal footprint)(equal footprint)

• Increased outer beam confidenceIncreased outer beam confidence

• User control of swath width and User control of swath width and number of beamsnumber of beams

Multibeam SystemsMultibeam SystemsRecognized Limitations & DrawbacksRecognized Limitations & Drawbacks

• $$ High -- complete system cost … $$ High -- complete system cost … $70K to $400K$70K to $400K..

• $15K $15K single beam does the job … always has.single beam does the job … always has.

• Limited applications in shallow waters … Limited applications in shallow waters … < 15 ft< 15 ft..

• Don’t always need Don’t always need 100%100% coverage … high shoaling areas coverage … high shoaling areas

• Added processing time. Added processing time. New techniques being implemented to New techniques being implemented to reduce processing time 10 – 40 fold!!reduce processing time 10 – 40 fold!!

• Increased system calibration requirements.Increased system calibration requirements.

• Data quality in outer beams must be continually assessedData quality in outer beams must be continually assessed..

Multibeam SystemsMultibeam SystemsRecognized Limitations & DrawbacksRecognized Limitations & Drawbacks

• Is it Too much data ??Is it Too much data ??– Some ultimate data users prefer traditional sparse cross-sections.Some ultimate data users prefer traditional sparse cross-sections.– No Problem, just “cut” them out of the Multibeam dataNo Problem, just “cut” them out of the Multibeam data– End up throwing out End up throwing out 99%99% of collected data! of collected data!

Despite the above, acceptance & use of multibeam Despite the above, acceptance & use of multibeam technology is increasing … future standard surveytechnology is increasing … future standard survey system.system.

For Example: For Example: You collect Multibeam over an area and its You collect Multibeam over an area and its final XYZ data set is final XYZ data set is 400 MB 400 MB … then you cut Cross Sections … then you cut Cross Sections through this data set at through this data set at 250’ intervals 250’ intervals and choose to save a and choose to save a soundings soundings every 10’ every 10’ . When completed, your resulting . When completed, your resulting Cross Section XYZ file is now Cross Section XYZ file is now only 4 MBonly 4 MB. This is a reduction . This is a reduction of of 99%99% of the total data set. of the total data set.

Multibeam BasicsMultibeam Basics

• Transducer Mounting Transducer Mounting OrientationOrientation Biases Biases – [Patch Test for Roll-Pitch-Yaw Offsets][Patch Test for Roll-Pitch-Yaw Offsets]

• PositioningPositioning Antenna-Transducer Antenna-Transducer OffsetsOffsets [measured] [measured]• Positioning and/or MRU Positioning and/or MRU System Latencies System Latencies [Patch Test][Patch Test]• HeaveHeave [MRU and/or RTK] [MRU and/or RTK]• Sound Velocity Profile Sound Velocity Profile [periodic probe data][periodic probe data]• DraftDraft or Index biases [bar check] or Index biases [bar check]

After a slant range is determined for each beam in the After a slant range is determined for each beam in the array, the following array, the following correctionscorrections [and sensors] must be [and sensors] must be applied to determine a properly oriented depth for each applied to determine a properly oriented depth for each point on the bottom:point on the bottom:

Multibeam BasicsMultibeam Basics

• SquatSquat [squat calibration or RTK] [squat calibration or RTK]• Tide/stageTide/stage [gage or RTK] [gage or RTK]• Vertical datum Vertical datum adjustments [tidal models, geoid undulations]adjustments [tidal models, geoid undulations]• Horizontal datum Horizontal datum adjustments [transformations]adjustments [transformations]

Slant Range Corrections (Continued):Slant Range Corrections (Continued):

These corrections will be discussed in later PresentationsThese corrections will be discussed in later Presentations