Ongoing Developments in Side Scan Sonar

The pursuit of better Range, Resolution and Speed

Nick Lawrence

EdgeTech

Advances in Seafloor-mapping Sonar Conference

30th November 2009

Company Profile

• EdgeTech (formerly EG&G Marine Instruments)

• Started in 1966 by Doc Edgerton

• Offices in Massachusetts and Florida

• Sister division to ORE Offshore

• 3 main product lines:

– Side Scan Sonars (SSS)

– Sub-Bottom Profilers (SBP)

– Combined SSS/SBP Sonar Systems

Ongoing Developments ?

• Technologies that have been around long enough that we just accept them as normal.

• Technologies that are currently available

– Not all of which are widely adopted

• Where next ?

Range

• Definition of maximum Range?

– See an echo from a target (and how large?)

– Delineate shadows behind the target, so as to aid identification.

• Either way, it‟s a matter of signal strength and noise.

What affects the Range ?

• Signal Strength

– Absorption (depends on frequency, water temperature and salinity).

– Altitude (grazing angle – reflected energy)

• Noise

– Water Depth (surface noise /backscatter)

– Ambient Noise

– System Noise

Absorption Regional Differences

Absorption

Improving Signal Strength

• Wideband, Low amplitude, linear FM chirp pulses

– Provide high energy signals, with superior resolution

– CW: Tx power typ. 1kW, Pulse length ~ 50us

• Energy = 1e3 * 50e-6 = 0.05 Joules

• Resolution = 3.75cm (depends on pulse length)

– CHIRP: Tx 200W, pulse length ~3ms, BW = 40Khz

• Energy = 200*3e-3 = 0.6 Joules

• Resolution = 1.8cm (depends on bandwidth)

Reducing Noise

• Digital Towfish – no cable noise

• Linear FM chirp pulses (again!)– Implementation of matched filter processing

• out of band rejection > 70dB

– Wideband front end analog electronics

• excellent linearity and no phase distortion

More Signal…

Less Noise….

Better Range

Resolution

• Across Track Resolution is really not the issue

– Frequency dependant

– Typically side scan sonars have plenty of across track resolution

• Along Track Resolution is the real issue!

Resolution Along Track:The „simplified‟ case

• Beam width is determined by length of the sonar array

• Longer arrays with narrower beams provide finer spatial resolution

• Definition of beamwidth– See App note on

www.edgetech.com

Resolution Along Track:A better model

Resolution Along Track:Even that is not everything !

The classic dilemma: Near Field vrs Far Field

• Best resolution is limited by the length of the array

• But remember what happens with a short array!

Resolution: Multi-beam SSS

d

Transmit beamReceive Beams• Use highest

possible frequency

• Longest possible arrays

• Beam forming techniques

Resolution: Synthetic Aperture

• SAS overcomes the array length limitation by synthesizing an extremely long array– Coherently combines data from

many successive pings– Resolution is no longer a

function of physical array length

• Because array length is no longer an issue, neither is frequency– Can use lower frequencies to

increase range

“Synthetic”Aperture

“Physical”Aperture

Sub-Element

The EdgeTech solution:Dynamic Focussing

• What if we segment the array…

• … and stay in the near field as long as possible...

• …so we will get the best resolution possible at a given range…

The EdgeTech solution:Dynamic Focussing

Take Advantage of this Multi-Element Array in Another Way

• What if we curve the segmented the array?

• Why do we want to do this?

– Because sound does not come back to the array in a flat plane…

…it‟s curved slightly!

Application of Dynamic Focusing

• Various configurations have been built

– Different Frequencies

– Different number of segments

– Variable length segments

– Not necessarily symmetrical

300Khz, 8 Elements

Non Focused

Focused

Speed

• Why do we care ?

– Better imaging with more „hits‟ on a target

– Specifications

• IHOManual on Hydrography M-13 Ch 4

– the minimum number of returns to make a discernible mark on the trace is taken as five

• NOAA– The hydrographer shall tow the side scan sonar at a speed such that an object 1 m on a side

on the sea floor would be independently ensonified a minimum of three times per pass.

The general rule is 3 „hits‟ on a 1m target per pass

Speed – Conventional Sonar

• Conventional side scan sonar tow speeds typically limited to 4 - 5 knots or less...

• Low probability of target detection or recognition even at moderate (4 knot) speeds…

T

x

T = (2*Rmax)

C

T

x

Range Scale = Rmax

Rx Period

Speed – Conventional SonarAim is for 3 hits on a 1m target

• Based on travelling a distance d between pings

– Typical: d = 0.33 m, Range R = 125 m

– V1p <= 2 m/s (3.9 knots)

• At longer ranges, you have to go even slower….

Multipulse Transmit Sequence

Multipulse Timing

T

x

A

T = (2*Rmax)/C

T

x

A

T

x

B

T

x

Btp

T

x

C

T

x

D

T

x

C

T

x

C

T

x

D

Range Scale = Rmax

Rx A Period

Rx B Period

Rx C Period

Rx D Period

tp

tp

tp

Speed – MultipulseAim is for 3 hits on a 1m target

• Based on travelling a distance d between pings

– Typical: d = 0.33 m, Range R = 125 m

– V1p <= 2 m/s (3.9 knots)

– V2p <= 4 m/s (7.8 knots)

– V3p <= 6 m/s (11.7 knots)

– V4p <= 8 m/s (15.6 knots)

• Or more hits on the same target size, at the same speed….

Multipulse – Technical Issues

• The sonar system must be able to:– Process receive data whilst transmitting

– Handle returns from all ranges simultaneously (so no analogue TVG can be applied)

– Provide separate output data streams for each transmit pulse

• Once again it is coded pulses and matched filter processing that is required (Chirp !)

Receive while Transmitting

Pulse Separation

Receive data Processing

• The receiver

– uses digital (DSP) replica correlation techniques to reject out of band echo energy.

– correlation processing provides high resolution, pulse compressed, high signal to noise, output data.

– provides an output data stream for each pulse.

Pulse Separation Processing

Multipulse

Data Examples

Results – Comparison 1

Image of a shipwreck,

Conventional sonar

Across

Track

Range

(meters)

Fwd

Multipulse Image of a shipwreck, 3 Pulses

Results – Comparison 1

Image of a 2m x 2m target, 1 Pulse

Fwd

Multipulse image of a 2m x 2m target, 4 Pulses

3 pulse versus 1 pulse

Four pulse image of sand waves and rocks

Four pulse image of sand waves and rocks Zoomed

One pulse image of sand waves and rocks Zoomed

4700 Data Examples

4700 , 600Khz , 1 pulse, Speed 3.7 knots

4700 , 600Khz , 1 pulse, Speed 7.5 knots

4700HF, 2 pulses, same area as 1 pulse, speed 7.0kts

4700, HF 3 pulses, same area as 1 pulse, speed 7.3 kts, first screen,

4700HF, 3 pulses, same area as 1 pulse, second screen, 7.3 kts

Products – technology table

Chirp Dynamic Focusing

Multipulse

4125 CW SP

4125 SP

4200-SP SP

4200-MP 2

4700 DFX 3

AUV / ROV Systems

Option Up to 3

4200 Series

4200-SP (Single Pulse)

Digital dual simultaneous frequency side

scan sonar system

4200-MP (Multi-Pulse)

Digital dual simultaneous frequency side

scan sonar system

with added High Speed Mode allowing tow

speeds of up to 10 knots while meeting 3

hits on a 1 m target.

100/400, 300/600 or 300/900 kHz

Choice of rack mount, portable or 3rd party interface for topside processor

Choice of stainless steel (2000m rated) or lightweight aluminum (500m rated) towfish

Pressure, temperature, magnetometer, acoustic responder,

depressor and custom sensors available as options

4200 Series

4200 Series

What next ?