1. Introduction to Sonars

2. Learning Objectives How do sonars work?
Materials used to make transducers
Resolution
Pulse length and range resolution
Beamwidth and angular resolution
Sonar beam patterns
Effect of array diameter to wavelength ratio
Effect of array length to wavelength ratio
Frequency review
High frequency: high resolution; poor range
Low frequency: low resolution; good range
Common sonar settings and what they do
range, power, gain, pulse length/width.

3. Brief History of Sonar
Initially a tool for detecting icebergs (shortly following Titanic 1912)
Quickly developed by navies in response to submarines.
SOund Navigation And Ranging
Active Sonar (transmit and receive)
Passive Sonar (receive only)

4. Basic Active Sonar Principle
Transmit Sound
Measure round trip travel time.
Use sound speed to get distance

5. Transducer Materials
Transducers are typically composed of Piezoelectric (crystals) or electrostrictive (ceramic) materials
Quartz crystals
Barium titanate
Lead zirconate titanate (PZT)
Polyvinyldene flouride (PVDF)
But explosives, air guns, and other sound sources are used in some applications.
A typical house speaker uses a magnetic coil to translate electrical signals to mechanical vibrations. Piezoelectric crystals do much the same thing.
A transducer is a combination of a receiver and a projector- a combination of a microphone and a speaker.

6. Transducer Materials 1-3 Composite (Materials Systems Inc.)
PZT Ceramic shapes (EDO Corp)
Basic point here, is transducer materials come in all shapes and sizes.
PVDF polyvinyldene fluoride (Airmar Corp)

7. Pulse length/width Sound is transmitted in pulses, not continuously
Naval Echo-ranging (WWII)
Sound is transmitted in pulses, not continuously
Pulse length and width often used interchangeably
Length of time of pulse
Width of pulse in the water
Pw = c x Pl
Killer Whale Feeding
There is typically no reason to continuously transmit a pure sin wave. You can’t transmit information or measure anything with it. Especially for echosounding, we want to transmit a short pulse of acoustic energy.
Think of the echo in the canyon example.
Sonar sound is from
Killer Whale is from NOAA.

8. Pulse Length Pulse Length
When a transducer produces a signal, it sends out a preset number of wave cycles. This is usually measured by the transmit time or “pulse length”. This single pulse length is typically what is referred to as a “ping”. In this example, the pulse length is 200 micro seconds or seconds long. The frequency is 100 kHz, so there are 20 cycles in this pulse.
The same relationship exists between pulse length in time and pulse width in space as between the period and the wavelength of a wave. The speed of sound relates the two quantities.
It is easy to get confused between pulse length and wavelength.

9. Pulse Length and Resolution
Objects separated by less than ½ the pulse width cannot be resolved as independent objects.
incident sound pulse d 2d
Pulse length limits the range resolution of the sonar.
This is a bit counterintuitive. The ½ comes from that 2 in the denominator of the sonar equation. You can see from this example that by the time the leading edge of the return from the second object gets back to the first object, the return from the first object will be just ending.

10. Pulse Length-Example
Pulse Length = speed * time
PL = 1.5x103 m/s * 93x10-6 s
PL = 140x10-3 m
PL = 14 cm
A typical launch sonar (8101) setting for pulse width is 93 micro seconds (93µs)
Take sound speed to be 1500 m/s.
but resolution is ½ pulse length
so:
Range Resolution = 7 cm
Let do the math for some typical settings.
Resolution is not necessarily the range accuracy- just the ability to resolve that there are two things there rather than one.

11. Pulse Length and Resolution
Long Pulse
Short Pulse
Good signal to noise ratio
OK signal to noise ratio
High Energy in the water
Less energy in the water
High maximum range
Shorter maximum range
Low range resolution
High range resolution
Good for deep, flat areas
Good for shallow, feature rich areas
Think about being lost in a forest. If you wanted someone to hear, you would yell for HEEEEEEEEEELLLLLLLLPPPPPP. Not help.
With some of these factors (like the signal to noise), what is long depends on the wavelength. So what is long for a high frequency sound might not be so long for a low frequency sound.

12. Beamwidth and Angular Resolution
Yellow: Wide beamwidth
Blue: Narrow Beamwidth
Objects separated by less than the beamwidth cannot be resolved as individual objects.
So we have looked at the range resolution, and how it is related to the pulse length, but what about the spatial or angular resolution?
Beam width is an angular measure, usually given in degrees.

13. Beamwidth and Resolution
Advantages of Wide Beam
Better Coverage
Typically smaller and cheaper transducers
Advantage of Narrow Beam
Good spatial resolution
Higher effective range
Sonar marketing will often have you believe that narrower is always better, but this may not be always true. A wide beam will cover a wider area. If you are looking for shoals or fish, a wide beam may give you a better result.
Think of a adjustable beam flashlight.
There are some beamwidth effects that we commonly see in singlebeam traces. Discuss why fish returns are inverted arcs. If we take the first return as our depth, discuss shoal biasing effect of wide beams.

14. Interference and Beam Formation
The central region of constructive interference is the main lobe
The off center regions of constructive interference are called side lobes
The regions of destructive interference are called nulls.
How do we even go about forming a beam? Consider two coherent sources- that is they are in phase with each other.
If the path length between the two is the same. The arriving signals will be in phase, and will constructively interfere. The most obvious place this happens is on a line between the transmitters.
If we start to move away from the centerline, the path lengths the two signals need to travel will start to differ. The phase will no longer be the same, and the signals will start to destructively interfere. When the path lengths are such that the difference is ½ a wavelength, the signals will be out of phase and will cancel each other out.
As we move farter out still, the difference in path length will become equal to one wavelength. The signals will again be in phase, and will constructively interfere.
This pattern will repeat- and we get a series of light and dark fringes.

15. Beamwidth- Fix Analogy
Closely spaced sources gives a wide beam
Just like closely spaced radar targets give a imprecise fix
Here, the arcs represent regions of the wave that are close to the same phase and will constructively interfere.

16. Beamwidth- Fix Analogy
Widely spaced sources gives a narrow beam
Just like widely spaced radar targets give a more precise fix

17. Effect of increasing radius / wavelength ratio
Beam Patterns
Effect of increasing radius / wavelength ratio
Here is a different view of beam patterns from a cylindrical piston. Note that the bigger the transducer, the narrower the beam. Big is again dependent on the wavelength.
Discuss the different representations of the beam pattern. Discuss sidelobes. This will take some drawing on the board.
SE 3353 Imaging and Mapping II: Submarine Acoustic Methods
© J.E. Hughes Clarke, OMG/UNB

18. Anatomy of a beam pattern from a circular plane array
Beam Patterns
Anatomy of a beam pattern from a circular plane array

19. Non-Symmetrical Beam Patterns
Effect of increasing length / wavelength ratio
We can consider what the beam pattern would look like if the transducer were not rotationally symmetric.
A long narrow transducer will produce a beam that is narrow in the dimension that the transducer is long, but wide in the dimension that the transducer is narrow.
SE 3353 Imaging and Mapping II: Submarine Acoustic Methods
© J.E. Hughes Clarke, OMG/UNB

20. Beamwidth, wavelength and transducer size
The shape of the beam is governed by the dimensions of the transducer.
Big and small are relative to the wavelength of the transmitted sound.
For a desired beamwidth, if we double the wavelength (halve the frequency), the transducer needs to double in size.
So far we have covered the basic sonar equation, pulse length, transducers, beam patterns, and how transducers effect the beam pattern.

21. Beamwidth, wavelength and transducer size
Consider the difference between a 12kHz deep water system and an 455kHz shallow water system.
To achieve the same angular resolution as the 455kHz system, the 12kHz transducer needs to be about 40 times the size.

22. Beamwidth, wavelength and transducer size
455kHz shallow water system, 0.5º beam
Photo: Reson
While these are not single beam sonar systems, the beam width/ transducer size principle holds.
Low frequency, narrow beam width requires big transducers.
12kHz deep water system, 1º beam
Photo: NAVO

23. Sonar Frequency Review
High Frequency
High resolution
Shorter pings
Smaller scatter size
Small transducers
Poor range
Low Frequency
Good range
Big Transducers
Generally poorer resolution
If you can walk away with understanding this, you will have made a good start.

24. Sweep Sonar Systems
So now we understand the basics of a single beam. Where can we go from there?
One pretty obvious solution is to just add more single beam transducers. They will need to operate on slightly different frequencies so they don’t interfere with each other. This is actually a very effective solution for very shallow water surveys.

25. Side Scan Sonars
Rather than just looking at the time of first return, we can look at the entire time series return, and image the bottom acoustically.
Sidescan sonars typically use a towed sensor, although they can be hull mounted
Sidescan sonars transmit and receive signals on a linear array of transducers.
Sidescan sonars are used for locating and identifying targets (i.e. Dangers to Navigation) based on the strength of the returned signal
The will be a bunch of information later on side scan. A basic side scan doesn’t give you any depth information, but can give an acoustic picture of the seafloor.

26. Multibeam Sonar Simultaneous beam formation from one transducer array.
We will get into multi-beam in greater depth during the next lecture. But basically, we can form 100’s of individual beams from one transducer array.

27. Phase Differencing Sonars
Bathymetric Sidescan Sonars
Able to give depths as well as imagery
Benthos C3D
Klein 5410
GeoSwath
Phase Differencing (or interferometric) sonars are similar to side scans, but can extract depth information from the phase of the returned signal.

28. Sonar Settings Range Gain Power Pulse length (pulse width)
Time Varied Gain (TVG)
Fixed
Power
Pulse length (pulse width)
These will be discussed in much more detail later, but as an introduction…

29. Knobs Shared by All Sonars
Range
How long the sonar listens for a return
Determines how frequently to ping
Pulse repetition rate (PRR)
At a given speed, determines along track ping spacing.

30. Range Incorrect range setting increases noise
Set too short – the outer beams aren’t long enough to reach the seafloor
Set too long – ping rate is reduced and the sonar ‘listens’ for too long, increasing the noise in the return signal
Good Too Low – Bad
Outer Beams are Lost

31. Power
Power
How loud to project the sound
volume on the speaker

32. Power This setting should be set as low as possible
When power is too high, fliers increase and there is a halo around each ping on the display
There is the possibility of a double return
May need different power settings for different bottom types

33. Gain
The gain function controls how much the returned sonar signal is amplified
In manual mode, gain settings between 4 and 12 yield the best results
In auto mode, settings Auto 2 through Auto 4 are typical
Most NOAA vessels use manual gain

34. Time Varied Gain
Compensates for spreading and attenuation by increasing gain for more distant signals
Basically listening harder to the signal that comes later in the return.
Receiver gain = (2 α R) + Sp logR + G
α = Absorption loss (dB/km)
R = Range (m)
Sp = Spreading loss coefficient
G = Receiver gain

35. Transmit Pulse Width Variable depending on sonar
Measured in microseconds or milliseconds
The smaller the number, the shorter the pulse width – typical setting between 70 and 120
Lower frequency system have longer pulses
High frequency, high resolution systems have short pulse lengths
The shorter the pulse, the better the resolution, the longer the pulse the better the range performance (more energy in the water) but the poorer the resolution

36. Common Issues
Q: What’s wrong with this data? What could be done to improve it?
A: It is overpowered. The transducer power needs to be turned down

37. Common Issues
Q: What’s wrong with this data? What could be done to improve it?
A: The range is too low. Noise is apparent in the data.

38. Common Issues
Q: What’s wrong with this data? What could be done to improve it?
A: The range is too high. The outer beams are lost. The range should be lowered.