Introduction to Sonars

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.

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)

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

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.

Transducer Materials 1-3 Composite (Materials Systems Inc.) http://www.matsysinc.com/im.html PZT Ceramic shapes (EDO Corp) http://www.edoceramic.com/Materials_Shapes/ Basic point here, is transducer materials come in all shapes and sizes. PVDF polyvinyldene fluoride (Airmar Corp) http://www.airmar.com/

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 http://www.liverpoolmuseums.org.uk/maritime/collections/boa/asdic.asp Killer Whale is from NOAA.

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 0.0002 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.

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.

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.

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.

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.

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.

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.

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.

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

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

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

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

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.

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.

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

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.

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.

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. http://woodshole.er.usgs.gov/operations/sfmapping/sonar.htm

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.

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.

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…

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.

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 1 2 3

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

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

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

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

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

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

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.

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.