AND SONAR HOW IT BUILDS A SPATIAL MAPPING THROUGH THE USE OF SOUND Mark Russell (306093731) Spatial Audio, DESC 9137, Semester 1 2007 Graduate Program in Audio and Acoustics Faculty of Architecture, Design and Planning, University of Sydney
ABSTRACT This report will explore the use of sonar and its ability to create an image through the use of sound. Sonar uses a varity of infrasonic to ultrasonic frequencies to build a spatial image of what the surrounds are. Often known for its use in under water solutions, sonar can be found in almost every environment. This report will mainly focus on the underwater uses of sonar but could also apply to above ground uses. 1. INTRODUCTION become both a scientific instrument and indispensable navigational tool. With civilian applications of monitoring biomass and sea fishing benefiting from the technology. Side scan sonar which is used to generate images of the sea floor became one of the major tools in marine geology after its invention in the early 60s. The efficiency of sea bed acoustic mapping increased dramatically with the emergence of multibeam echo sounders in the 70s. When these two technologies were combined in the 80s, they allowed the collection of quality maps, which were able to measure both the topography of the sea bed and its acoustic reflectivity. The oil industry benefited greatly forms these developments and techniques of acoustic monitoring. It has even been suggested to use sonar to monitor the evolution of the average temperature of large ocean basins on a permanent basis. This is part of global climate studies.[1] 2.2. Sonar applications and different types With the various requirements of sonar needed in todays world, different types of sonar have been developed. This is due to the increase and development in the large scientific programs of environmental monitoring as wells as offshore engineering and industrial fishing. The bathymetric sounder specializes in the measurement of water depth. They transmit a signal downwards inside a narrow beam. The time delay of the sea bed echo is measured, and a distance is calculated and displayed Fishery sounders are designed for the detection and localization of fish shoals. They are similar to the bathymetric sounders, but they support additional tools to detect and process the echos coming from the entire water column. Side scan sonars are used for the acoustic imaging of the seabed. They allow high accuracy observations. They are placed on a platform which is towed close to the seabed; the sonar transmits a short pulse which sweeps the bottom. The signal, reverberation as a function of time, yields an image of irregularities, obstacles and changes in structure. This type of active sonar is mainly used in the survey of sunken ships and in marine geology. Multibeam sounders are used for sea floor mapping. They are installed aboard oceanographic or industrial survey ships to map the topography of the sea bed. A fan of elementary beams, transmitting athwart ship, rapidly sweeps a large swathe of seabed and measures its relief. If the angular aperture is large enough, the sounder can also provide acoustic images, like side scan sonars. This type of sonar provide a very detailed and accurate image. Sediment profiles are used for the study of the stratified internal structure of the sea bed. These single beam sounders are similar to the bathymetry. The frequencies used for sediment profiles utilize much lower frequencies, enabling penetration of tens and
Sonar, an acronym for Sound Navigation and Ranging is a system used for the detection of objects. It is most effective when used underwater; this is due to waters ability to propagate sound efficiently. There are two forms of sonar, active and passive. Active sonar utilizes under water reflections of sound waves. It achieves this by using a submerged radiating device and a sensitive microphone, or hydrophone. Active sonar can build a spatial image of the environment by radiating ultrasonic pulses which are then reflected back to the hydrophone from potential objects. The measured time delay is used to estimate the distance between the sonar and its target. The measurement is complete with a calculation of the angle of the arrival of the signal. Further analysis of the echo allows identification of more characteristics of the targets (e.g. its speed using the Doppler effect). Passive utilizes listening devices with out transmitting and often use a large data base of known sounds to detect and decipher a sound. This paper will investigate active sonar. 2. ACTIVE SONAR
By 1918 both Britain and America both had developed active sonar systems, and by the outbreak of WW2 both countries had incorporated these systems into their navies. Present day active sonar can be found in technologies such as fish finders, private, commercial and military vessels. Sonar used in civilian applications lacks the output capacity of the sonars found in military applications. Though some geological survey ships, which operate in deep water or require deep penetration of the sea floor require large output power levels which come close to those used by the militaries. 2.1. Civilian developments in active sonar In parallel with military developments, civilian oceanography developed very quickly. Acoustic sounders quickly replaced the traditional lead on line to measure the depth of water. Sonar has
even thousands of meters, depending on the sea floor. Sediment profiles use explosive or percussive sources and long antennas or receivers. Acoustic Doppler systems use the frequency shift of echoes to measure the speed of sonar (and its supporting platform) relative to a fixed medium. This type of sonar is used for scientific measurements.[1] There are other forms of sonar which are used around the world. These consist of: acoustic communication systems, positioning systems and acoustic tomography networks.
2.3. Devices of active sonar Active sonar converges a number of different devices to generate an image which contains the relevant information .A diagram which depicts the basic layout of an active sonar can be seen in Figure 1.
directly behind. This keeps the unwanted noise from the vessel from affecting the efficiency. Also sound attenuating material is used to reduce this affect. The array is set up to reduce the beam width in the vertical direction, as the transducers should generally receive information from the downward direction only and not directly ahead. This is due to the sound from the surface which could impede the sonars detection devices. The receiver collects the energy received from the transducer array. The power levels are then compared to the noise levels and a result is deduced. The receiver will also demodulate the signal if the pulse was modulated by the transmitter. The synchronizer provides the coordination and timing for the complete system and acts as the system clock and coordinates the resetting of the system for each transmission. This may include resting the display for each image being generated Display This combines all of the information and converts it to a visual form. There are several forms of display which includes A-scan and PPI (plan position indicator). [2] 2.4. Transducers array The transducers which are used in sonar are designed and assembled to meet a particular design objective. The common requirements in the specification are: Selection of transmission frequency Section of transmission bandwidth (or Q factor) Selection of power drive capability Transducer electrical to acoustical conversion efficiency
Figure 1. Active sonar functional diagram. Active sonar, as seen in Figure 1 has 7 main components which are vital in its ability to function. The transmitter generates a pulse which is some times referred to as a ping. The transmitter also controls the width, modulation and carrier frequency of the pulse. This information varies with the application of the sonar. The duplexer is incorporated into the system to protect the receiver from the transmitter power. After the pulse is generated by the transmitter it travels through to the duplexer. The duplexer can be thought of as a switch which changes between the transmitter and the receiver. The transmitter may generate such a powerful pulse, which would have detrimental effects to the system, if it were to be directed straight into the receiver. The duplexer prevents this from happening. The beam forming processor receives the transducers input signal. Time delays are added to the output signal so that a narrower bandwidth can be transmitted. By reducing the bandwidth, a higher directivity factor is achieved, which intern provides results of a higher degree of detail. The sonar operator can decide the degree of directivity applied by the signal, as varying degrees would be required in certain situations. The transducer array is possibly the most important device in the system. The array is made up of individual transducers, arranged in an array which improves the directivity index. Generally the array is arranged in a semi circular pattern which provides a complete coverage with the exception of the area
Sonar that requires the use in deep water or long distances, would require a transducer which had the capabilities of producing low frequencies. Sediment profilers are an example of a type of sonar which requires traduces with low frequency capabilities and high output power handling 2.4.1. Array designs
The simplest array is the linear hydrophone array (plane arrays). It consists of a line of equally spaced, equally weighted (unshaded), Omni directional elements. The elements are Omni directional because they are dimensionally small when compared to the wavelength of which they are propagating. Although the hydrophone array elements are presumed to be Omni directional, the array its self will exhibit strongly directional properties normal to its direction of extension. This type of transducer array would commonly be used in marine seismic survey. The advantage of plane arrays is their simplicity of design and realization. But they suffer from a major problem; increasing the dimension (e.g. to increase the power transmitted) leads to a narrowing of the directivity lobe, at a given frequency. A curved array produces a wide directivity and is much better suited to a wider range of activities. Curved arrays directivity is imposed by their geometry rather than by the dimension/ wavelength ratio. The most common geometry is cylindrical (or part cylindrical). These types of arrays are found on submarines, where a semi sphere or part cylindrical is used. By using such a design the transducers are only able to receive in the forward direction, which eliminates sound made by the ships aft.[3]
being in the outer beams. This can result in an increase in the output power needed. 3. VARIANTS WHICH EFFECT THE EFFICIENCY OF ACTIVE SONAR 3.2. Propagation of sound through water The speed of sound is defined by the rate at which vibrations propagate through a medium. Frequency and wavelength are related by the formula in equation (1)
Sonar has to contend with a number of different variants in order for it to build a successful image. The requirements of the scan will decide the output power required and the frequency needed by the sonar device. Attenuation of the signal effects the efficiently and detail of the received data. 3.1. Sound absorption and reverberation in sea water Three formulae are available for the calculation of absorption due to sea water. They rely on the chemical composition the water and can become highly involved due to various chemicals which can be present. The main chemicals which affect the formulae are: boric acid, magnesium and sulphate pure water. The three formulae are the Fisher and Simmons (1977), Francois and Garrison (1982) and Ainslie and McColm (1998). These formulae are all reasonably accurate within certain frequencies. Francois and Garrison estimate their model to be accurate to within about 5% for frequencies of 10KHz-500 kHz. The Ainslie and McColm formula retains accuracy to within 10% of the Francois and Garrison model between 100 Hz and 1 MHz. A graph which plots the absorption affects in the different oceans can be seen in Figure 2.
=
c f
(1)
Where f is the frequency, c is to the speed of sound in the medium and is the wavelength. The speed of sound is roughly 1500m/s in water, though in air it is only 340m/s. This would mean that a 20Hz tone in water is 75 meters long, when the equivalent in air is 17m long. This example shows just what effect the medium of water has on the propagation of sound. In water, temperature, salinity and pressure have effects on the speed at which sound travels. These factors will affect the sonars ability in providing an accurate image.
Temp Salinity Depth
Amount of change in Unit + 1 degree + 1ppt + 30meters in depth
Change from 1500 +3meters/sec +1.2meters/sec +0.5meter/sec
Table 1. Factors which change the speed of sound in water. As seen in Table 1, temperature has the biggest effect on the speed of sound in water. The worlds water temperature varies from -2C to 32C; this has major effects on how sonar works and detects distances. Between the depths of 30 to 100 meters is where the biggest noticeable difference in varying temperature is seen, this is called the thermocline, which divides the warmer surface water to the colder deeper water. The salinity of the worlds oceans can vary from 32 to 38parts per thousand. Differing salinity in the worlds oceans is very minute, though changes are generally seen around coast lines and ice burgs. The change in speed of sound due to depth as a result of pressure is relatively small, though when working at great depths this small variant can have significant effects. When sonar is being used, the temperature and salinity remain generally constant and only change when a large distance is traveled, this means that depth is always a constant variant and has to be monitored regularly.
Figure 2. The affect of absorption in the different oceans around the world As seen in Figure 2, which depicts absorption as a function of frequency, the amount of energy that is attenuated in water is frequency dependent, higher frequencies exhibit much larger attenuation than lower frequencies. Conditions that cause high reverberation in the ocean can be compared to situations seen outside. Like a brick walled room or stair well, deep oceans with hard sea floors can cause a substantial amount of reverberation which can cause problems in the use sonar. Sonar is not designed to indicate to the operator the amount of reverberation, though reverberation can be seen in increased amounts of noise levels in the sonar and perhaps a narrowing of the effective swath due to the noise
A formula which takes into accounts all of these variables and gives a close approximation is:
4388 + (11.25 temperature(in F )) + (0.0182 depth(in feet ) + salinity (in ppt ) (2)
This is an empirically derived approximation. It can be applied in relatively normal temperatures, salinity and most oceans depths. Though if you were trying to calculate the speed of sound in waters where it was very deep, very cold and experienced large changes in salinity, the formula may have a certain amount of error as it is an approximation which is based and calculated for normal circumstances.
3.3. Thermocline and mixed surface layers effects The mixed surface area is the first 30 meters of ocean. The major variations on the speed of sound are caused by wave motion and rapid changes in temperature. After the first 30 meters you enter what is known as the thermocline zone. This zone divides the warmer zones and the colder zones. Sonar originating on one side of the thermocline and penetrating through the zone can often suffer from a bent or refracted wave, which is caused by the sudden change in temperature. An Increase in sound speed can cause the sound wave to be refracted away from the region of higher sound speed. The mathematical model of refraction is called Snells law.
Where SL is source level, TS is target strength, PL propagation loss and N is noise or the amount of sound energy from other sources. The signal begins with a source level from a transducer array which is shown by the letters (SL). The level is reduced during the process of propagation to the sea floor target which is expressed by the letters (PL). An amount of the signal is reflected off the target which is expressed as (TS). This target signal is then reduced again by propagation loss back to the transducers (PL). The received signal is then reduced by the ability to decipher it from the surrounding noise which can be called (N).
4.
SOUND INTERATCTION WITH THE SEA FLOOR AND FREQUENCY SELECTION
Figure 3. Shows the effect of a wave propagating through two mediums. The diagram above shows Snells law[4] which sates that the cosine of the initial angle of incidence divided by the initial sound speed is a constant as the sound passes from regions of differing sound speeds Cos (A)/ Co = Constant Therefore, if a new region has a lower speed of sound, the cosine of the angle of incidence must be less than that of the previous region. Hence the new angle of incidence is less. This can be explained by imagining that the sound is bending towards a regions of lower speed, the angle that the sound travels is steeper, and travels "away" from regions of higher sound speed in the sense that the angle of incidence is shallower. This helps derive the complete path because once you know the angle that a sound wave left its source, and the sound speed profile of the medium it passes through, you can then calculate the path that the wave takes. 3.4. Signal to noise ratio Problems with sonar with background noise affecting the results are common. With water being so efficient in propagating sound, every sound under water can be heard. This can create a very noisy environment. The mathematical equation which describes the signal to noise ratio can be expressed by the equation:
SNR = SL PL + TS PL N
There are many factors that affect the way sound interacts with the sea floor. It is dependant on the grazing angle (the angle of incident), the composition of the sea floor (the smoothness), the material and the frequency. A good reflection is achieved if it is bounced off a flat surface normal to the sound waves path of travel. Scattering can occur when large rocks or non linear objects reflect sound. This can some times induce a weak reflection back to the receiver, but can enhance it when the angle of interception with the surface would otherwise reflect most of the sound energy away. The frequency which is used and the composition of the material of s...