Sonar – General Information
Sonar is an acronym for Sound Navigation Ranging.
In simplest form, a sonar sends out sound waves. These reflect off a target and the sonar records the echo. By calculating the time it takes for the sound wave to travel to its target and bounce back, a distance between the sonar and the target is established.
Distance “r” to a single target is determined by the following:
1- Measure travel time of acoustic signal from transmission to the reception of its echo.
2- Multiply the time calculated in (1) by the speed of sound in the medium being used
3- Divide the result by two (there and back!)
1- 0.5 seconds total time
2- 0.5 * 1500 m/s (speed in water) = 750 meters traveled
3- 750 m / 2 = target is 375 meters away
Applications for sonar mapping
· Seafloor mapping
· Offshore Oil and Geological surveying
· Non-Destructive Testing
· Remote sensing of Atmospheric Conditions
· Robotic Navigation
First recorded use by people: Leonardo da Vinci in 1490 placed one end of a tube in water and placed his ear to the other end, allowing him to hear passing ships. First patent for Sonar filed in 1912, initiated thanks to the sinking of the Titanic earlier that year. The motivation was to create an underwater collision-avoidance system. World War 1 further accelerated advancements in the field.
Two kinds of Sonar… Active and passive
Active - Transmits and Receives (speaker + microphone)
Uses sound to determine relative positions of submerged objects and the sea floor, by submitting a sound signal and recording its reflection.
Passive - Receives only (microphone)
A listening device that can determine the presence, characteristics, and direction of marine noise sources. These may include biological and human generated sounds. Since this system doesn’t introduce sound into the environment, it doesn’t disturb marine life. It is also more stealth than a active system.
A transducer converts energy from one form top another (i.e. from electricity to an audio signal or vice versa). Can be comprised of one unit for smaller systems, or several units in an array for larger systems.
Piezoelectrics are often used as transducers because there are no moving parts and they are very sensitive. When current is applied to a piezoelectric, it changes in volume, producing a pressure wave (like a speaker). Conversely, when pressure is applied to a piezoelectric, it produces an electric charge (like a microphone). In some (though not all) cases, the piezoelectric transducer used in the sonar acts as both speaker and microphone.
A piezoelectric element Model 872 Yellowfin sidescan sonar incorporates piezoelectric sonar
Sonar – Output & Input
The output power of a sonar system is determined by the application in question. Sound dissipates over distance, so subjects that are farther away from the sonar require greater amplitudes of signal.
Output power levels vs. received levels
As sound travels in waves, it dissipates as the waves spread, loosing energy in the process.
High power - Military applications, commercial sidescan sonars, deep water echo sounders and fish finders.
Low Power - Shallow water applications and when subject is closer.
Frequency used can range considerably based on environment and subject…
Lower frequencies - (below 20,000 Hz) have greater range due to lower rates of sound attenuation over a given distance, BUT cannot distinguish small objects/fine detail.
High to very high frequencies - (above 100,000 Hz) provide excellent resolution of fish and other small objects, including sea-floor imaging, but suffer from signal loss over distance from the source. These systems are only practical in shallow water or for short range detection of objects near the source. In the case of sea floor mapping, this means maintaining a certain offset between the equipment and the seafloor as scanning takes place.
Speed of Sonar
Sonar operates at the speed of sound. Underwater, this varies with salinity, temperature and pressure while when traveling through the air, its speed varies with temperature and humidity. Rough estimates are around 1500 m/s in saltwater and 343 m/s in air.
In sonar, as the audio signal hits the subject, the signal is scattered in all directions but not uniformly. The sonar image from the returning echo is composed only of the backscattered energy that returns to the receiver. The intensity of this backscatter, and hence the brightness of the image, depends on both the properties of the system, and the characteristics of the terrain.
Properties of the system - These include the frequency of the signal, signal duration and power of the signal.
Properties of the terrain - Backscatter is most strongly influenced by objects that are at least ½ the wavelength in size or larger. The frequency used must therefore be proportional to the size of the detail required.
Other terrain characteristics that can influence the quality (intensity) of the backscatter are texture or roughness, and the inherent reflectivity of the surface. The reflectivity is governed by the acoustic impedance of the material, which in turn is determined by the physical properties of the material including porosity, pore fluids, grain composition, and structure.
In most cases, a strong backscatter means the material is relatively hard or contains a lot of texture.
Sonar – Applications and resulting data
In most cases, the raw data is placed through algorithms which compensate for various conditions during the echo recording. This allows the system to provide accurate images, despite constantly changing conditions. Data formats for the raw data vary depending on the system being used.
To store processed data, the Shapefile format (.SHP) is quite common, as it is vector based. It can contain points, polylines, and polygons These elements, in turn can have attributes associated with them, like a name or temperature. Shapefiles actually consists of a set of at least 3 files which work together (.SHP;.SHX;.DBF).
Other file formats for output include XTF, SEG-Y, CSV, TIF, GEOTIFF, XYZ. Some software also supports translation to KMZ, allowing the data to be uploaded directly to GoogleEarth.
Used for shorter distances and much finer resolution than normal Sonar.
Ultrasonography (Medical Sonography) - Uses a transducer to bounce sound waves off the inside of the body, detecting muscles, tendons, and organs. Operates in the 1,600,000 - 10,000,000 Hz range. Can offer greater detail than x-rays and is safer. However, it is believed to heat up soft tissue, and can create an inflammatory response in some people. While most often used to monitor pregnancy, it is also used to detect cysts and cancerous cells including prostate cancer.
2d ultrasound 3d ultrasound
Ultrasonic testing (a.k.a. Non-destructive testing) - Uses sonar with waves in the 100,000 - 15,000,000 Hz range to detect internal flaws or to characterize the material. Can also detect wall thickness in objects (i.e. used to measure pipe corrosion). Most commonly used on metals and alloys, but also used on concrete, wood and composites. The subject is often immersed in oil or water to improve the transmission of the signal.
Two basic modes…
Reflection (a.k.a. Pulse-echo) mode - A typical sonar setup. The transducer acts as both transmitter of the wave and receiver of the echo. The echo comes from a geometric interface within the object, such as the back wall of the object, or from an imperfection within the object.
Attenuation (a.k.a. Through-transmission) mode - A transmitter send the wave through the object on one side, and a separate receiver detects the wave on the other side. Imperfections in the object reduce (dissipate) the amount of signal received, thus revealing imperfections in the object.
A man with a blue hat, testing the wall thickness of a steel pipe
Sonar equipment is either installed directly on the ship or towed in the water at a controlled depth behind the ship.
Single Beam Sonar
In this basic system, a single pulse is emitted directly below the transducer with a narrow footprint at specific intervals. Typically dose not provide continuous coverage of the seafloor (think of a pixilated image, rather than a continuous tone). Resolution depends on footprint size, sampling interval, sampling speed, and distance between samples. The result is a two-dimensional image.
A Single Beam Sonar system
More sophisticated sonar for large scale mapping (called “swath” mapping; swath = a long, broad strip). Projects a narrow fan shaped beam from the sides of the unit which illuminates a swath. The image is produced as the instrument travels, sweeping the swath it produces along its subject’s surface. The width of the swath for offshore scanning is usually several times wider than it is deep (in oceans, about 500 meters wide; for shallower waters, it can be down to 150 meters). Much like mowing the lawn, this is done in successive passes. Then the information is stitched up into a final image. Multibeam sonar can produce 2d or 3d images.
Side-scan sonar in operation
Strips of data (a “sidescan mosaic”) prior to stitching
Example of a submerged ship, the Swedish “S/S Nedjan”, in 105 feet of water captured with sidescan sonar.
The ultimate in positional accuracy. Uses an array of receivers (hydrophones) that focus reception on very narrow angled paths. This allows the Multibeam system to scan for distance and depth, giving a very accurate 3d images. Absolute measurements down to 10 cm. The position of the head is calibrated using GPS and additional equipment on shore. Millions of XYZ points are collected and any unwanted noise is filtered out, resulting is very high definitions. Multibeam sonar is extensively used for bathymetry (underwater topography). These systems are normally installed in the hull of the boat.
geometry of a typical Sidescan sonar on the left and a Multibeam sonar on the right
The British shipwreck
HMS Royal Oak, captured in its final resting position off the seafloor in
Bathymetric Multibeam sonar image looking west
from Georgian Bay at the submerged Niagara escarpment and the sill between the
These units are able to distinguish both the sea floor and fish. Fish are easy to visualize with sonar because their air-filled bladder have a different density than the surrounding water. High-end Fish Finders operate between 50,000 and 200,000 Hz. Good to a depth of about 750 meters. Other uses of Sonar in fishing include sonar sensors attached to the bottom of nets that measure the distance from the net to the sea floor, and systems that measure how many fish are currently in the net.
This aint your daddy’s Fish Finder! the Lowrance’s Broadband Sonar, released in 2008, is the latest and greatest in consumer grade sonar.
Off-Shore Surveying for Oil
An array of air-guns is towed behind a ship. The array sits just below the surface of the water. The guns emit a low frequency (10-300Hz), high intensity (215-250 dB) sound every couple of seconds. This venting of high pressure generates seismic waves in the earth’s crust beneath the sea, which can travel hundreds of kilometers from the source. Sound waves bounce off boundaries between different types of rock in different ways. The resulting echo can then be studied to show geological structures of types often associated with petroleum deposits.
On-Shore Seismic Surveying uses seismographs to measure the resulting vibration of the earth rather than the wave itself. They are therefore not strictly considered Sonar and are not covered in this research.
Setup for Oil Surveying
Sonic Detection and Ranging (an upward-looking in-air sonar) is used for atmospheric investigations, operating in the 4,000 Hz range. These collect wind speed at various heights above the ground, along with the thermodynamic structure of the lower layer of the atmosphere. The frequency shift of the echo varies according to the wind speed, while the intensity of the echo varies according to thermal conditions. SODAR is also known as echo sounders or acoustic radar.
DSDPA.90-24 Complete Doppler SODAR System
An array of transducers found inside a SODAR system
Data obtained using a SODAR system. Angle of the lines indicates direct of wind. In the image on the right, length of the line indicates wind speed.