Sidescan Sonar 101 by William Crayton Fenn
Sidescan sonar is an acoustic survey device which is used in underwater imaging. It is most commonly used to image marine substrates. For example sidescan is used for mapping sub-sea geological features, corridor or hazard surveys for pipeline and cables, and mapping archaeological sites. It is an effective tool for searching for downed aircraft, sunken ships/boats and drowning victims.
Sidescan sonar is most commonly deployed from a surface vessel though it can also be incorporated on autonomous underwater vehicles (AUV), submarines and deployed from oil platforms and helicopters. Sidescans range in frequency from 30kHz to 2400kHz.
The sidescan transmits a very narrow vertical fan shaped beam of acoustic energy (sound) from two transducers - one on the port and one on the starboard side of a torpedo like device commonly referred to as a towfish. The acoustic energy travels through the water and reflects off of things such as the seafloor or items on the seafloor. The acoustic energy that reflects off of the seafloor and other items returns to the transducers on the towfish. The data that is received at the towfish is sent up the tow cable and through the winch, deck cable and into the CPU.
The CPU takes the travel time and intensity of the reflected acoustic energy and then plots the data points across a single line for each return or echo from each ping or transmission of acoustic energy. There is a new line created for each ping of the sonar. These lines are assembled next to one another and patterns in the points of data become evident allowing targets to be detected and resolved. The ping rate or transmit pulse of the sonar can be controlled manually or automatically. Typically a NEMA 0183 speed over ground data stream from the DGPS is used to control the ping rate. It is important to have synchronization between the actual towfish speed over ground and the ping rate or record rate of the data. If the record rate and the fish’s speed are not synchronized the aspect ratio of the sonar data will be incorrect.
Components:
The components that commonly make up a sidescan sonar system are, a central processing unit (CPU) or data acquisition system, handling system made up of winch, tow cable, deck cable, string block, A-frame or davit and a towfish (see Figure 1). The central processing unit (CPU) or data acquisition system controls things such as power to the towfish and its trigger or ping rate. It also acquires and stores sonar and navigation data. The CPU has software that allows the viewing of the sonar data as it is being collected and many other tools such as measuring and plotting targets in the sonar data. The handling system and deck winch facilitates the launching and recovery of the towfish. It also controls the altitude or depth of the towfish by paying out or taking in the tow cable. The tow cable couples the towfish to the deck winch. The tow cable, with aid of the deck winch slip ring and deck cable, allow sonar data and commands to flow back and forth between the towfish and CPU.
Figure 1. Survey skiff with string block and towfish on bow
The towfish is a weighted hydrodynamic torpedo shaped towed vehicle that houses, protects and provides a stable platform for the sonar transducers. There are various types of towfish utilized for survey depending on survey conditions. For instance, there are lightweight, hand-deployable towfish that are suitable for shallow water conditions. Heavy towfish are suitable for deep water or high speed surveys and neutrally buoyant towfish are suitable for rough water applications.
Field Operations Summary:
Assuming that you are on the survey site and conditions are suitable and safe for running your sidescan survey, the sonar operator will first inspect the deck gear. The operator would then start the data acquisition system, assure that the sonar transmits and receives and that the GPS interface is active, and verify that a survey plan with survey track lines has been prepared. Then the sonar operator will need to select the correct range setting depending on the survey criteria and the frequency of the sonar. For instance, if you are conducting a Dungeness crab pot survey and want to not only detect a one meter target but also resolve it as a crab pot, you might choose to use a 600 kHz towfish and run it on a 50 meter range. If are you mapping larger targets such as geology you might choose 150 kHz towfish and run it on a 400 meter range. The range or slant range of the sonar refers to the distance from the towfish to the outer edge of the recorded data. A 50 meter range setting indicates 50m to port and 50m to starboard or a swath of 100m. One can think about frequency as the physical length of a pulse or burst of fixed sine waves of acoustic energy (there are six sine waves in a pulse). The higher the number of sine waves that contact or sonify an object the higher the chance of resolving that object. The higher the frequency the better the resolution but the tradeoff is a lesser effective range. Likewise, the lower the frequency the lower the resolution but greater the range.
When the vessel’s captain determines that conditions are safe for deployment of the towfish, the fish can go in the water. With the vessel in motion at survey speed, the towfish is lowered to an altitude of approximately ten percent of the slant range of the sonar. For example if the sonar is set to sonify or image 50 meters to port and 50 meters to starboard, the towfish should be approximately 5 meters above the seafloor. Once the towfish is at its desired altitude the sonar operator needs to make adjustments to the gain or attenuation to get the desired image and then calculate the layback and offset of the towfish in relationship to the GPS antenna. This can be done in shallow water surveys (30 meters or less) with a simple calculation (see Figure 2). In deeper surveys where a larger quantity of sonar cable is deployed it is necessary to acoustically track the sonar towfish. The operator can now enter the layback and offset data into the software which georeferences the sonar data. This is a critical step to ensure returnability to your targets. The sonar operator flies the towfish following the contours of the seafloor by paying out or taking in the tow cable. Each time there is a change in cable length or speed of vessel, the layback will need to be reentered into the software.
Figure 2. Layback Calculation
When surveying in waters where thermoclines exist or in an areas such as a river delta or near shore where changes in temperature and salinity are present, the sonar’s acoustic energy transmitted by the towfish may be bent or distorted due to a change in sound velocity. (see Figure 3). The speed of sound in fresh water is about 1435 m/s and 1500 m/s in saltwater. These numbers vary depending on salinity, temperature and pressure. The speed of sound in seawater changes about 20-40 m/s for every 10 degrees Celsius. This change in velocity degrades the data and reduces the effective range of the sonar. When experiencing this effect it may be necessary to reduce the range of the sonar and the spacing of the survey track lines.
Figure 3. Thermocline Example
Figure 3 shows an example of where the towfish passes through a thermocline. Starting at the bottom of the image, the towfish is 17M above the sea floor. At the top of the image, the towfish is 7M above the sea floor. It passes through the thermocline at which point the speed of sound changes from approximately 1445M/S to 1455M/S. This change causes the sonar beam to be distorted or deflected. Once the towfish has passed through the lens or thermocline at approximately 8M off the sea floor, the sonar can image without much distortion.
The sidescan can also sonify objects in the water column like schools of fish or krill and waves, whitecaps and vessels on the surface of the water (see Figure 4). These things can obscure the seafloor data being collected. Sidescan sonar will not pass through gas charged materials such as bull kelp, eel grass, mill foil and other vegetation (see Figure 5). These gas charged objects act as a bubble curtain both absorbing and reflecting the acoustic energy. These facts should be taken into consideration when planning your survey.
Figure 4. This image was acquired with 600kHz, 50M range.
Figure 5. This image was acquired with 600kHz, 20M range.
Some other factors that may affect data includes interference from AC power that is not properly grounded, machinery noise from the vessel/engines, propulsion (jet or prop wash), sea state and hull slap. Like most tools, the results obtained with sidescan sonar will vary dramatically depending on the skill level of the sidescan sonar operator and the captain’s ability to stay on course.
Sidescan sonar has been utilized for fish habitat studies, derelict fishing gear surveys for crab, lobster, shrimp pots, gill and seine nets (see Figure 6 and 7). Thousands of crab pots have been located with sidescan sonar and then recovered by divers. Consequently, thousands of crabs have been kept from certain death in these pots. In just one day on one dive boat the diver released 693 live Dungeness crabs from recovered pots. Sidescan is currently being utilized to map geology, glacier till and man made structure (net habitat) that snag fishing nets.
Figure 6. Image of two crob pots. The upper pot is a 2 ft. square sport pot and the lower pot is a 3 ft. round commercial Dungeness crab pot with line trailing off to the left. The pocked texture of the bottom indicates gas charged sediments that have collapsed.
Figure 7. These are 600kHz images of monofilament gill nets and boulders.
Most of the derelict nets have been located by harvest divers or with drop camera surveys. Drop cameras have a narrow survey swath limited to the field of view, light and water clarity. A good drop camera and light package may be able to image a 5M swath of the sea floor with enough resolution to resolve net or lead line. Whereas, a sidescan swath is hundreds of meters with the capability of detecting net and lead lines. With the aid of historical fishing effort data it has been possible to plan surveys by prioritizing the areas with the highest fishing effort. Where there are high relief substrates or ships in these areas there are typically nets.
We have chosen Marine Sonic Technology’s heavy duty commercial grade sonar system for our surveys. This system includes a 126 lb. stainless steel towfish and clutched hydraulic winch with heavy cable. Derelict gear surveys present a high probability of collision and entanglement of the gear and the survivability of the heavy towfish is superior. We have found Marine Sonic’s system to be reliable and the image quality is second to none.