Title: Author(s): Document owner: Recommended operating guidelines (ROG) for sidescan sonar Dave Long (BGS) Dave Long (BGS) Reviewed by: Janine Guinan (MI) 07/09/07 Workgroup: MESH action: 2.1 Version: 2.0 Date published: 08/7/2005 File name: Language: Sidescan sonar ROG in wrapper.doc English Number of pages: 9 Summary: The document reports on the use of side-scan sonar systems for mapping benthic habitats, including the deployment and testing of such systems. Reference/citation: Keywords: Side-scan sonar, interference, data acquisition, seabed features Bookmarks: Related information:
Change history Version: Date: Change: 2 15/1/07 Updated template to include change history 1 8/1/07 Initial version of document
Recommended operating guidelines for side-scan sonar 1. Methodology Side-scan sonar is a method of underwater imaging using narrow beams of acoustic energy (sound) transmitted from the side of the towfish and across the seabed. Sound is reflected back from the seabed and from objects in the water to the towfish. Certain frequencies work better than others: high frequencies such as 500 khz to 1MHz give excellent resolutions but the acoustic energy only travels a short distance. Lower frequencies such as 50 khz or 100 khz give lower resolution but the distance that the energy travels is greatly improved (system ranges, frequencies and wavelengths are detailed in Table1). Variation in side-scan sonar systems Sidescan Sonar System Type Frequency Wavelength Range Low 100 Hz 15 m >1000 km Low 1 khz 1.5 m >100 km Low 10k Hz 15 cm 10 km Low 25 khz 6 cm 3 km Medium 50 khz 3 cm 1 km Medium 100 khz 1.5 mm 600 m High 500 khz 3 mm 150 m High 1 mhz 1.5 cm 50 m Table 1. Variation in side-scan sonar systems (table from USGS). For benthic habitat mapping, short ranges are used (i.e. 100 m or less) which allow relatively small objects to be detected. Such high resolution is also used in archaeological surveys 2. Equipment A side-scan sonar system consists of a topside processing unit, a cable for electronic transmission and towing, and a subsurface unit (a towfish) that transmits and receives acoustic energy for imaging. The towfish may be a single unit or a double where a depressor unit is used to get the transmitter/receiver down close to the seafloor; this removes much of the motion from surface waves.
The towfish generates one pulse of energy at a time and waits for the sound to be reflected back. The imaging range is determined by how long the towfish waits before transmitting the next pulse of acoustic energy. The image is thus built up one line of data at a time. Hard objects reflect more energy, causing a lighter signal on the image, while soft objects that do not reflect energy as well show up as darker signals. Very dark areas imply an absence of reflected sound, indicating a shadow behind an object. These are very useful for estimating the height of features on the seafloor. A vessel should be used that is of suitable size for the survey area. For shallow-water surveys, a vessel with shallow draft, adequate dry space to house electronic equipment, and a suitable power source should be used. It should also be big enough to deploy a side-scan sonar safely. For deeper water surveys the draft of the vessel is not an issue; the key factor is ensuring that there is enough deck space to accommodate a side-scan sonar winch with adequate cable. Although most modern systems support direct digital recording, it is often good practice to have a thermal recorder and digital acquisition and processing system interfaced together during data collection, as this provides data backup and aids online quality assurance and control. When data are recorded on thermal paper, gain changes should be kept to a minimum. For low-budget surveys where only an overview of the seabed is required, a survey undertaken with only a thermal recorder will be sufficient. If more detailed examination of individual targets or mosaicking of the data are required, however (e.g. for seabed classification), a digital acquisition and processing system should be used. 3. Operations at sea Side-scan sonar data are adversely affected by poor sea conditions, particularly in shallow water. To obtain good quality data it is recommended that data are not collected when the sea conditions are worse than sea state 4. 3.1 Testing Before side-scan deployment, a series of tests should be performed. Before leaving port and while the survey vessel is secured to the dock, the towfish assembly should be lowered into the water to check that system seals are watertight and that the mechanical deployment systems are functioning properly. The system should be turned on and the recording data acquisition inspected.
Prior to deployment a rub test should be performed to determine the integrity of the system. The side-scan system is turned on with the gain set to maximum. The transducers are lightly rubbed by hand until a dark line appears on the paper record and/or on the monitor screen. In this manner, the system circuitry is checked, confirming that port and starboard side-scan transducers are functioning properly. To improve the acoustic coupling of the transducer heads to the water, detergent should be brushed on the transducer faces. In addition, a series of calibration tests should be undertaken to check equipment settings and interfacing; this is particularly relevant for nondedicated systems. These checks may include the following: Compass calibration Acoustic underwater positioning system calibration Navigation system check and calibration Side-scan sonar navigation check (survey a known point or object in opposite directions) Trial runs over the survey area to adjust gain settings. 3.2 System deployment Side-scan systems are deployed either from a crane or davit on the side of the vessel, or more often from the stern of the survey vessel, usually through an A-frame. If the side-scan is kept low by a depressor unit, then deployment will require a two-stage operation. 3.3 Surveying Optimum survey speed for many systems is about 2.5-3 knots, particularly for high-resolution side-scan systems, providing an along-track horizontal resolution of 7 cm. At this speed, however, many survey vessels cannot maintain an accurate heading and seabed coverage is slow, whereas the horizontal resolution at 4 knots is about 15 cm. At such speeds, side-scan sonar data can be collected simultaneously with seismic profiling. This allows complementary data to be examined when interpreting the results of the survey. For benthic habitat mapping, short ranges tend to be used (100 m or less), which allow relatively small objects to be detected. For seabed reconnaissance, individual survey lines are collected over a broad area. In mosaic mode, a pattern of survey tracks is run at specific line spacing. The line spacing is less than the swath width (i.e. twice the range) of the sonar, so that range overlap occurs. This design ensures that the area of seabed being surveyed is completely insonified and that the loss of resolution at the outer
limit of the range is compensated for. As a rule of thumb, in areas of relatively smooth seabed, a line spacing of between 75% and 50% of the swath width will provide the necessary overlap. For large areas, a corridor survey may be applicable if 100% coverage is not possible (see Review of Standards and Protocols for Seabed Habitat Mapping Acoustic systems techniques Side-scan sonar 3.1). Data acquisition using modern digital side-scan systems follow standard procedures based on the operating criteria outlined by the manufacturer of the system. Data are normally recorded through the side-scan manufacturer s own software or through third party software such as CODA or Triton Elics. The type of side-scan specified for any survey must take into account the nature of the seabed, water depth and sea conditions, and the area to be covered. The size of the survey vessel will also have a bearing on the choice of side-scan. A small lightweight towfish, such as C-MAX, would be applicable for a small boat. 4. Record interpretations 4.1 Checking quality Like any other type of acoustic system, side-scan sonar is susceptible to interference from a number of sources, but with experience most of these can be recognised in the data. The sources of error to watch out for are: Survey vessel noise. Regularly spaced dark lines across the record are often due to energy sources on the survey vessel and these need to be removed if possible. These commonly occur when the sonar is close to the vessel (typically <50 m); simply increasing the horizontal distance between the towfish and the vessel will often eliminate the noise; Navigation drop-out of the signal will give rise to errors in the speed correction of the record, causing distortions. Depending on the system, this may be evidenced by areas of no data in the record or as interpolated bands (Figure 1);
Figure 1. Navigation drop-out (Davies et al. 2001). Interference may also be caused by schools of fish or a porpoise, as illustrated in Figure 2, which shows the body undulations travelling in the direction of the sonar; Figure 2. Interference caused by a porpoise (Davies et al. 2001). Other significant effects are caused by changes in seawater temperature, density and waves. In Figure 3, wave effects are evident as dark banding across the sonograph; note how the effect is more apparent towards the centre line of the record. Banding owing to acoustic interference tends to be more evident towards the edge of the sonograph.
Figure 3. Interference caused by heave on the towfish as a result of waves (Davies et al. 2001). 4.2 Data interpretation (See Review of Standards and Protocols for Seabed Habitat Mapping Acoustic systems techniques Side-scan sonar 3.3.) The height of objects on the seafloor can be calculated using the following equation: H o = (H ss *L) / R Where H o is the height of the object, H ss is the height of the side-scan sonar towfish above the seabed, L is the length of shadow cast by the target, and R is the distance through the water between the towfish and the end of the shadow cast by the object. 5. Data recording A crucial aspect of side-scan sonar surveying is establishing where the towfish is positioned. For single-unit systems an estimate can be made by calculating the towfish layback using the following equation: L=2(C 2 -D 2 ) ½ In the equation, L is the layback, C is the amount of in-water cable and D the depth of the towfish. This does not take account of the catenary effect which
lessens the lay back, but this becomes more of a problem for long cable deployments. With improvements in acoustic positioning systems, a beacon can be placed on the towfish and its position detected from the survey vessel and integrated into the survey navigation data stream, so greatly increasing the accuracy of towfish positioning (see Review of Standards and Protocols for Seabed Habitat Mapping Acoustic systems techniques Side-scan sonar 2). Good quality survey and data processing logs should be maintained throughout a side-scan sonar survey. All equipment settings and offsets used on the survey vessel should be logged. The survey logs should also include information such as the time of start and finish of each survey line and the vessel heading, even though these data are normally logged in the navigation software. Brief descriptions of the weather conditions and sea state should also be included. These logs will allow the navigation data to be crosschecked and enable the data processor to correctly process the data and quickly find any faults. Much of these notes have been developed from The Natura 2000 Marine Monitoring Handbook which contains a section on procedural guidelines for side-scan sonar (see http://www.jncc.gov.uk/pdf/mmh-pg 1-4.pdf). Fish and Carr (1990, 2000) also provide a useful guide. Manufacturers websites are also good sources of information; some, such as Klein Associates (http://www.l-3klein.com) are particularly useful, with practical and clear advice on operations and basic principles. For the mosaicking of the data to produce area wide coverage see http://www.jncc.gov.uk/pdf/mmh_pg1-5.pdf. References Davies, J., Baxter, J., Bradley, M., Connor, D., Khan, J., Murray, E., Sanderson, W., Turnbull, C. & Vincent, M., (2001). Marine Monitoring Handbook, 405 pp. JNCC. Peterbrough, UK. Fish, J.P. & Carr, A.H. (1990). Sound Underwater Images: A Guide to the Generation and Interpretation of Side-scan Sonar Images, 189 pp. American Underwater Search and Surveys Ltd, Lower Cape Publishing Co., Orleans, MA, USA. Fish, J.P. & Carr, A.H. (2000). Sound Reflections: Advanced Applications of Side Scan Sonar, Lower Cape Publishing Co, Orleans, MA, USA.