The Sonar Method
Sidescan sonar uses backscattered acoustic energy received from the ocean bottom to image the sea floor. Early sonar systems were developed during World War I, by the British, French, and Americans to locate underwater targets, such as submarines and icebergs. The Second World War saw the refinement of sonar systems in terms of signal strength and coherency. The advances that have led to our modern day systems began in the 1960's.
|A sidescan sonar system was used on the Surveyor during 1965 to explore for offshore extension of the San Andreas Fault. (Courtesy NOAA Photo Library)|
The first sidescan-sonar systems were analog systems that allowed only a qualitative, though extremely valuable and insightful, view of the acoustic character of the sea floor. Developments in digital signal acquisition and processing have expanded the sidescan-sonar capacity to encompass a degree of quantitative analysis and inter-system comparison that was not previously possible (after Johnson and Helferty, 1990).
Sidescan-sonar systems are configured with a linear array of transducers, (devices that convert electrical impulse to pressure wave and pressure wave to electrical impulse) mounted on either side of the tow vehicle. An electrical impulse (signal) is applied to the transducers, causing them to flex, or change shape. This flexure produces a pressure wave that propagates away from the tow vehicle. The emitted acoustic pulse is wide in the across-track direction and narrow in the along-track direction. The energy propagates through the water column and ensonifies a strip of the sea floor. The energy intercepts the sea floor at an angle (the grazing angle), which images small irregularities (the roughness) of the sea floor better than energy that is directly incident, such as that propagated by echosounders. Acoustic energy is then scattered by the sea floor, with a small percentage of the acoustic energy traveling back through the water column and received by the sonar. The backscattering process is complex; it is dependent on the frequency of the outgoing acoustic energy, the arrival (grazing) angle of the energy relative to the sea floor, the bottom characteristics, the acoustic impedance contrast (defined by the bulk density and the velocity of sound in the medium) between the water column and the sea floor, and dimensions of the sea floor target relative to the wavelength of the outgoing pulse (Lurton, 2002).
|Graphical representation of selected sidescan-sonar terminology.|
After emitting an outgoing acoustic pulse, the transceivers enter a “passive” mode and “listen” for the returning backscattered energy. After a defined period of time, the system enters the “active” mode again, and the transducers then emit another ping of outgoing acoustic energy. This begins a new cycle, which is on the order of milliseconds. The cycle, or ping rate (length of time between pings), also determines the range of the sonar system.
|Diagram of the elements of sidescan sonar acquisition. A research vessel, equipped with navigation devices (GPS, Differential, RTK systems) tows a "towfish" from the stern. The outgoing energy is depicted by the yellow, fan-shaped area. The yellow shaded area of the sea floor depicts the previously ensonified area of the sea floor.|
A sidescan sonar image is created by assembling, or mosaicking, each of these lines of data into a composite that represents the acoustic character of the sea floor.
|Example of sidescan-sonar mosaic collected by the WHSC during several surveys off New York Bight. Bright regions represent areas of high backscatter, here, older rocky deposits near the surface. The darker regions represent less backscattered energy, mostly muds, channel fill, and some sand. (Schwab et al., 2000)|
For a more detailed discussion of the interpretation process, please click on Sidescan Sonar interpretation on the tree to the right.
This brief history is not intended to be a complete one. It is an overview from which readers may further explore the topic in libraries or on the web, using the topics presented above as starting points.
Johnson, H.P. and M. Helferty, 1990, The geological interpretation of sidescan sonar, Reviews of Geophysics, v. 28, no. 4, pp. 357-380.
Schwab, W.C., Denny, J.F., Butman, B., Danforth, W.W., Foster, D.S., Swift, B.A., Lotto, L.L., Allison, M.A., and Thieler, E.R.. 2000. sea floor Characterization Offshore of the NewYork-New Jersey Metropolitan Area using Sidescan-Sonar: U.S. Geological Survey Open-File Report 00-295