Imaging the sea floor with a sidescan-sonar system is accomplished by towing a sonar “tow-fish” over the study area. The tow-fish is equipped with a linear array of transducers that emit, and later receive, an acoustic energy pulse in a specific frequency range. The acoustic pulse is specifically designed such that it is wide in the across-track direction and narrow in the along-track direction (depicted by the bright yellow fan-shaped area in the diagram below).
|Towfish is shown at apex of across-track beam of energy (yellow fan-shaped area). Previously ensonified area shown as light grey area, with corresponding image to features depicted below. Brown stripe below towfish shows the along-track dimension. Figure adapted from Able,1987.|
The acoustic energy received by the sidescan-sonar tow vehicle (backscatter) provides information as to the general distribution and characteristics of the surficial sediment and outcropping strata. In general, if all other parameters are constant, a rougher surface will backscatter more energy than a smooth surface and therefore, return higher amplitude signals. Shadows result from areas of no energy return, such as shadows from large boulders or sunken ships, and aid in interpretation of the sonogram (after Urick, 1983; Fish and Carr, 1991).
In the schematic to the right, strong reflections (high backscatter) from boulders, gravel and vertical features facing the sonar transducers are light gray; weak reflections (low backscatter) from finer sediments or shadows behind positive topographic features are dark gray.
The USGS Woods Hole Science Center typically uses sidescan-sonar systems in the 100 to 500 kHz frequency range. Swath width can range from <100m to ~500m within these operating frequencies. The resultant sidescan sonar image is created by assembling, or mosaicking, each swath of data into a georeferenced composite that represents the acoustic character of the sea floor within the study area.
|Mosaicked imagery from Lake Mead, 2001:High backscatter is represented by light tones within the imagery; low backscatter by dark ones. Water body is aqua and surrounding land mass, brown.|
|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|
The areal coverage, desired resolution, and other parameters required for a given survey are considered in selecting the type of sidescan sonar system to be used. For example, the frequency range of the outgoing acoustic energy determines the operating range and resolution. Low-frequency sound energy is characterized by a long wavelength and corresponds to a long pulse length (amount of time the sonar emits an acoustic pulse). Long ranges are possible with low-frequency sound, but with limited resolution. High-frequency sound has short wavelengths, and corresponds to shorter pulse lengths, providing greater resolution, but over limited ranges. The table below shows this relationship.
Below are views of a sidescan sonar mosaic and associated swath bathymetry created from data collected in the South Essex Sanctuary, offshore northern Massachusetts by the USGS in 2004-2005 aboard the R/V Rafael using a 100/500 kHz dual frequency system. The data were acquired as part of a research effort to characterize the surface and subsurface geologic framework along coastal Massachusetts.
Able, K.W., et.al, 1987, Sidescan-Sonar as a tool for detection of demersal fish habitats, Fishing Bulletin, v. 85, n. 4.
Fish, J.P. and H.A. Carr, 1991, Sound Underwater Images, A guide to the generation and interpretation of sidescan sonar data, second edition, Lower cape Publishing, Orleans, MA,
Lurton, X., 2002, An Introduction to Underwater Acoustics, Principles and Applications, Springer in association with Praxis Publishing, p.3.
Urick, R.J., 1983, Principles of Underwater Sound, Peninsula Pub., 423 pp.