|The schematic above (derived from Fish and Carr, 1990)|
The operation of a sonar towfish and associated electronics is shown schematically in the diagram to the right. The topside sonar acquisition computer displays the received sonar data in a water fall display, and also monitors various telemetry from the towfish, such as depth, pitch, roll, and altitude above the bottom. This computer also controls the firing rate of the sonar towfish, as well as any time-varied gain (an amplifier which has a gain that increases nonlinearly with time after the start of a sidescan cycle in order to correct for signal losses due to absorption and beam spreading, Johnson and Helferty, 1990), pulse widths (pulse duration), as well as power and initial setup of the towfish. This information is transmitted from the computer down the coaxial cable to the towfish electronics, which in turn instructs the firing electronics when to operate the transducers and at what pulse length and corresponding frequency. The received backscattered signal from the sea floor is picked up by the transducers, which are monitored by the receive electronics and from there transferred to the TVG electronics for applying gains to compensate for signal loss. The signal is then transferred back up the coaxial cable to the acquisition computer for display and monitoring.
The WHSC operates three sidescan-sonar systems. Two are dual-frequency systems, operating at 100 and 500 kHz, the other, an FM (frequency modulated) chirp sidescan system, operates in the range of 90 to 110 kHz. Energy propagates a much greater distance with the lower frequency systems, but resolution is less. For example, a 100 kHz operating frequency allows for up to 500-m range, or 1000-m swath width. The third system is a chirp sonar and sweeps through the frequency range, 90 to 110 kHz on the port channel and 110 to 90 kHz on the starboard channel in order to lessen noise and cross-talk. This system also has a chirp subbottom profiler. Chirp systems signals potentially have minimal noise and distortion due to the application of a matched filter that correlates the signal (known signal plus noise) with the known signal reversed in time to maximize recognition of the returned signal from noise (after Urick, p. 384).
The weight of the three systems varies greatly. The combined sonar/chirp system requires a winch and crane for deployment; the dual frequency sonar systems can be deployed and retrieved by a single individual. The smaller systems can be operated from a small vessel for lake and estuary work and other shallow environments.
Maximum tow depths are compatible with continental shelf water depths.
|Sidescan-Sonar winch and crane||Deployment of the combined sonar and chirp system||Dual-frequency system on deck attached to a tow harness.|
The Woods Hole Science Center has collected sidescan sonar data over a great variety of bottom types, water depths, geologic frameworks, and host environments. When collected with other high-resolution geophysics (seismic-reflection) and ground-truth data, a comprehensive interpretation of the sea floor map is possible. All three WHSC sidescan systems can be used in all continental shelf depths, and one of the systems, the chirp sidescan sonar/subbottom profiler, is rated to 1000 meter water depths. During operations, the critical decisions are what ping rates to use in a particular area (thus controlling the swath width being used), and the objectives of the study: Are small scale features being mapped, or is a regional overview what the study warrants? For large-scale regional studies, we utilize a setting that obtains a 400 meter swath width. This allows our group to insonify a large area in a relatively short period of time. Ship’s speed while towing a sonar is kept to 5 knots, or about 2.5 m/s. Above this the strain on the data cable becomes too great, and the towfish unsteady resulting in marginal data quality. It is also harder to keep the towfish at depth at higher speeds. For smaller scale, detailed studies, we will run the sonar at anywhere between the 25 and 100 meter range, resulting in swath widths of 50 and 200 meters, respectively. This enables much higher resolution in the data imagery, and much smaller scale bedforms are discernible and able to be mapped. Below is an example of a regional study from the New York Bight Apex, 1995-1998. The data were acquired using our swept frequency sidescan sonar. In this image, high backscatter areas are shown as light tones, low backscatter as dark tones.
Sidescan-sonar imagery collected within the New York Bight Apex, 1995 -1998, acquired using a 90 - 110 kHz swept-frequency sidescan-sonar system. High backscatter areas are shown as light tones, low backscatter as dark tones. The sea floor appears very inhomogeneous on scales of 100's of meters. In the nearshore area off Long Beach, there are sharply defined, linear, shore-perpendicular to slightly shore-oblique, high-backscatter features. In the central part of the study area, seismic profiles confirm the interpretation of Cretaceous-age coastal plain strata outcropping on the sea floor, covered in places with a thin veneer of sediment, producing a particularly complex pattern of high and low backscatter. In the eastern segment of the study area, a series of curvilinear sand ridges is the dominant feature on the sea floor, appearing as areas of relatively low backscatter separated by relatively higher backscatter troughs. From Schwab et al, 2000.
Sidescan-sonar mosaic collected in Bear Lake, Utah-Idaho by the USGS in 2003. A dual-frequency sidescan-sonar system was operated for ease of deployment from the USGS R/V Rafael and to acquire high-resolution sonar data. Areas of high backscatter are shown in light tones and those of low backscatter in dark tones. Within Bear Lake, it was discovered that variation in backscatter was primarily related to the concentration of gastropod (a type of mollusk) shells, the higher concentrations correlating to high-backscatter regions. Additional interpretations of more subtle features of sublacustrine springs, a fault scarp, fan deltas, and a relict beach are possible. After Denny and Colman, 2003.
Sidescan-sonar data collected offshore of Myrtle Beach, SC, 1999 2003 using a smaller, dual-frequency sidescan-sonar system for nearshore surveying <5m water depth, and a chirp sonar for regions >5m. The USGS worked collaboratively with South Carolina Sea Grant, conducting geophysical surveys to define geologic framework within LongBay, SC in order to better understand factors controlling coastal erosion within the region. In this image, high-backscatter is represented by light tones, low-backscatter is represented by dark tones. Surficial grab samples and video show areas of low-backscatter to be characterized by fine-medium sand, and silt. Areas of high-backscatter are shown to be characterized by coarse sand, shell hash, outcropping strata, and gravel. (Baldwin, et al., 2004).
Baldwin, W.E., Morton, R.A., Denny, J.F., Dadisman, S.V., Schwab, W.C., Gayes, P.T., and Driscoll, N.W., 2004, Maps showing the stratigraphic framework of South Carolina's Long Bay from Little River to Winyah Bay, U.S. Geological Survey Open-File Report 2004-1013.
Denny, J.F. and S.M. Colman, 2003, Geophysical surveys of Bear Lake, Utah-Idaho, September 2002, USGS Open File Report 03-150, CD-ROM.
Fish, J.P. and H.A. Carr, 1990, Sound Underwater Images, A guide to the generation and interpretation of sidescan sonar data, Lower Cape Publishing, Orleans, MA, p.23.
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.
Urick, R.J., 1983, Principles of Underwater Sound, Peninsula Pub., 423 pp.