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SIDESCAN SONAR!

SIDESCAN SONAR!. Some terminology. Footprint: Seafloor area sampled by a single acoustic signal pulse. Fish: An instrument towed behind a ship -- e.g. sidescan sonar and its casing. Hull: The main body or structure of a ship. Sidescan sonar. Multibeam bathymetry. Hull-mounted vs towed.

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SIDESCAN SONAR!

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  1. SIDESCAN SONAR!

  2. Some terminology Footprint: Seafloor area sampled by a single acoustic signal pulse. Fish: An instrument towed behind a ship -- e.g. sidescan sonar and its casing. Hull: The main body or structure of a ship.

  3. Sidescan sonar Multibeam bathymetry

  4. Hull-mounted vs towed • Hull-mounted (i.e., most swath bathymetry systems): • Are continually mounted on the ship; don’t require repeated deployment • Can collect data while ship is being used for other purposes (e.g., physical oceanographic cruise) • Fast surveying (ship can travel at ~10 knots) • Acoustically noisy (near-surface turbulence and ship noise) • Resolution constrained by near-sea-surface location (i.e., can’t go lower to get more detailed picture of seafloor) • Hard to access for repairs • Can only be used with certain hull shapes

  5. Hull-mounted vs towed • Towed (i.e., most sidescan sonar systems): • Easier to move from ship to ship • Operate in quieter water at greater depth • Must be deployed each cruise • At deployment, ship speed must be near zero • Location of fish with respect to ship must be measured or calculated

  6. Basic physics: Sidescan sonar • A set of transducers on the instrument transmits acoustic energy through the water column • Energy encounters seafloor • Energy can be: • Reflected • Refracted (Snell’s Law) through substrate • Scattered/reradiated from seafloor features

  7. Basic physics: Sidescan sonar • Sidescan instrument collects the backscattered energy returning from the seafloor. • Produce images of the topography or roughness of the sea floor based on the strength of the energy backscattered off of the seafloor. • Images kind of look like aerial photographs, but are produced using sonar rather than light. “Illuminated features.” • “Reflectivity” is often used in place of “backscatter” (although strictly speaking, reflected energy is different from backscattered energy).

  8. Instrument design/data collection • Two outgoing sonar signals, one to either side of the instument. Beams expand with distance from the instrument. • Directly beneath instrument = “nadir” = no signal. • Slightly different frequencies on each side (11 kHz for port and 12 kHz for starboard) to decrease interference between return signals on each side. • Strength of returned signal is assigned value 0 to 255 (0 = black and 255 = white). Translated into pixels.

  9. Image interpretation • Strong returned energy (hard bottom, no sediments) = dark on a light background. “Like shining a black flashlight on the seafloor.” (GLORIA and TOBI are opposite.) • Picture of roughness of the seafloor. Sedimented abyssal plains = low roughness. Faults, fissures, slopes, new volcanic features = high roughness/”high reflectivity.” • Lots of geometric (angle) corrections. Requires computer processing. Fish depth, bathymetry, look angle, speed of survey, frequency, repetition of pings, acoustic noise, track curvature. • Data duplication/deletion at turns.

  10. Image interpretation • Features perpendicular to shiptrack are deemphasized (“underensonified”), while features parallel to shiptrack are overemphasized (“overensonified”). • “Simulated sun at a low grazing angle.” Shadows (sonic, not light).

  11. GLORIA

  12. GLORIA GLORIA = Geological Long-Range Inclined ASDIC ASDIC = Anti-Submarine Detection and Investigation Committee Southampton Oceanography Centre and USGS Towed 200 m behind the ship. Tow depth: 30-60 m Depth range: up to 11 km Emits a pulse every 30 seconds. Survey speed: 12 knots

  13. DSL-120 (DSL = Deep Submergence Laboratory (WHOI))

  14. DSL-120

  15. SeaMarc II No current photo available

  16. HMR1

  17. Coaxial Segment, Juan de Fuca R.

  18. Coaxial segment,Juan de Fuca Ridge

  19. Coaxialsegment,Juan de FucaRidge

  20. Coaxialsegment,Juan de FucaRidge

  21. Puna Ridge Kilauea first began erupting in 1983. Lateral protrusions of magma (dikes) = rift zones Southwest Rift Zone. East Rift Zone: 55 km from summit to shore, then another 75 km underwater. Pu’u O’o: site of current Kilauea eruption (20 km from summit) Puna Ridge = submarine portion of East Rift Zone R/V Thompson (U.W.) and DSL-120. Explore submarine magma transport processes.

  22. Finding shipwrecks s/s Paula Faulbaum Oct. 18, 1941, headed just south of Stockholm, Sweden Raining, wind blowing at 18-20 m/s Hit aground, took on water. Crew abandoned ship. Next morning, ship had sunk. Bow is 40 m deep, stern is 70 m deep.

  23. Finding shipwrecks

  24. Finding shipwrecks

  25. Finding shipwrecks s/s Nedjan. Built in Scotland in 1893. 64 m long. January 1954: Departed Sweden. Overnight, steering malfunctioned and she started to drift. Lost off the coast. No survivors (crew of 17). Found in 1996 at 32 m depth using sidescan.

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