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1144 10 th Avenue, Suite 200 Honolulu, HI 96816-2442 (808) 539-3706 firstname.lastname@example.org Software Services Systems Oceanic Imaging Consultants, Inc. Professional seafloor mapping software, created by surveyors, for surveyors
Sound never looked so good… Introduction to SONAR
Sidescan Systems AMS-120, SM 30 Atlas Deso Benthos / Datasonics SIS150X, 3000, C-3D EDA EdgeTech 272, DF1000, FS-AU EG&G GLORIA Imagenex Innerspace Klein 595, 2000, 3000, 5000, 5500 RDI - BSSS SeaBeam S-150 SSI TAMU TOBI Tritech SeaKing Ultra Electronics Deepscan OIC-supported Sonar Systems Multi- Beam Systems Atlas Hydrosweep Elac Odom Echoscan Reson SeaBat SeaBeam Simrad EM12, 950, 300, 3000 Interferometric Systems AMS120 (Woods Hole DSL) SeaMARC Ultra Electronics Deepscan *GeoDAS can be customized to support any underwater sensor system. Optical Systems Raytheon Laser Linescan SAIC Laser Linescan Electronic Still Cameras Arctic Streak LIDAR
SONAR Sidescan data Beam width Ping Towfish Waterfall Overview of Terms Bottom tracking Altitude Oscilloscope Slant Range Ground Range Imaging Artifacts
Introduction to Sonar SONAR: Sound Navigation And Ranging Early sonars returned single values for range to target, Sidescan Sonar extends the beam of sound laterally, in a fan, mapping a swath across the seafloor. ~What direction is the target, and how far away is it?
Speed of Sound in Salt Water = VOS =~ 1500 meters / second Function of pressure, temperature and salinity Pulse Length = time (in milliseconds) in which the sonar is actively transmitting energy Ping Period = time (in seconds or milliseconds) in which the sonar listens for echoes before it pings again. Recall: SONAR = SOund Navigation And Ranging i.e., What direction is the target and how far is it What direction is the target? Straight down, out to the side, or somewhere in between. Without angle measurement, can only know range How far is the target? Distance = Rate * Time General Terms and Definitions
Defining a Ping Each sonar pulse illuminates an area on the seafloor - the echoes from which we record, and call a “ping”. The pulse length and beam width define the sonar’s ability to resolve items on the ground. Shorter pulse lengths and narrower beam patterns give higher resolution. 100% coverage requires that we go slow, or ping fast, so that no gaps occur between pings. This implies a trade-off between survey speed, sonar range and survey coverage.
Where is the sonar? Hull-mounted or in a “Towfish”. Vessel position calculated from a DGPS (hopefully); and sonar towfish position calculated relative to vessel, via estimates of offsets, depth, cable-out and relative bearing to ship (or USBL) Sidescan “towfish” Sonar Placement
Sonar Placement (cont.) Here’s another perspective…
Sidescan & Data Transfer Let’s begin with the sensor… A sidescan sonar towed behind the boat that sends out a pulse (ping) of sound waves in a 180-degree arc.
Next… All sound waves that come in contact with a solid object are reflected or echoed back toward the sonar.
Then… The sonar receives each return pulse (which travels at about 1,500 meters per second) and sends it to the boat.
Finally… Instruments on the boat measure and translate each pulse’s strength, slowly building a “picture” of the objects or sea floor below as the boat passes over them.
In a nutshell… Each ping illuminates an area of the seafloor beneath and perpendicular to the sensor (a “slice” of the seafloor). The backscatter data for each “slice” is received by the sensor and sent to a shipboard waterfall display, such as seen in GeoDAS, and provides a display of the seabed slices in consecutive order, forming a scrolling image of the seafloor.
What is a “waterfall”? Purpose of a Waterfall The waterfall display is what appears on the computer screen via a data acquisition software program such as GeoDAS. A waterfall display is used to portray the recorded sonar pings as a vertically scrolling image of sequential scan lines abutted to portray a continuous image of the bottom, optionally corrected for vessel speed.
Bottom Tracking What is “bottom-tracking”? The process of detecting the arrival time of the first echo from the bottom is known as “bottom-tracking” and is mandatory for successful operations. With an estimate of the sensor’s height above bottom, or altitude, and the assumption that the bottom is nominally flat across-track within the coverage of the ping, we can convert the raw time-domain (slant range) imagery of any target at point “P” to ground range images, and represent the actual ground range to the target on the bottom, via the Pythagorean Theorem.
Detecting the first return, as shown by the black lines superimposed on the raw data, gives us the sonar’s height off seafloor bottom also know as “altitude”. GeoDAS uses its own built in oscilloscope to detect this first return and begin bottom tracking which is important to properly apply some of the image enhancement routines. Oscilloscope Sonar echoes (port and starboard) from one “ping” Detection of 1 st return gives alt.
Bottom Tracking Given the speed of sound in water, the arrival time of each acoustic backscatter sample produces a direct measure of the range to the reflecting target. This allows GeoDAS to track the seafloor bottom. Bottom-tracking is indicated in GeoDAS by the red lines in the bottom-tracking waterfall and in the sidescan waterfall. Bottom-tracking is most helpful in avoiding dredging…
Slant-Range vs. Ground Range Slant range is the raw time-domain from the sonar to the target. As mentioned in bottom-tracking slide, the slant-range can be converted to ground range by using Pythagorean’s Theorem.
; Slant - Range Imagery: A “raw” display of sonar data, object shapes may be distorted Image without Bottom-Tracking Image with Bottom-Tracking applied Ground-RangeImagery: Requires knowledge of terrain, slope or the "flat - bottom assumption“ Slant-Range vs. Ground-Range
Uniformity of sidescan imagery is dependent on uniformity of illumination. Major limits on uniformity are: Spreading Losses: Intensity (2R) = Intensity (R) / 4 (1 over R squared rule); approximate, but reasonably close; often estimated Fix these with TVG Beam - Pattern Variations (side - lobes): Illumination by sonar is not perfectly even; can cause severe image banding Fix these with AVG Imaging Artifacts
Review SONAR:Acronym for SOund Navigation And Ranging Sidescan data: Arrive as time series (port & stbd) of echo amplitude Beam-width:Footprint of the sonar ping on the ground, usually in deg. Ping:All the samples recorded from 1 active transmission Towfish:Hydrodynamic towbody containing sonar electronics Waterfall:Display of sequential pings as grayscale images Bottom-tracking:The process of detecting the first return (gives altitude) Altitude:The height of the sonar off the seabed Oscilloscope:Backscatter sampled by sonar in 1/SAMPLE_RATE Slant Range:Cross-track resolution is constant in time Ground Range:Cross-track resolution is constant in space Image Artifacts:Distortions in sonar data