SIDE SCAN Theory and Operation Sidescan Survey Overview
A sidescan sonar can be used for a wide variety of survey operations. Search and recovery Geological Identification Pre / Post dredge surveys Target verification and location It is also a useful tool for correlating and verifying bathymetric data Shipwreck with shadow (in white) Sidescan data showing sediment change What you find with a Sidescan Record
Shadow Water Column How the sonar sees the object: Water Column -Provides information about towfish height. Target-features off the seafloor will produce a shadow. Bottom sediment -Signal will differ based upon return angle onincident and seabed absorption. 3 What happens at Nadir region
There is no White Area under the fish. The secondary lobes ensonify the nadir. However: The incidence angle makes this area not really useful The importance of the Shadow
As the towfish gets closer to the seafloor, the shadow size will increase The importance of the Shadow
The shadow could be more helpful that the shape of the object. The importance of the Shadow
Same object with his shadow. Take a look at this image. What is it? The Shadow Effect Imaging the seafloor from above will cast a shadow of the object.Sometimes the shadow can give you more information that just the data on the seafloor What you find with a Sidescan Record
Received Signal T0 (Ping) T2 T1 (First return) T2 (range) Signal VS Time Time T2 Time T1 T0 T1 T2 What you find with a Sidescan Record Understand the Amplitude VS Time Signal
The waterfall view is the amplitude coded in color level. Detection of first return for bottom track Sidescan Theory Sonar system components
The sonar system (towfish) has a transmit and receive component takingthe signal (digital) and sending to the acquisition computer for display andstorage. The echo distance is determined by time, similar to other sonar systems Display Transmitter / receiver Sidescan Signal Distance = time (round trip) x Sound Velocity 2 Note the use of Sound Velocity. It is needed to compute the distance.The default of 1500 m/s will give an error Bottom echo Time of pulse What is a Sidescan Resolution.
Across Track Resolution depending of the frequency pulse. Along Track Resolution depending of the aperture Horizontal Aperture (0.25 to 0.8). The Along Track Resolutionis not linear along the swath. The Across Track Resolution is the same along the swath The resolution used to qualified a sidescan is the Along Track Resolution Across track range resolutionWhy sonars work better at the outer ranges
Distinguishing two independent objects can be referred to asthe sonar resolution. If two objects are too close, they will appear as one on the sidescanrecord. Getting these objects further apart will show them asindependent objects. How close can they be? Half the pulse length. Example: A 500khz system has a pulse length of 1 cm. In practice, that wont be seen, and cannot distinguish an object thatclose.We can approach this value further away from nadir. The footprint of sonar (arriving energy) is longer near nadir due to theangle of incident. Conversely, the further away, the more the bottomfootprint approaches the pulse length. At the very far end of the signal, the best resolution would be obtained First return Along track resolutionwhy objects in the far field cannot be distinguished
The resolution in the along track direction is dependent on the sonar Horizontal beam width. The image below represents what happens when targets in the far field are inside the angular resolution of the sonar. They become indistinguishable and look as a single object At the near field, these objects can be distinguished. Horizontal beam width First return Image quality vs. location
Targets is these areas might be missed or cannotbe measured accurately Ideal location for targets The nadir return is degrading causing by the incidence. The sidescan image typically degrades at the far end of the swath When running survey operations, the best image will be in the center of the swath Survey Speed The survey speed has to take in account the limitation of the sidescan. Speed < Max Speed Coverage 100 % Speed > Max Speed Coverage < 100 % Height of Contact (H) = L * A / R
Measuring a contact: Three points along the image are needed altitude (A) , shadow length (L), total distance (R) Each target will have a shadow; use the tools in Sidescan Survey or HYSCANto measure contact Height of Contact (H) = L * A / R 20 Sidescan operation Survey planning: 100% and 200% coverage
100% coverage: At 100 meter range scale, line spacing would be set for 160 meters. This provides a 20 meter off-track error while surveying. 200% coverage: This survey plan ensures that the bottom is covered twice by the Sidescan, covering the nadir region with the second line pass Line spacing 160 meters Line spacing 80 meters Swath width 200m Note the overall swath width is twice the range scale 22 Collecting sonar data is different than echosounder data
Survey operations will change when running a sidescan sonar Vessel speed (slow is best 4 to 5 kts is typical) Vessel handling (you cant stop when towing a sidescan) No turns during collection Line spacing is fixed (unlike multibeam collection) Nadir area is obscured. Visual display of waterfall gives immediate results (sometimes that is good enough for folks) Sidescan files are large, more storage needed But. Even with all that, sidescan by itself or part of a surveyoperation provides very good results. Sidescan positioning Getting the position of the towfish is critical.Seeingan object but not knowing where it is, will not be veryuseful. Cable out (layback) Acoustic positioning system (USBL) ROV (inertial systems on board) Ship mounted (uses on board GPS with offsets) Which one is best?Dependent of survey operationsand water depth. How to do a self check of object detection
During survey operations, set the range scale of thesonar to your operating range. Run past a know object. A buoy block or sand waves willwork Ensure that you can see the entire swath. If you cant,shorten the range scale until you do. If you dont see a known object, chances are you wont find the unknown object. Sidescan Deployment Towfish deployment Sidescan sonar transducers can be fixed to a vessel for shallowwater deployment or towed when operating in deeper water Operating a fixed mount (on survey vessel)
RANGE SCALE Optimum Fish Height 50 meter 4 10 meters 75 meter 6 15 meters 100 meter 8 20 meters Too shallow, high angle A rule of thumb to get the best results: Use 8% to 20% of range scale for fish height Why 8 20% or range scale? This altitude will provide the best angle of incidence (reflection of an object) for a sonar return. When using a fixed mounted sidescan the image may degrade in water depths greater then 40 meters. The angle is too steep Too high, high angle 27 TOWED Sidescan Operations
TowFish It is critical that the towfish is close to the seafloor. Remember the 8-20% rule TOWED Without USBL tracking, the navigation of the fish is an estimation.Using a layback driver based on the cable out, the fish position is not precise, especially on turns Hull Mounted For a hull mount, you know the exact position of the side scan. This is typically used for shallow water applications. Angle of incident becomes too sharp when working in deep water when hull mounted Hull Mounted For a hull mount, you must be careful that the side scan is aligned with the vessel keel.Otherwise, you must make an adjustment to the yaw offset for the sidescan.