1. SIDE SCAN Theory and Operation

2. 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

3. 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 on incident and seabed absorption. 3

4. What happens at Nadir region
There is no « White Area » under the fish.
The secondary lobes will ensonify the nadir.
However: The incidence makes this area not really useful

5. Take a look at this image. What is it?

6. 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

7. The importance of the Shadow
As the towfish gets closer to the seafloor, the shadow size will increase

8. The importance of the Shadow
The shadow could be more helpful that the shape of the object.

9. The importance of the Shadow
Same object with his shadow.

10. 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

11. What you find with a Sidescan Record

12. Understand the Amplitude VS Time Signal
The waterfall view is the amplitude coded in color level.
Detection of first return for bottom track

13. Understand the Amplitude VS Time Signal
The waterfall view is the amplitude coded in color level.
Detection of object on seabed

14. Sidescan Theory

15. Sonar system components
The sonar system (towfish) has a transmit and receive component taking the signal (digital) and sending to the acquisition computer for display and storage.
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 

16. 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 Resolution is 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

17. Across track range resolution Why sonars work better at the outer ranges
Distinguishing two independent objects can be referred to as the sonar resolution.
If two objects are too close, they will appear as one on the sidescan record. Getting these objects further apart will show them as independent objects. How close can they be? Half the pulse length.
Example: A 500khz system has a pulse length of 1 cm.
In practice, that won’t be seen, and cannot distinguish an object that close. We can approach this value further away from nadir.
The footprint of sonar (arriving energy) is longer near nadir due to the angle of incident. Conversely, the further away, the more the bottom footprint approaches the pulse length.
At the very far end of the signal, the best resolution would be obtained
First return

18. Along track resolution why 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

19. Image quality vs. location
Targets is these areas
might be missed or
cannot be 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

20. 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 %

21. 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 HYSCAN to measure contact
Height of Contact (H) = L * A / R 21

22. Sidescan operation

23. Sidescan Deployment
Towfish deployment
Sidescan sonar transducers are typically mounted on a towfish. For shallow water operations they can be hull mounted.

24. 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 24

25. Collecting sonar data is different than echosounder data
Bold statement, but it is true.
Survey operations will change when running a sidescan sonar
Vessel speed (slow is best – 4 to 5 kts is typical)
Vessel handling (you can’t 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 survey operation provides very good results.

26. Sidescan positioning
Getting the position of the towfish is critical. Seeing an object but not knowing where it is, will not be very useful.
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 operations and water depth.

27. How to do a self check of object detection
During survey operations, set the range scale of the sonar to your operating range. Run past a know object. A buoy block or sand waves will work Ensure that you can see the entire swath. If you can’t, shorten the range scale until you do. If you don’t see a known object, chances are you won’t find the unknown object.

28. 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 28

29. TOWED Sidescan Operations
TowFish

30. TOWED
Without USBL tracking, the navigation of the fish is an estimation. Using a layback driver based on the “cable out”, the fish position could be degraded.

31. Hull Mounted Hull Mounted
For a hull mount, you know the exact position of the side scan. It is more difficult to determine the height of objects above the seafloor. This is typically used for shallow water applications.

32. 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.