Motion Tolerance in SonarWiz

Chesapeake Times, Vol 9 | April 2022

In the latest release, SonarWiz is now more able to import and process the Motion Tolerant sidescan packet from Edgetech sidescan sonars.  The data packets, stored in subsystem 70 and 71, are collected by the Edgetech 6205s and 4205 Sidescan sonars. Until this update, SonarWiz relied on the data file having either the standard CH1-2 records (subsystem 20 and 21) or having the CH1-2 having already been replaced (using an option in EdgeTech Discover software) by the Motion Tolerant variant. Now, with this update, the sonar operator does not have to decide during the survey as to which channels to keep/store to JSF, but can keep all channels and decide later in post SonarWiz processing.

While using a sidescan, having the fish travel with no external motion forces – pushing the fish left and right or up and down, will typically produce the best imagery.  We can see this type of stability on AUV’s, as these are very stable platforms. However, a survey operation with either a fixed mount sidescan or a towed sidescan, these movements will affect the data.  While towing, this is heightened when operating in shallow water, when the ship motion is transferred as a damped wave down the tow cable.

Motion of a towfish with yaw forces acting up the data. The resultant image will show the streaks of data in the across track direction

The yaw and pitch will induce these artifacts in the data, and when the motion is too rapid the receive beam can end up with no signal.  This can be seen as the streaks in the across track data. It is more noticeable in the longer ranges and narrow beamwidths. In a simplistic term this will appear when the Yaw rate > (½ x BeamWidth )/time  

Edgetech implemented a hardware solution to compensate for this motion; by having different transmit and receive bandwidths, namely the wider receiver beamwidth goes a significant way to compensate for the motion of the fish, in return for negligible loss in resolution.

The small increase in bandwidth of 25% to 40% will capture a 3-fold increase in the yaw rate, in the area where this banding occurs.

In SonarWiz, we updated the reader to capture this data packet.  Sometimes the hardest thing to do is add a new feature without breaking anything, while making this feature easy to use. The motion tolerant data can be either low frequency or high frequency.  However, we already have a spot for the LF (CH 1-2) and the HF (CH 3-4).  The solution was for the Motion Tolerant data to be imported as the channel 5-6 pair. 

Over the past two years, this CH 5-6 pair has been used in SonarWiz to handle the tri-frequency channel and to take in the Gap Fill data.  Now this pair has a third option, for the Motion Tolerant.  To clean things up a bit, we added a user selection in the File Specific Option page for the user to select how this channel pair will be used.

Data from a survey done shows the difference between the two data results. In the first image, the data is the standard low frequency side scan, with the survey line running east-west.  The second image is from the same low frequency side scan channel but the Motion Tolerant variant.  

Survey line, CH1-2, without the Motion Tolerant feature
Survey line, CH 5-6, with the Motion Tolerant feature

This feature doesn’t mean you can go out and survey in sea state 5 on a small boat.  There are limitations to how much the data can be corrected.  Take as a rule-of-thumb, roughly doubling the 2-way Rx beamwidth in turn should allow for around a 300% increase in the yaw rate at which the banding might start to occur.  The image would only get worse when the rate is exceeded, as the Rx is looking completely elsewhere when the signal is returned.  The absence of the banding in the Motion Tolerant image makes interpretation significantly easier.

Getting the perfect mosaic sometimes involves tools from both the hardware and software side.  This was the case for handling the EdgeTech motion tolerant data.  In other situations, we need to rely solely on the tools in SonarWiz, using a combination of gains and filters. There is no one single solution to clean data.  We continue to add more to our tool kit to improve the quality of the sidescan mosaic image.