Top 10 multibeam issues

The MAC, technicians, and colleagues encounter several common factors that limit data quality across a wide variety of platforms.

Top 10 common issues

Here are ten all-too-common complications to consider when planning, collecting, and processing multibeam data:

1. Inaccurate vessel offsets (or incorrect interpretation)
   1. Data quality depends fundamentally on correct sensor configuration; see Dimensional Control
2. Inadequate sound speed profiling and/or mismatches at the transducer
   1. See Sound Speed and SmartMap
3. Higher noise levels due to biofouling and changes in machinery
   1. Run pre- and post-shipyard RX Noise tests to examine this
   2. For Kongsberg systems, see the Transducer Cleaning, Fairing, and Painting Procedure
4. Inappropriate runtime parameters
   1. Automatic modes still need monitoring by experienced watchstanders
   2. For instance, the depth gates mean business!
   3. Scanning acquisition parameters can help to identify the root causes of bad (or missing) data
5. Infrequent calibrations
   1. Routine patch testing can rule out some biases
6. Interference from other acoustic or electronic systems
   1. Is that 12 kHz bridge fathometer really secured?
   2. Synchronize your scientific echosounders
7. Sea state, aeration, and bubble sweep along the hull
   1. Work is underway to adjust ping cycles around washdown events
   2. Meanwhile, testing RX Noise vs. swell direction can help to identify quieter/better survey orientations for each particular vessel
   3. Mapping is often the 'back up plan' when other work is on hold due to sea state!
8. Waterline errors
   1. Like other sensor offsets, this directly affects the reported depth
   2. Waterline impacts refraction correction by changing the 'starting point' in the sound speed profile
   3. The value depends on the manufacturer's conventions and is not always equivalent to the draft
   4. Use the Waterline Worksheet to calculate this parameter for Kongsberg systems
   5. Sound Speed Manager plots the transducer sound speed value and depth; this can be extremely helpful in verifying the waterline configuration
9. Infrequent operation
   1. It takes longer to identify issues when the systems are not operated routinely
   2. When issues do arise, they are under more 'critical' circumstances and become 'emergencies'
   3. Opportunistic testing and transit mapping helps to maintain operator familiarity and catch problems early
10. Outdated software and firmware
    1. Over the 10+ year hardware lifespan, manufacturers routinely release software and firmware updates to fix real issues with operation
    2. While some of these might be simple user interface updates, some address fundamental errors in TX or RX processes
    3. Keeping systems up to date can improve data quality (e.g., reduce outliers, provide new warnings to users) and protect hardware health (e.g., adjust duty cycles or power limits)

Uncommon multibeam issues

Here are a few examples of issues that severely impacted data quality and took a while to sort out, partially because they may are not common problems.

1. Transducer anti-fouling paint (over-application)

   1. If an array is painted, it must follow the manufacturer spec
   2. Adding mass (paint) to the transducer can drastically change its frequency response
   3. This reduces TX power and RX sensitivity, while increasing acoustic attenuation
   4. The net results are very poor coverage and accuracy for the incorrectly painted arrays

2. Array orientation in sensor setup (incorrect rotation)

   1. Some systems allow 180-deg rotations of the arrays to fit various configurations
   2. Incorrect 'rotation' in the sensor configuration (i.e., array heading) can be applied at the flick of a toggle button
   3. Incorrect 'rotation' of either array can lead to fundamental mismatches between the pulse forms transmitted at ping time and expected during the RX cycle
   4. The net results are wildly inaccurate soundings (or no bottom detections at all)
   5. These symptoms are often less severe with CW sectors and extremely severe where FM is used (a telltale sign of incorrect array rotation!)
   6. Array installations must be documented with pictures showing the cable orientations (and module numbers) to confirm setup in the software

3. Array module order (cabling out of sequence)

   1. Some arrays are made of multiple modules which must be installed in a particular order
   2. Installing or cabling the modules out of order leads to fundamental beamforming errors (TX, RX, or both!)
   3. When TX modules are cabled out of order, there is a risk of the radiated TX beampattern deviating severely from the intended shape and amplitude
   4. The net results are poor bottom detection, scattered distributions of soundings, and mistracking
   5. Mistracking is sometimes more clearly evident on slopes, where TX sidelobes ahead or behind the main lobe are providing stronger returns