Sounding Better!
Processing Multibeam Data from Rock Cuts
By Mike Kalmbach
For obvious reasons, extra care needs to be taken with survey and processing of rock cut
channels. USACE Philadelphia district is preparing for just that; blasting and channel
deepening, and they’ve asked for some HYPACK training. Given that I haven’t thought about
this for a while (and “Sounding Better” articles are due), a writeup seems the natural way to
clear the cobwebs. Here it is.
FIGURE 1. Oh no! Image courtesy of Jim Mrozek, Soo Ste. Marie Area Office.
SURVEY PLANNING
Two simple procedures to optimize accuracy:
1.
Use a +/- 45 degree multibeam swath. This is helpful to control depth errors due to
sound speed profiles that don’t fully match conditions. A particular problem in estuaries
where tidal changes are difficult to keep up with. The 45 degree limit has been in place
over rock as long as I’ve been involved with USACE and multibeam, 20+ years. I don’t
believe it’s changed but you can refer to the Engineering Manual to check.
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Processing Multibeam Data from Rock Cuts
Use planned line spacing for 200% survey coverage. At +/- 45 degree swath, line
spacing = water depth * tan(45) = water depth. The 200% coverage allows all detections
above grade to be sounded twice. Helps distinguish between fish (detected once) and
rocks (detected twice).
Here I’m going to stress the importance of 200% coverage. When processing and analysis
time comes, you (or someone else) will be thankful.
2.
PERFORMANCE TESTING
The battery of HYSWEEP performance tests should be applied to your survey boat. Beam
angle test, single beam comparison, bar check, etc. These tests determine accuracy and
repeatability of the multibeam system. Modern systems are repeatable within 0.2’ (6
centimeters). Surveyors typically compare multibeam to single beam over a reference area,
the single beam being the standard for accuracy. These tests are described in the USACE
Engineering manual.
TPU predicts system uncertainty from individual component specifications (GPS, MRU, etc.).
TPU matches the empirical tests in the cases I’ve looked at.
Figure 2 shows a test within one of the Soo Lock chambers. Boat was stationary and lead line
gave depth of 33.7’. As we are sounding over flat concrete the uncertainty is easy to see,
matching the 0.2’ from performance testing and TPU. Beam 150 is trouble - it will be tossed in
processing.
FIGURE 2. Soundings in a flat lock chamber. Water depth in the lock is 33.7 feet. The spread of sounding
depths is a nice visualization of system uncertainty.
PROCESSING AND ANALYSIS IN MBMAX64
Multibeam processing and the graphs shown in the remainder are from our MBMAX64 editor.
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Processing Multibeam Data from Rock Cuts
FILTERS
Filters are useful with absolutes such as beam angle and depth limits (figure 3). No room for
statistical issues with these. Filters based on statistics should be avoided when looking for
rock hazards. Better to make the decisions yourself.
FIGURE 3. “Safe” Filters I Would Use on Rock Cut Soundings. (Depth filters based on statistics might
remove soundings that are better off kept. Refer to Figure 1.)
USING CHANNEL INFORMATION IN EDITING
Three things you can do, shown in figures 4-6.
1.
Color by Shoal Depth: In Color Settings, select the Shoals palette and set the project
depth level. Soundings above project depth are drawn in danger red, below project depth
in safety green.
FIGURE 4. Setup for Sounding Colors Above or Below Project Depth.
2.
Project Depth Line in the Profile View: The bold white line is drawn at project depth
(Figure 5).
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Processing Multibeam Data from Rock Cuts
FIGURE 5. Project Depth Line.
3.
Overlay Channel Plan in Profile View: If a CHN file is available you can use it in editing
to reference soundings and make better edit decisions. Use File menu, Overlay Channel
Plan.
FIGURE 6. Channel Plan Overlay.
SEARCHING BY HIT COUNT
This carries out the “3 Hit Rule” from the USACE Engineering Manual. Setup is shown in
figure 7. Enter the Hit Count (soundings per matrix cell) and Project Depth then Search.
MBMAX64 will locate the next matrix cell at or above the hit count. Search does not require
soundings to come from differing survey lines.
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Processing Multibeam Data from Rock Cuts
FIGURE 7. Setup for Searching by Hit Count.
MANUAL EDITING IN THE PROFILE WINDOW
To my way of thinking, this is the best way to inspect and edit rock surveys. Three things I like
to keep in mind.
This editing is to find rocks above project depth. Therefore, anything below is
unimportant. Project depth line or plan overlay is very useful here. Editing for volumes is
different – in that case you are interested in all the soundings.
2. As a general rule, soundings from a single survey line are, by themselves, not of
great importance. It’s only when soundings from overlapping lines are seen that you
have to pay attention.
3. We are not creating a Picasso! The tendency is to over-edit. Resist that and pay
attention to project depth and overlap.
1.
Figure 8 shows a hit using criteria 1-3 above. Although there might have been over editing.
Point flags are useful if you see something you want to come back to later. Golden soundings
are useful to prevent a shoal, once detected, from getting accidentally removed. The F5 key
marks a HYPACK target.
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Processing Multibeam Data from Rock Cuts
FIGURE 8. Shoal Detection from Multiple Survey Lines.
OTHER EDIT WINDOWS
The Sweep, Cloud, Cell and Imagery windows provide different ways of looking at soundings.
Sometimes useful, sometimes not. The windows are kept in sync so that the cursor point in
the profile window will match the cursor in the others.
Visualization in the Sweep Window (Figure 9) shows a sounding to clearly be bad. It may not
be as obvious in Profile or Cloud windows. That’s why the data cursors in MBMAX64
windows are always kept in sync.
FIGURE 9. Visualization in the Sweep Window
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Processing Multibeam Data from Rock Cuts
NEW METHODS – MULTIPLE DETECTION AND WATER
COLUMN
Multiple detection allows more than one sounding per beam. It’s for high detail sounding of
structures and “seeing behind” structural elements. As such it does not seem useful with rock
surveys.
Water column shows the analog multibeam data – equivalent to a single beam echogram
times 256 (or however many beams you use). It might be useful when the survey boat returns
to verify a target. The high data volume of water column isn’t much of a problem when used
in this limited way.
FIGURE 10. Water Column Display in MBMAX64 Editing.
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