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 CSL Heron Squamish Operations
April-October 2017

Survey Update Plan

John E.Hughes Clarke, Anand Hiroji, Liam Cahill Ian Church,  Angela Gillis
Center for Coastal and Ocean Mapping
University of New Hampshire, USA
 Ocean Mapping Group
University of New Brunswick, Canada

This page lists the ideal set of standard lines that should be run for each survey update epoch.

There are 4 sets of data to collect:


The South Channel Corridor


The main focus of the 2017 survey is the section of the south channel, from where it cuts through the fossil levee in ~ 120m of water, to its distal limit. We have now done a standard set of three lines along the channel  including one in the axis, and two offset to the north or south (see figure below showing actual navigation). The aim is to both gets good bathymetry AND get backscatter imaging.

The issue with the backscatter imaging is that, at high grazing angles (close to nadir), while the bathymetric tracking is usually best, the backscatter is dominated by near specular echoes. In that geometry, the backscatter is especially sensitive to small changes in seabed slope which change the grazing angle. Thus to avoid this, the offset lines were added (this has already been done for the Bute channel surveys since 2014).

racetrack

These lines need to all be done in "Very Shallow Mode". Additionally there are specific sector constraints:

The asymmetric swath, allows more beamforming channels to be focused into the channel to improve the oblique bathymetric definition.
The three lines should be Aldebaran. In case they are not here is a list of coordinates

All done at no more than 6 knots.

While running these lines, I had expected the centre line to provide excellent bathymetry. However, the sidelobe echoes off the inward facing channel walls actually provided some very bad bottom tracking.  As a result, I set up in Aldebaran (but never used) a series of cross-lines. The idea was that by running across the channel, those sidelobe mistracking events should be absent And I added heavy overlap so that there would be lots of redundancy. Furthermore, by keeping the swath width wide (fixed to +/-250m), I could look at the backscatter of the channel floor from a range of differing grazing angles, facing either up channel or down channel.

I've reproduced the line plan here:

cross lines

I'm not sure what spacing exactly I used in Aldebaran. But I'd like to have 125m spacing (same as the along lines). And to be using +/-250m swath - forced to Very Shallow mode.  For these cross -lines, you do not want to be logging the corners. And try to be steady on line with the yaw stabilization reduced when you start logging.


The coordinates of the lines in the figure above can be found in this list of coordinates. I've included 15 lines. With the turns, this is going to take a fair bit of time (6 knots or less). If you are short of time, you can just do every second line. But I'm hoping that the heavy overlap and redundancy will give me the best image possible.

If you have excess time (I doubt it), I'd like to try running the cross lines with a +/-125 swath width - to get better sounding density for the bathymetry.
The Distal Lines

Beyond the South Channel, there is an interest in defining the distal deposition and backscatter modulations. To address this, there are already a series of survey lines in Aldebaran.  Note that the original lines were every second one, but in 2015, I added the splitting lines. This meant heavier overlap, allowing backscatter imaging from a single azimuth with nadir data suppressed.
The lines of interest for the 2017 program do not need to go down to Britannia Beach (or on to the sill at Porteau Cove). We will do that extension in October.

distal

This covers the area where there are pronounced backscatter modulations that clearly are related to active flow:

mos

The reason the line density is so tight is so that we can replace all the nadir backscatter data with oblique data from the adjacent line.

Note that to the south of Watts Point, the seabed backscatter goes up markedly. This is an active Environment Canada approved dredge spoil disposal site.


And the depth differences clearly show that there is net deposition in this region (with a pronounced flow transverse undulation).:

depth differences

Note that the depth difference implies that the active flow is all within the first 4 lines and only extends out part way to Woodfibre. I've added the southeastern two lines so that we show  that the flow stops and you have a reference area that is homogeneous in backscatter for beam pattern calibrations.  


The lines should already be in Aldebaran. In case they are not here is a list of coordinates:

  The system should be free to chose mode - it will switch between Shallow and Medium mode at 200m.

Originally  I used to collect data without swath width constraints. As a result the total width was typically +/-400+m.  For the more recent data, however, I have taken to forcing the swath to 300m per side to increase the ping rate and trim the poor quality outer beam tracking.

The Proximal Prodelta and Lip


As always, an update of the rest of the prodelta needs to be done. The area involved includes everywhere you haven't already been with the south channel corridor and the distal lines.

plan

Note that, the prodelta area overlaps the south channel area. It is actually quicker just to steam over the south channel area than turn at the edges. I would rather these lines be run roughly along the contours (the dashed lines in the figure show approximate orientation).   The line running should be freehand with Gord running on the edge of the previous swath. You can start at the delta lip or out at the distal area.
The sonar should be set to automatic (it will switch from Very Shallow to Shallow).  And hold the swath to +/-300m (for most depths this will not restrict a +/-65 deg sector).

The delta top

and using an available high water, a subset of the delta top would ideally be covered. The extent should ideally match what we achieved in June in the last survey (figure lower to right).

My hope is that those cracks in the deltatop are marking the new delta lip when you get there. We shall see....


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JD 158 2017                  
                   JD163 2017


Update MRE for Aldebaran

It would be good to give Gord an updated MRE for Aldebaran for the JD163 survey. For that you need two files:
You need to put these of the aldebaran PC in C:\Charts\SQ_2017 (or a directory with as name like that).

Then you need to run the MREconverter software - select the tif and it should then make an MRE in the same directory.
restart Aldebaran and you should see it.

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