NRCan Makkovik Bank 2004

CCGS Amundsen EM300
Makkovik Bank Survey, Labrador Sea
October 5th-6th 2004

John E. Hughes Clarke and Kristian Llewellyn
Ocean Mapping Group
University of New Brunswick

Executive Summary

On October 5th-6th 2004 (JD's 278,279), a bathymetric survey was conducted on the top of Makkovik Bank (Labrador Shelf) using the Simrad EM300 mounted on the CCGS Amundsen. The survey was conducted on behalf of Gary Sonnichsen of NRCan (GSC-A). The aim was to use the available time (up to 24 hours of ship operations) to try and fill in as much as possible of an area bounded by two prior surveys conducted in previous years by Fugro Jacques Geosurveys. The vessel was already transiting through this area on her way home.

The depths in the area of interest ranged from ~ 135m to 95m. The system was operated with a +/- 60 degee swath (135 beams distributed in an equidistant manner). The line spacing used was 300m. The survey commenced at 1700 on October 5th in low seas (winds ~15 knots). Knowing that a storm was coming, the speed was raised to 14 knots to collect as much good data as possible. Over the next 13 hours, the winds increased to 30 knots, forcing the vessel to slow down, and ultimately to abandon the survey.

The two major ancillary concerns were sound speed variability (this location is right on the edge of the Labrador Current) and tidal control (there being no available tide gauges in the vicinity). To alleviate a number of new approaches were undertaken:

for sound speed, the MVP-300 was deployed continuously, profiling at ~35 minute intervals. Every 2nd SVP was loaded into the system in real time whilst cornering.
As the hull mounted surface sound speed (and the keel CTD actually) was broken, we used the sound speed from the fish whilst towed at keel depth (~ 7m) for beam steering calculations.
In the absence of a reliable tidal signal , the C-Nav RTG ellipsoid elevations were recorded to reduce the vertical variations.

The figures below illustrate the main points of the survey. A set of 3 geotiff files are available of preliminary data to compare the feature detection capability to that achieved in the previous years adjacent surveys. The data has yet to be reprocessed for the full sound speed variations or for the tidal signature.

The MVP 300/1700 is installed in the fantail of the CCGS Amundsen. The towbody contains a Seabird 911 CTD, from which sound speed information was derived. The towbody bridle had been damaged earlier in the Arctic field season, which had restricted the tow speed to 4 knots. A replacement bridle was transported by OMG staff from BOT and installed. With the new bridle the towbody was able to collect data at speeds of over 12 knots to depths of at least 300m.

As there were only two survey staff on board during the survey (watches of 1 person). The towbody was not left in automatic control. The towfish was released evey ~ 30 minutes under human observation. The SVP profile was transfered to the EM300 system, with evey second profile being entered in real time. All the profiles are available for post-application with the capability of recomputing steered angles (new surface sound speed) and interpolataion between time-adjacent profiles.

Prior to arriving at the Makkovik survey site, the MVP was experimentally deployed at 30 minute intervals whilst steaming in transit up the continental margin at 12 knots. The system cycled to 300m depth for areas with greater depths. In shallower depths, the towfish was set to recover at a depth of 20m above the seabed. These data were used to examine the transition from the Labrador Sea water masses (surface temperatures up to 9 degrees C) across the edge of the Labrador Current.

The transit deliberately extended slightly past the Makkovik survey site at the request of NRCan.

The Labrador current was particularily evident with surface salinities of as low as 30.5 ppt and deeper temperatures as low as 2 degrees C (as opposed to a toasty 9 degrees out in the centre of the Labrador Sea). The front between the two water masses is very abrupt, occuring within a zone around 10 km wide. The location of the front is extremely close to the site of the desired bank survey indicating that the spatial variation in oceanographic properties were likely to be a cause of concern in the refraction correction of the EM300 multibeam data.

Converting the CTD data to density and sound speed, one clearly sees that the temperature signature dominates the sound speed structure, whereas the salinity variations dominate the density field (that in turn controls the baroclinic component of the circulation).

The sound speed varies vertically over ~ 12 m/s within the Labrador current, and more significantly by over 20m/s laterally across the eastern edge of the current. Thus for this survey (and probably any further operations), the sound speed structure will have a significant influence on the achievable vertical accuracy. This in turn will limit analysis of repetitive surveys, looking for evidence of new iceberg scouring.

Interestingly, standardly MVP profiles are only taken at intervals of 6 hours or more. Yet the entire survey area lay within no more than ~2 hours of the location of the edge of the Labrador current. The same concern will probably extend to all central and outer Labrador Shelf surveys all the way up to Saglek Bank.

The survey consisted of 9 parallel lines. The SVP data is presented as a function of time, to illustrate whether there was a systematic variation in the sound speed structure as a function of time of day. What is most noticeable however, is that the sound speed structure appears to vary more by distance, as one moves east-west.

The C-Nav RTG ellipsoid height signal clearly indicates that the survey was conducted through the local high water period. In addition however, as the local ellipsoid-geoid slope is particularly strong in this area of the Labrador shelf, one can see a zig-zag vertical signal as one moves up and down the ellipsoid traveling east and west respectively. In order to convert this data to mean sea level (MSL), closely approximating the geoid, the 3D geoid-ellipsoid slope will have to be taken into account. The EGM96 model is currently being investigated to perform this transformation.

The sound speed data are herein presented, projected by east-west distance. From this image, it is apparent that the thermocline was generally deeper to the west and the deeper water changed quite abruptly at the eastern end of the line.

Looking at cross-sections of the initial real-time refraction-corrected data, it is clear that significant (> 2%) refraction artefacts are present in the +/-60 degree swath data towards the end of each line (the real time SVP's were only entered at the start of line). The data will have to be re-processed using the high density MVP data and include interpolation from one watermass to the other.

The final area covered within the 13 hour period extends ~25 km E-W and 2.5 km N-S. The data is presented as a 5m resolution terrain model and backscatter mosaic. At this time the data has NOT been corrected for tides.
mosaic geotiff (-35 to -10 dB)	bathy (sun 000) geotiff	bathy (sun 090) geotiff
As can be seen from the data, the quality degraded significantly from south to north as the seastate rose. The data most impacted are the backscatter data at wind speeds over ~ 20 knots. Above 25 knots the bathmyetric data significantly degrades also.

The figure to the left shows a close up of a N-S section across the survey. One can clearly see the sensitivity of the data to seastate.

At wind speeds of 15 knots or less, the ship can operate at speeds of 14 knots (actually in the Arctic is was clear that 16 knots was also acceptable, although the fuel costs become unrealistic).
As the winds rise up to 20 knots, the bathymetric data remains acceptable, but one starts to see bubble washdown events in the backscatter data.
As winds rise up to 25 knots one starts to see a degradation in bathymetric data quality. At this point, the backscatter data is essentially useless.
As winds rise up to 30 kntos, all the data is unacceptable.

It is particularily noticeable that the data is worse steaming to the east when the seas (from the SE) are coming at the starboard forward quarter. It should be noted that the array is actually on the port side of the hull tilted 6 degrees to port. Lines in which the seas are on the port rear quarter are significantly better.

The data and observations presented here are preliminary and for the purpose of updating the NRCan staff on the likely quality of the data. Further processing of the data to a final product will proceed in due course.

created by JEHC, October/November 2004