Sunday, October 2, 2016

Tan 1611 - Post 1

On Thursday morning at 4am, 60 hours after leaving Wellington, we finally made it to the south end of the research site. This is c. twice the distance from New Zealand shores than the distance for the current world record open ocean swim (~225 km). For a short period the sun poked its head out of the clouds to shine on almost glassy sea conditions, and a few of the crew were had the fortuity to catch a magnificent sunrise.

Fig. 1.  An ocean sunrise over calm seas at 33ยบ South.
One of the key components of the data collection is gathering information about the patch of ocean we are sailing across.  This requires the use of a machine called a sound velocity profiler (SVP for short), which measures the change in the travel speed of a sound wave with increasing depth by transmitting an acoustic wave between two points on the SVP unit (Fig. 2.).  This mostly relates to changes in temperature, salt content (also called salinity), and pressure, and the information is used to calibrate the measurements of seafloor topography made using a multi-beam sonar (more on this at a later date).

Fig. 2. The base of the SVP unit housing the velocity measurement tool (indicated by red arrow).
To make these measurements with a SVP the boat must stop while the unit is dropped to the seafloor and then retrieved, a process that will take about 1 or 2 hours.  This is a fairly simple operation but it is time consuming if done often.  After retrieving the SVP from the seafloor it became clear that something wasn’t working quite right - the unit had apparently lost power during the 2 hour deployment and the vital data from the water column was potentially lost.
Tim Kane, the multi beam technician from NIWA made several phone calls throughout the day in an effort to fix the SVP and retrieve the data while the boat carried on making measurements using a data set from 2015.  With a little luck and a lot of perseverance Tim was able to diagnose the problem as a faulty cable connection inside the machine housing and he was able to retrieve the new water column data.

As we are surveying a major submarine ridge care must be taken to check that the water properties are the same on both sides.  A major ridge could deflect currents and cause changes in water salinity or other properties that would affect the topography measurements made with the multi-beam. 

Fig. 3. Tim Kane (at right of first image) preparing the SVP unit (black cylinder in the cage) for deployment.  Deck crew then attach the unit to a winch cable and lower it over the side with the hydraulic arm in the far right image.

 For the second SVP measurement Fabio attached a bag full of polystyrene cups and skulls that his children and their friends had decorated.  The pressure at 2000m water depth is significantly higher than at the surface but is still consistent from all directions which causes the polystyrene objects to shrink while retaining their shape.  The SVP was deployed and retrieved without a hitch and after a quick inspection of the cups and skulls we were on our way again.


Fig. 4  The polystyrene skulls (top left before and top right after) and cups (large cup didn’t go down with the others).  The skulls didn’t shrink as much as we had hoped but the cups, having more open space at their centre, shrunk significantly.



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