Wednesday, October 12, 2016

TAN1611 - Post 6

Dredging

We are now nearing the end of the voyage with just 3 days to go, one of those in transit back to Auckland.  At this point we have hang up the geophysical equipment and, for the most part, turned off the multi-beam system to focus on dredging for seafloor rock samples.  This is final scientific objective for this voyage and is crucial for ‘ground truthing’ the geophysical data.

Pic of rock dredge emptied on the deck

The rock dredge is essentially a long, narrow steel basket with some teeth at the front much like those you might see on the bucket of an excavator.  The teeth dig into and break apart seafloor rocks, and with any luck a decent portion of these freshly broken rocks will be funnelled into the basket and stay there while the dredge is retrieved!  Periodically new teeth must be welded onto the dredges because, as one might expect when towing a large piece of metal behind a ship and across rocks, they have a tendency to snap off.  The ships engineers are a godsend when a bit of crafty welding is required.

The general process for dredging is to use the already collected geophysical and seafloor topography data to identify key areas of interest - something like a large volcano might be of interest if it sticks out from the surrounding terrain.  On this trip our main focus has been the Colville Ridge feature so this is where dredging will be focussed.  Upon reaching a site picked for dredging the ship slows down to 1 knot while the dredge is sent to the seafloor, a process that takes about an hour per thousand metres. While the ship creeps forward at a lazy one knot (one knot/h is equal to 1.85 km/h) the dredge (hopefully) collects the all important rock samples.  We give the dredge maximal 15 minutes at the seafloor and then winch it back up to the ship where we gleefully sort, cut, bag, and tag our geological treasures.

This might sound like a laborious task, and it certainly is - this is the most action the ship will see during this three week voyage, it is however key to a good understanding of what is hiding several thousand metres below us.  As I talked about previously, the geophysical and multi-beam data allow us to say something about the rocks making up this patch of seafloor, but there is no substitute for the hard evidence provided by actual rock samples.  With rock samples we can make accurate and extremely detailed observations of the rocks chemical composition, age, and hydrothermal alteration (think Rotorua geyser areas).  This information is used in piecing together a first approximation of the geological history for the region.

We will be dredging for about 50 hours after which everyone will likely go for a well-deserved kip while the ship begins its 30 odd hour journey back to port in Auckland. 

Monday, October 10, 2016

TAN-1611 - Post 5

The rough seas have subsided and the ship and data collection are ticking along nicely now with just one week left of our voyage.  Today I thought I would move away from the science to talk about about what day to day life is like on board a research vessel like the RV Tangaroa - it certainly isn’t what I expected.

Most people only have limited experience on any kind of ship, and ideas of ship life might be more akin to that shown in movies or TV shows - this is far from the reality of life on a research vessel.  Fortunately, I would say that, in general, ship life is far better than what one might expect! 
Sleeping quarters are plentiful for this particular voyage because we don’t have a full crew - the RV Tangaroa can take 40 people at full capacity but we have only 23 for this voyage.  This means everyone gets their own room; a bed, small table, chair or bench seat, and a full ensuite.  Given that people are up at all hours of the night and day for different shifts, it is quite nice to have your own room.


 The meals are always a feast fit for kings, not the unappetising gruel that movies and books illustrate on a pirate ship or navy vessel.  The cooks are probably the hardest working of anyone on the ship, making sure there are three cooked meals a day at 7:30am, 12:00pm, and 5:00pm, as well as fresh fruits and sometimes fresh baking for morning and afternoon tea.  There are always slices, cookies, and cakes on hand for those with a sweet tooth, and there is a freezer stocked with ice-cream as well.  We were lucky enough to have a full American style thanksgiving lunch yesterday - turkey, mash, carrots, stuffing, gravy, and cranberry sauce!

Free time on the ship is spent in a variety of ways depending on your interests.  Every night there is an hour or two of card games played in the lounge area, but there is also a fairly extensive selection of DVD’s and a small library to keep the crew entertained.  There is also a small gym on board so you have a chance to work off some of the ice-cream and cake.  When the weather permits it doesn’t get much better than sitting out on the bow of the ship to soak up some sun and read a book.  Many of us have research work or study to do as well, the temptation to leave this in favour of a movie or a good book and some sun is a difficult one!


There are also regular drills performed by the ship crew to keep them on their toes.  We scientists haven’t been involved in many of these drills on this trip but it is comforting to know that the crew will be ready in the event of an accident. 

Thursday, October 6, 2016

TAN-1611 - Post 4

Gravity and Magnetics

For the last few days the seas have been a little on the rough side which generally means that the boat has to slow down; the multi-beam data generally suffers in these conditions because the ship is rolling around a lot more.  We are hoping that this will pass soon and we can get back to full speed.  Regardless, everyone is happy and ship-shape.  I find it surprising that the cooks on board are still able to provide fresh fruit and vegetables everyday, along with copious amounts of baked goods and hot meals.  We are very lucky indeed!

Moving on from last weeks blog on the multi-beam system I thought I would talk about the geophysics side of data collection today.  This involves measurements of gravity and magnetism at the seafloor using two instruments: a gravimeter, and a magnetometer, and is run by Fabio Caratori Tontini (marine geophysicist), along with Christian Timm and Rachel Barrett.







 The role of the gravimeter is to measure variations in the Earth’s gravitational field at the seafloor which occur due to differences in density and thickness of rocks.  Different rock types have different densities; Earth’s crust has a density of roughly 2.9g/cm3 whereas rocks at 100km depth have a density off roughly 3.3g/cm3.  So by measuring the variation in the gravitational field we can understand something about the different rock types present at different depths.  Gravitational anomalies can also be related to variation in the thicknesses of Earth’s layers (like the upper and lower crustal lithosphere).

The magnetometer is a  one and a half metre long torpedo shaped object that is towed behind the ship at a distance of 300 metres to make sure the metal of the ship does not interfere with measurements.  From its position behind the ship it measures changes in the seafloor magnetic field which is related to a variation rock types and age.  Different rock types have their own characteristics, a volcanic rock characteristically contains a number of iron based minerals which are magnetic, sediments on the other hand do not, generally speaking.  This means that the volcanic rocks will have a stronger magnetic signal than the marine sediments and we can see this in the magnetometer data. 



Fig. 4.  An image showing magnetic anomalies around Macauley Island. Red/purple indicates higher magnetism and blue shows lower magnetism.  The map shows how complicate the pattern of magnetism could be on the seafloor.

The data also allow Fabio to gain an understanding of relative age - the age of something relative to a reference with a known age.  A magnetic signature can only develop when a rock forms, so before a volcano erupts the magma cannot become magnetised because it is like a thick liquid and at a very high temperature.  After an eruption, minerals in the magma cool below their Currie temperature - the point at which they can take on a magnetic direction (called a magnetic polarity).  Importantly, the Earth has gone through several changes in the polarity of its magnetic field, where the position of magnetic North flips to the opposite direction.  It is these magnetic reversals that we compare the rocks against to give us an idea of the time the formed.



The aim of making these measurements is to say something about the variation in rock types, the variation in the age of formation, and to use alongside a geological model to understand the shape, composition, and thickness of the Earth’s outer layers.  The measurements also help Christian Timm, the voyage petrologist, to make a decision on where we should dredged for rock samples - a tough job if the rocks are hidden under several thousand metres of ocean!

Tuesday, October 4, 2016

Tan 1611 - Post 3

As of Tuesday morning we have completed a third of the voyage period, with 8697 km2 of seafloor mapped with the multi-beam.  Seeing as it constitutes a large part of this voyage, I thought that I would take the time to go into a bit more detail about how the multi-beam system actually works and what we can learn from the data it produces.  During a research voyage like this one, equipment must be monitored constantly to make sure that there are no gaps in the data and that what is being recorded is correct.  This means that a full 24 hour rotation is split into shifts of 12, 8, or 4 hours depending on what your role is onboard.  The rotation for the multi-beam is 4 hours on and 8 hours off, with three people (Tineke Stewart, Tim Kane, and Susi Woelz) rotating over the 24 hours.

The multi-beam unit (a Kongsberg EM302 ) is mounted on the hull of the RV Tangaroa.  It works by shooting and receiving acoustic (sound) waves in a fan shape (called a swath).  The swath has 288 individual beams that travel down through the water column to the seafloor where they are reflected back toward the receiver.  The system can then calculate the depth the each beam reached by using the time taken for an individual beam to leave the multi-beam unit and return to it.  This is where the SVP data I talked about in an earlier blog post comes into play.  The information from the SVP allows the system to calculate how fast one of the acoustic waves will travel through the water and translate that into a distance.  As the multi-beam data is collected it is cleaned and interpolated to remove any points that look particularly out of place with the rest of the data.
The Kongsberg EM302 is a fairly new unit (installed in 2010) and has a few other interesting capabilities.  Not only can it map seafloor depths, it can also record information about the water column and seafloor density.  The water column data allows scientists to look for features like fresh water plumes or hydrothermal vents while the density information can provide a basic understanding of the variation in rock and/or sediment composition on the seafloor.

The final product is a detailed depth map of the seafloor at a 35 metre resolution, in other words, a 35 m2 area is given one elevation value so any feature is accurate in shape and size to within a 6 by 6 metre square.  Although this may not sound great it is a huge advance on the first bathymetry maps in the late 1800’s.  These early bathymetry maps had poor resolution, sometimes with only one data point for 1000 km2 and used clocks and lead-lines, a rope with a weight at one end, to measure seafloor depth profiles.  Even today there is only about 10% of the ocean floor that has been mapped to around 200 metre resolution, so there is much to be discovered !

Tan 1611 - Post 2

As of Tuesday morning we have completed a third of the voyage period, with 8697 km2 of seafloor mapped with the multi-beam.  Seeing as it constitutes a large part of this voyage, I thought that I would take the time to go into a bit more detail about how the multi-beam system actually works and what we can learn from the data it produces.  During a research voyage like this one, equipment must be monitored constantly to make sure that there are no gaps in the data and that what is being recorded is correct.  This means that a full 24 hour rotation is split into shifts of 12, 8, or 4 hours depending on what your role is onboard.  The rotation for the multi-beam is 4 hours on and 8 hours off, with three people (Tineke Stewart, Tim Kane, and Susi Woelz) rotating over the 24 hours.

The multi-beam unit (a Kongsberg EM302 ) is mounted on the hull of the RV Tangaroa.  It works by shooting and receiving acoustic (sound) waves in a fan shape (called a swath).  The swath has 288 individual beams that travel down through the water column to the seafloor where they are reflected back toward the receiver.  The system can then calculate the depth the each beam reached by using the time taken for an individual beam to leave the multi-beam unit and return to it.  This is where the SVP data I talked about in an earlier blog post comes into play.  The information from the SVP allows the system to calculate how fast one of the acoustic waves will travel through the water and translate that into a distance.  As the multi-beam data is collected it is cleaned and interpolated to remove any points that look particularly out of place with the rest of the data.
The Kongsberg EM302 is a fairly new unit (installed in 2010) and has a few other interesting capabilities.  Not only can it map seafloor depths, it can also record information about the water column and seafloor density.  The water column data allows scientists to look for features like fresh water plumes or hydrothermal vents while the density information can provide a basic understanding of the variation in rock and/or sediment composition on the seafloor.

The final product is a detailed depth map of the seafloor at a 35 metre resolution, in other words, a 35 m2 area is given one elevation value so any feature is accurate in shape and size to within a 6 by 6 metre square.  Although this may not sound great it is a huge advance on the first bathymetry maps in the late 1800’s.  These early bathymetry maps had poor resolution, sometimes with only one data point for 1000 km2 and used clocks and lead-lines, a rope with a weight at one end, to measure seafloor depth profiles.  Even today there is only about 10% of the ocean floor that has been mapped to around 200 metre resolution, so there is much to be discovered !

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.