Tuesday, October 4, 2016

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 !

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