Written by Dr Helen Bostock and Peter Gerring at NIWA
Position: 48.888683˚S, 167.759479˚E
Sea state: 3-4 m swell – rough!
Yesterday, just before the rough seas started and most of the science team took to their beds suffering with sea sickness, the geology team started running the multibeam seafloor mapping system.
We are collecting some opportunistic seafloor data on the voyage transits to and from Antarctica. We also hope to map the continental shelf around the Mertz Glacier to add to the data that was collected on previous voyages. These detailed maps will be used to understand the oceanographic flow, evidence of past Antarctic ice extent (using the ice berg scours on the sea floor), as well as the distribution of biology and sediments.
How do we map the seafloor with the multibeam?
Let’s start with a bit of history. Back in the good old days, the positions of features on the seabed were plotted using soundings taken by lead-line.
The position of the sounding was fixed using a quadrant, compass bearings off land features, and later on a sextant. Charting was extremely labour-intensive and not very accurate. More commonly, directions and hazards were handed down through the generations by word of mouth or learned by experience – not always good ones!
After World War II, single beam echo sounders became widely available. Echo sounding is a technique for measuring water depths by transmitting an acoustic pulse (or ping) from a transducer mounted on the hull of a vessel and listening for the reflection (or echo) from the sea floor. The time taken for that ping to travel to the seabed and back again is converted into a depth by halving the time taken between transmission and reception, and multiplying that time by the speed of sound in water (somewhere near 1500 metres per second).
Most of the charts that currently map the world’s coasts and oceans were made using these soundings, and these single beam echo sounders are commonly found on even small boats.
However, single beam sounders don’t provide 100% coverage of the seafloor. Lines of sounding are recorded and these soundings are used to generate depth contours. In between the soundings are areas which potentially contain large rocks or holes. So, in 1964, a technique for multiple narrow-beam depth sounding was patented by SeaBeam. This system allowed survey vessels to produce high-resolution coverage of wide swaths of the ocean bottom.
The multibeam system fitted to the Tangaroa is a Simrad EM302. It has 432 beams. These are sent out from the ship in a fan shape (see this video explaining how it works), and can cover an area of seafloor up to 5 km or more in width with each pass of the ship, although the coverage is much smaller for shallower depths.
Of course things are a little more complicated than this – aren’t they always? For one thing, the speed of sound through the water is not constant. This leads to the beams being bent or refracted as they travel to and from the bottom. To compensate for this, we have to know the sound velocity of the water which changes with density. We can use the data from the CTD (see the previous blog post in this series) to calculate this.
Secondly, the ship is not stationary – it is constantly in motion, which is why people get sea sick. The ship’s position must be precisely known at all times and this is achieved using GPS (Global Positioning System; the same as in your phones or satnav), which can tell us where we are within a metre or less. We also have to correct the swath data for the ship’s roll, pitch and yaw.
The end result of all this is that we can create very accurate maps of the seabed, which can be used for a whole range of science.