Posts Tagged seafloor mapping

Antarctic voyage: Mud, mud, glorious mud Guest Work Feb 28

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Date: 27/2/2013
Location: 66.5914˚S, 148.064901˚E
Weather: 10 knots and clear
Sea state: Calm

…Nothing quite like it for cooling the blood. Especially when the air temperature is below freezing.

Multibeam data for canyons. [Helen Bostock; data supplied by Alix Post,Geoscience Australia]

We have successfully collected a couple of cores from the Antarctic slope in between some of the CTD stations. The slope is made up of canyons and ridges, some of which have been previously mapped by other voyages using the multibeam (see blog post 6: Multibeam mapping of the seafloor). We have been filling in some of the gaps in the multibeam coverage and mapping new areas, where previously there was only limited bathymetry (depth) data.

We’ve been targeting the flanks of the ridges, rather than the canyons, for the cores because the bottoms of the canyons are often highly eroded by the water and sediment pouring down off the shelf. Some of this sediment settles out of the water on the flanks.

3.5khz chirp section. [Helen Bostock]

As well as using detailed multibeam maps to target the core sites, we also use a sub-bottom profiler or chirp system. This also works like an echosounder, but the frequency of the chirp system allows it to penetrate the upper sea bed and bounce back off any layers that have a change in density (which cause a change in sound velocity, a type of seismic line). We pick coring locations where we can see lots of layers in the sub-bottom profiler. Unlike most of our sounders the chirp system is audible to humans, so the “chirping” has become a constant companion on the ship.

The gravity corer has a 6.5 m barrel (pipe) on it and the longest core we have recovered on the slope has been a whopping 6.2 m – a new record!

Once the gravity core is on board the boat, we remove the plastic liner with the mud inside from the metal core barrel, and measure the total length.

We then cut the core up into sections of about 1 m and split each section lengthways so that we can visually describe (log) the core. We describe the colour, texture (grain size – mud, sand or gravel),and  any fossils or structures (laminations, deformation, erosional features or burrows) that we can see. We also make a note if the changes in the core are abrupt or gradual transitions.

Molly Paterson and Courtney Derriman measuring the magnetic susceptibility of the cores. [Helen Bostock]

Next we measure the sediment in the core for its magnetic susceptibility. This gives a relative measure of the changes in the amount of grains that are magnetic (so it picks out iron or magnetic rich layers). We use a long probe that looks like a ray gun straight out of a Star Wars movie, and we analyse the core every 2 cm down its length. This gives us an idea of changes in the amount of sediment that has come off the adjacent coast (or evidence for volcanic ash), as these layers often have more iron in them than the carbonate or silicate marine microfossils that make up the rest of the sediment.

The cores are then wrapped up and stored in a fridge to stop them drying out. When we get back to the laboratory we will undertake a whole suite of analyses on the cores. The exact analyses that we undertake on the core will depend on the question that we are trying to answer, but can include: X-rays (same as you get when you break an arm of leg ) to look for structures that are not always evident to the eye ; detailed grain size; carbonate and opal (biogenic silica) content; density; visual identification of microfossils or other grains down the microscope; and a whole array of geochemical proxies such as organic biomarkers, rare earth elements and stable isotopes.

We hope that the analyses on these cores from the slope of Wilkes/Adelie Land will reveal natural changes such as the extent of sea ice and amount of Antarctic bottom water formation (see blog post 21: The formation of the Antarctic bottom water) over glacial/interglacial cycles (which occur approximately every 100,000 years).

Timing is everything. How old is a 6.2 m core? We won’t know until we date the sediment. Hopefully we can date the cores using biostratigraphy (fossils that are known to originate, or go extinct, at a certain time), or radiocarbon dating (if there is sufficient organic carbon or carbonate and it is younger than 50,000 years).  Once we know the age of the major changes in the cores we can determine if they are simultaneous with other cores analysed from around Antarctica and the rest of the world. In this way we provide a small piece of information to the giant 4 dimensional puzzle of understanding earth’s ever-changing oceans and climate.

And who wouldn’t want to play with mud? It’s every little kid’s dream.

Antarctic Voyage: Multibeam mapping of the seafloor Guest Work Feb 07

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Written by Dr Helen Bostock and Peter Gerring at NIWA

Date: 5/2/2013
Position: 48.888683˚S, 167.759479˚E
Weather: cloudy
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.

NIWA marine ecology technician Mark Fenwick in the multibeam lab, running the system. Credit: Helen Bostock

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.

Undersea New Zealand, a high resolution image of the complex and diverse marine realm around New Zealand. Credit and copyright: NIWA

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