May 24, 2017

World map from the HMS Challenger voyage
Serious deep-sea exploration began in the 19th Century when the HMS Challenger expedition (1872–1876) set out from Portsmouth, UK, on a three-and-a half-year journey around the globe that included south-eastern Australia. Challenger carried 144 miles of hemp rope for depth sounding and 12.5 miles of piano wire for seabed sampling. Exact depths were determined by lowering a 100 pound weight over the side of the ship until it touched the bottom. One depth measurement could take a whole day. Soundings taken during the expedition provided the first map showing the true outline of the ocean basins. The deepest measurement was 8200 metres in the Marianas Trench, now known as the deepest area of ocean in the world. Image: By Malby & Sons [Public domain], via Wikimedia Commons

Blogging the abyss iconDay 10: Asher Flatt: onboard communicator

Historical sounding equipment and RV Investigator sonar illustration
Left: The Baille sounder/sampler used on the HMS Challenger could bring up two feet of sediment. Image: Zera Luther Tanner [Public domain], via Wikimedia Commons. Right: An illustrated representation of multibeam sonar emitted from the transducer beneath the RV Investigator. Image: Marine National Facility

Battle ships and submarines are things that people usually think of when they think sonar, and they are not far wrong. The first use of sonar, however, was to detect icebergs back in 1906. It wasn't until World War 1 and World War 11 that sonar was reappropriated to detect enemy submarines and, with military necessity on its side, the technology made great leaps forward. Once peace had returned to the world this advanced technology was turned once more to the seafloor, not to look for submarines, but to map the land beneath the waves.

Back in the days before sonar, the only way to get an idea of ocean topography was to do ‘soundings’ with a weighted line being lowered to the seafloor. When the line slackened it had hit the bottom and an estimate of depth could be gained by measuring the amount of line used.

With the advent of sonar, there was no longer a need for long lines and manual labour: one can simply send out a wave of sound and measure how long it takes to hit the sea floor, bounce back and return to the ship. Like throwing a bouncy ball against a wall and measuring the time between it leaving your hand and you catching it again.

The composition of the seafloor can also be mapped in this way by measuring the strength of the returning sound wave. In a soft sediment, more of the sound is absorbed so the return signal will be weaker than with a hard substrate such as rock, where most of the signal will be bounced back.

Present day sonar has progressed even further with multi-beam sonar arrays such as the swath mapper on board the CSIRO RV Investigator. This uses the same process, except instead of sending out one beam of sound it sends out many beams of sound, all at once. This allows for very accurate mapping of large areas of the sea floor, and with the magic of sonar the hidden folds and valleys of the world below the waves is slowly being revealed.

A scientists sitting at a data acquisition display, and an image of sonar backscatter
Bernadette Heaney is one of the GIS mappers on the RV Investigator voyage. Her job is to produce bathymetry maps from the multibeam sonar data, and to do soundings for substrate hardness, to identify rocky areas best avoided with the sampling equipment. Once 'safe' areas are identified for trawling, the captain assesses their suitability from the perspective of sailing conditions. The image to the right shows sound reflected off fish and plankton shoals in the upper ocean, (blue, yellow and green) and the black-lined path of the box corer (over time) as it is lowered to and raised from the seafloor (red).

There is nothing worse than waiting five hours for a trawl net to go down 4000 metres only to hit a rock on the ocean floor. So multi-beam mapping is being used on our voyage to determine the make-up of the ocean floor below us. This lets us see where there are areas of hard rocky substrate so we can avoid these and save our gear from disaster.

An image showing seafloor bathymetry
A bathymetry map created using multibeam sonar data collected by the RV Investigator. Image: Marine National Facility

Learn more about the multibeam sonar capabilities of the RV Investigator.

Voyage date: 
Wednesday, May 24, 2017