May 17, 2019

Let’s get (Geo)physical

Posted by larryohanlon

By Imbi Simpson (University of Tasmania), Mardi McNeil (Queensland University of Technology), and Nathan Teder (Flinders University)

Group photo! From left to right: trainer James, Nathan, Mardi, and Imbi

After spending a couple of days doing four hourly rotations around all of the labs, we got to choose which lab we would like to spend the next few days in, to collate all of the data for that lab, plot and analyse the data, and prepare a report for submission to the Chief Scientist for her to compile the overall Voyage Report. Some students chose their personal specialty area, while others chose a station where they would learn some new skills:

“I chose geophysics as one of my preferences due to needing to clean up my remote sensing skills, and to learn a bit more about how canyons were plotted as this is relevant to my masters project.”

“I was torn between the sedimentology lab and geophys, because carbonate sedimentology is my background and there was some really exciting science going down in the sed lab. But I chose geophysics because I work with a lot of multibeam and sub-bottom profile data in my PhD project, and I wanted to learn more about the data processing and gain some new spatial analysis skills.”

“Being trained as a geologist, geophysics was my second option after sediments. But I’m happy with the team I was placed in! I’ve had a lot of experience in using GIS (geographic information systems), and I knew that using these techniques on a ship would be an exciting experience. GIS has a great influence upon the ship, and how the science team decides on where to deploy equipment for data collection.”

A few of the 28 different screens for monitoring geophysical data acquisition and processing.

The Geophysics and Operations room is the nerve centre of the ship. From here all of the ships underway systems are monitored in real time, events are logged, CTD deployments are controlled, and acoustic surface and sub-surface data are collected and processed. There is always someone working at all times of the day and night. It is also the one area of the ship that affects a lot of the decision making process on where to go, and what to do in relation to ship deployments

But what actually is geophysical data and how is it used in marine science?

Multibeam swath mapping. (credit: Deepreef Explorer

On RV Investigator we use multibeam acoustic echosounders to map the surface of the seabed. Soundwaves are generated by a sonar on the bottom of the ship. These sound waves travel down to the seafloor, are reflected off the bottom, and travel back up to a receiver. From the amount of time it takes for the sound wave to return, we can tell what depth of water we’re in. We compile all of the soundings to create a swath map of the seabed.

Similarly, we can image the sub-surface sediments below the seafloor by using an acoustic echosounder at a much lower sound frequency. This low frequency sonar penetrates through the different layers of sub-surface sediment, and reflects back at different times according to the different layers of sediment. From this information we can build up a sub-bottom profile, which is like cutting a slice of cake and looking at the different layers side-on.

A slice of RV Investigator sub-bottom profile from our transit across the Great Australian Bight.

In marine geoscience, completing a geophysical survey of the study area is the first priority, because this data informs our decisions about where to collect samples and take sediment cores. We need to know the water depth and substrate type (e.g., hard rock or soft mud) before we can decide where to sample, and which equipment to use. In fact, on this CAPSTAN voyage, we changed our survey plan based on receiving some new higher resolution geophysical data. At the original planned site the slope was too steep and the substrate too hard for the sediment type we were looking for. So we moved to a better site!

Using a backscatter function on a seafloor map gives a proxy for what sediments are present at a location, due to the reflectivity of sound being influenced by the hardness of what it inevitably hits, with rocks bouncing back more sound than sand or mud.

Some geophysical survey data from our submarine canyon site. Bathymetry (water depth) on the left, and backscatter (sediment hardness or roughness) on the right.

Another service of the Geophys/Ops team is to produce digital maps for other lab groups, so they can display and interpret their own data. A lot of GIS is interpreting data and understanding how best to display it, working out the files types that can be used, and executing the final product.

During our time on RV Investigator we’ve learnt a lot about planning and executing a marine science survey. It’s been a great experience to see what goes on behind the scenes and we would like to thank our trainer James, and the Marine National Facility support staff especially Dave, Stuart, and Francis who ran the Geophys/Ops department during our voyage. This team has a wealth of experience which they were only too happy to share with us.