Seismic surveying at Discovery Deep. Photo: Andrew Gorman
Geophysical exploration at Discovery Deep
We are collecting a variety of geophysical datasets at Discovery Deep to better understand the configuration of ice, ocean and sub-seafloor geology along the west side of the Ross Ice Shelf.
To better constrain forecasts and models of change as Antarctic ice sheets respond to warming, we need information on present and past environments in the region. This requires drilling to collect ice and seafloor sediment cores. Within the Antarctic Science Platform, and through collaborative international programmes like SWAIS2C, our focus so far has been on drilling sites, which investigate contrasting regions of the iceshelf:
- Kamb Ice Stream sites KIS-1 (drilled in 2020/21) and KIS-3 (to be drilled in 2023/24) are located near the grounding zone, where ice transitions from grounded ice sheet of West Antarctica to freely floating ice shelf, typically over several kilometers. Here the seafloor sediments are overlain by a thin ocean cavity (~50 m thick) and the floating Ross Ice Shelf (~500 m thick). This ice stopped moving about 150 years ago.
- Kamb Ice Stream site KIS-2 (drilled in 2021/22) is about 50 km north of KIS-1 and investigates a location where a subglacial channel flows out from under the grounded ice sheet into the cavity below the Ross Ice Shelf.
- Discovery Deep is across on the far side of the Ross Ice Shelf, within sight of the Transantarctic Mountains. This is the deepest region of shelf-covered ocean (the ocean is about 1.5 km deep). We hope to sample seafloor sediments here from previous glacial and interglacial periods in Earth history that have been trapped in the deep basin.
Kamb Ice Stream and Discovery Deep drilling sites, as well as existing ice cavity access sites (e.g. J9, HWD-2, Coulman High).
Exploring Discovery Deep
It takes many, many seasons to plan and prepare for drilling. Discovery Deep won’t be drilled until a few years into the future, but the preparation has already begun. Our goal at Discovery Deep is to use geophysical surveys to characterise the sedimentary basin in this location so our paleoclimate team can find the best location to collect a core in future. The data also contribute to the development of a new bathymetric model for the Ross Sea, and helps to ground-truth the ROSETTA airborne geophysical survey, which was an international collaboration using airborne geophysics to survey the Ross Ice Shelf (2015-2018).
Geophysics techniques
Geophysical survey techniques being used at Discovery Deep include:
Active source seismology: Seismic surveying is used to map the configuration of different physical bodies (e.g., ice shelf, water column thickness, sedimentary strata) in the subsurface using reflected sound waves – much like how ultrasound imaging is used in medical applications. Our hot water drill is used to produce 25m deep holes in which chemical explosives are detonated to produce a sharp and distinct sound source that can then be recorded. Specialised recording gear is deployed along the line in an optimal position to record each shot. The data are recorded by a computer and processed afterwards to make seismic images of the subsurface.
Skidoo towing seismic acquisition system sleds with record the signals returned to the surface from the explosive shots. Photo: Andrew Gorman
Ice-penetrating radar images the structure of the ice using an electromagnetic (radar) source rather than a sound source.
Radio echo sounding: APRES (Autonomous phase-sensitive radio echo sounding) provides an accurate sounding of the layering structure and depth to the base of the ice. Together with oversnow profiling), APRES allows us to characterise ice-ocean interaction, basal processes (including grounding zone sedimentation), and subglacial hydrology.
Gravity methods enable us to remotely map the density of material below us (e.g., ice, water and geological units have different densities). Gravity measurements, undertaken in lines, produce cross sections of density variability. A base station ties these measurements in the field to a location where gravity is well known.
GNSS (Global Navigation Satellite System) surveying of geophysical lines and measurement points enables us to accurately position and map our data.
Jenny Black making a gravity reading to understand the wider distribution of rock types below the seafloor in the region of the seismic reflection survey. Photo: Andrew Gorman
Andrew using GNSS equipment to observe the location (and motion) of the measurement point. Minna Bluff is in the background.
Discovery Deep field campaigns
In the 2021/22 Season We imaged the sediments in this deep bathymetric depression for the first time. Using a hot water drill, the team collected a 30 kilometre long active-source seismic reflection profile. Results showed seafloor depths ranging from 1200-1450 metres below sea level, crevasses in the bottom of the ice shelf, and up to 200 metres of dipping and folded strata beneath the seafloor. But we didn’t find the bottom (deepest part) of Discovery Deep!
Planning for the next season At the end of our 21/22 campaign, we collected a short experimental survey to compare two different active seismic survey techniques. The first used the well-proven but labour and equipment intensive explosive- and-geophone method, explained above. We also trialled a novel approach using detonating cord deployed on the surface and an over-ice streamer on loan from the Alfred Wegener Institute (AWI, Germany). This method enables about four times as much data to be collected as the traditional approach. Results were promising, allowing excellent data recovery at an increased rate of acquisition.
In the 23/24 Season This summer, we’re heading back to Discovery Deep to continue our exploration using the AWI over-snow acquisition method. We plan to record seismic profiles totalling 80 km in length – and have a strategy in place that should find the bottom of Discovery Deep, where seafloor sediments are likely to be the thickest. This work will further determine the suitability of the site for deep drilling – as well as providing information on water column thickness to inform oceanographic studies of the region.
Testing equipment near Scott Base. Matt Tankersley melting snow in our "flubber tank" and Hamish Bowman controlling the boiler. Photo: Andrew Gorman
An interpreted seismic profile from the 2021/22 season highlighting the seafloor geology. Note that different horizontal and vertical scales result in an image with a large amount of vertical exaggeration. The layers interpreted with blue dashed lines correspond to sedimentary rock units that are broadly deformed into a wide anticline that has been eroded off at the seafloor. (Perhaps by past glacial episodes?) There is little evidence of recent sedimentation on the seafloor.