7 scientists, 3+1 projects and countless split pins – a short tour of geophysics

By Katharina Hochmuth | A couple of days ago, we featured the working group with the most members on board. Today I introduce the group with the highest quota of female scientists: Welcome to team Geophysics.

The geophysics working group during the deployment of the 600 m long hydrophone cable (streamer). Photo: Thomas Ronge

Shackles, tie wraps and split pins are a geophysicist’s bread and butter. The pockets of our overalls are filled with pocket knives, tape and screwdrivers. We are a long way away from sampling sediment and putting it in small plastic bags or looking through microscopes in search of diatoms or foraminifera. We like to think in three dimensions and for some we also grant the childhood wish of flying… but let’s be honest…

The seven members of the geophysics group of this cruise work in three different geophysical disciplines and additionally support the hydroacoustics team with watches on the sediment echosounder system.

A team of two acquires magnetic data via helicopter flights. They measure the Earth’s magnetic field, to be able to deduce the local magnetic anomalies. This was also done on previous cruises in 2006 and 2010, but every line gets us closer to a more complete picture of the tectonic framework of the Amundsen Sea and Pine Island Bay.

The helicopter’s magnetometer sensor waiting for its first deployment. Photo: Thomas Ronge

The whole team is involved in the collection of reflection seismic data. We tug a 600m long cable of hydrophones across the deck, thankfully with the help of the deck’s crew and hold on very tight to the supply cables of our airguns. After such a workout, we don’t need to visit the gym. We are most interested in the geometry of the sedimentary units and how they changed through time, for example by the retreat of the ice sheet or the changing paths of ice streams. We also aim to connect the boreholes drilled by MeBo with our data, to be able to date certain units and reconstruct the geological past. To achieve a higher resolution for the uppermost sediments, which can be drilled by MeBo, we additionally deploy a second hydrophone cable. This one, we call the “little green garden hose”. On the contrary to its “big brother” we have to deploy it by hand and it is towed on the sea surface behind the ship. Since we are passing through fields of sea ice on a regular basis, the “little one” is no stranger to being towed over ice floes. We hope that the use of two recording systems will help us to resolve the sedimentary column with different resolutions and depths of view. This allows us to use the data collected during drilling and extrapolate those findings along all our profiles to develop a regional perspective.

The third team of the geophysics group is measuring the geothermal heatflow within the sediments. An elevated heatflow from below points to tectonic or magmatic activity below the surface. It is speculated that within Pine Island Bay an elevated heatflow favors basal melting that facilitates flow of the glaciers. We use small temperature sensors, which are mounted on a probe. During the measurement, the probe has to stick in the seafloor for several minutes. Especially during the night, those minutes seem like a lifetime. Every time we pull the probe back in, we are quite nervous, since the sensors are crashed relatively unprotected into the seafloor. Did all sensors survive their harsh trip to the seafloor?

During the last days of our cruise, the geophysics program is still in full swing. The heatflow probe has already been disassembled with a lot of help from the crew, but we are still patiently waiting for helicopter flight number 15 to finalize the helicopter magnetic program. On our way back to the outer shelf, we aim to connect some seismic profiles of previous cruises to enhance our seismic grid. So we hope that the dense fog disappears and that there are no giant ice floes blocking our way to the North.