10,000 years ago, the hillside houses on Brännäsvägen in Mölndal would have had sea views. Today, they look out onto an uneven grassy field that was once an industrial area, but has been disused for several years. This is the perfect location for field work in geophysics.
Professor of Geophysics Erik Sturkell stands in the morning sun with his shirtsleeves rolled up, pointing east towards the cliff that runs along Brännäsvägen.
“This was the coastline 10,000 years ago.”
He turns and points west.
“And Änggårdsbergen over there was an archipelago.”
A great deal has changed since then. Uplifts have altered the landscape, sand has been washed down from the hills, and clay has settled in the former coastal bay. Today, researchers reckon that the old industrial area in southern Mölndal is covered by 40 metres of sand and clay before the bedrock begins.’
This morning, Professor Sturkell is accompanied by a group of master’s students from a course in applied geophysics. They will spend a full day studying the ground beneath them.
“On the first course in applied geophysics they read about how these sorts of measurements are carried out. Now they can come out and try the practical side. And they get to blow things up. Everyone likes blowing things up!”
THE STUDENTS ARE divided up into two groups to study the area using two different methods. One group carries out seismic profiling. They set off explosions using dynamite, creating waves in the ground. These waves are measured by 24 geophones that the students have set up along a line in the ground. A geophone is an instrument that measures small movements, and when a shock wave passes it gives a clear signal.
The students make holes in the ground using a skewer and press the dynamite charges down into the holes, after which they take it in turns to detonate them.
“It’s charged,” says explosives manager Erik Meland into his walkie-talkie to student Erik Ackevall, who is dealing with the seismic instrument to which geophones are connected.
The trigger makes a puff sound, and half a second later the dynamite explodes, the ground shakes, and a cascade of soil shoots up in the air. The waves sent through the ground by the dynamite are captured by the geophones, which transmit their signals to the seismic instrument.
“The signals travel at different speeds, depending on whether they’re passing through clay or rock,” explains Professor Sturkell. “The seismic instrument allows the students to get a picture of the ground deep down.”
ON THE OTHER SIDE of the field, the other group of students are working with another method, led by Carina Andersson. They position electrodes along a line in the field and then send a current through them. There are different degrees of electrical resistance depending on the ground structure, and in a similar way to the seismic group they get a picture of the ground beneath them.
When the students inspect the instrument, they see that things are not quite as expected. Something in the ground is disrupting the signal.
“There’s rubbish in the ground,” says Professor Sturkell. “We don’t know what’s down there. There could also be an electrical cable affecting the signal.”
THE APPLIED GEOPHYSICS field trip is one of student Jenny Preiholt’s favourite parts of the course.
“Geophysics is cool, because it’s hardcore geology. We get to use real instruments, and we can get an overview of a landscape. On the last course we worked with other people’s data. Now we can gather our own data to analyse. And we get to use explosives!” she laughs.