Ice a thickness can be measured using an electromagnetic induction device due to the fact that sea water can carry an electric current far more efficiently that ice and snow.
In order to understand how the EM device works you need to understand a little bit about electromagnetic theory. Electricity and magnetism are intrinsically linked i.e. you don’t get one without the other! To generate electricity we either take a conductor and move it through a magnetic field or place a conductor in a moving (changing) magnetic field. If a conductor and a magnetic field are stationary relative to each other then no current will be produced, the important factor is that one of them must be moving with respect to the other.
Conversely to this any current carrying conductor will have a magnetic field associated with it, for example a long straight wire with a current flowing through it has circular magnetic field lines around it as shown ion the diagram below.
The direction of the magnetic field lines depends on the direction of the current, and as you increase your distance from the conductor the distance between the magnetic field lines increases which means the strength of the magnetic field decreases. For more information about magnetic field fields and field lines follow the link to ‘Magnetism’.
The electromagnetic induction device works by producing an oscillating magnetic field which penetrates into the sea ice and the water beneath it. Basic electromagnetic theory states that a changing magnetic field will induce an electric potential, and therefore a current, in a conductor (this is how a generator works). So if a changing magnetic field is introduced to sea ice and sea water a current will be induced if either of the substances can conduct electricity. As already stated sea water is far better at carrying a current than sea ice. This is because the ions that carry charge are free to move in the sea water but are held in place by intermolecular bonds in ice.
There is now a current flowing in the water below the ice, so back to electromagnetic theory - a current carrying conductor will always have a magnetic field associated with it (the same principle applies to the electric motor). The electromagnetic induction device detects the magnetic field produced by the oscillating current in the sea water and the strength of this magnetic field allows the scientists to calculate how far away the water is from their instrument and hence the total thickness of the sea ice and snow layer. The processes behind the EM device are summarised in the diagram below.
There are 2 electromagnetic induction devices aboard the Aurora Australis, one is hand-held and is carried along the transect taking measurements every metre. The other is known as the Worbot and is suspended off the side of the ship taking measurements as the ship is moving through the ice. It has to be suspended quite a distance from the ship to avoid interference. The Worbot also has a laser mounted on it to measure where the top of the snow/ice is as in the ‘understanding altimetry’ section, as it takes a combination of these measurements to calculate the actual thickness of the ice floe rather than just the distance between the Worbot and the water.
|Hand-held EM Induction Device||The Worbot - ship based EM Induction Device|