To Norse mythology Midgard is a place that is impassable, surrounded by a world of ocean. Thor, the hammer-wielding warrior god often traveled across to Midgard, and one imagines evidence of his fiery power remains in the highly charged rocks that are left behind. Magnetized rocks containing Thor’s energy and the fiery touch of his lightning bolts.
We are soaring today 1500 ft. over the surface of the twisting branches of the Midgard glaciers. Patches of low lying clouds drape around the tops of the mountains, like smoke from Thor’s lightning singes, but as the sky opens we see row upon row of majestic peaks. It is hard to balance the icy cold of the Greenland exterior with the molten heat of Thor’s lightning. Midgard, is cold and impassable, yet it is evident why Thor was attracted to this land.
Greenland’s geology is diverse. Some of the oldest rocks on Earth are found in southwestern Greenland in the Isua Greenstone Belt, an Archean belt between 3.7 and 3.8 billion years of age. Today, however, we are flying over the opposite side of the country.
The Midgard glaciers hug the southeast of Greenland where the main rock is Archean gneiss, later reworked and cut through with a mafic, or iron rich, intrusion. Perhaps this occurred when Thor was traveling these peaks. We see the changes as spikes in our magnetic data, and visual features that appear as well.
Magnetic measurements are some of the many measurements being collected by Operation Ice Bridge. Rocks have different magnetic properties, so collecting magnetic data can tell us something about the type of rocks that are under the ice, assisting in refining our understanding of how the overlying ice will interact with what is below.
Measuring the magnetic field can be challenging from a metal plane, however the P3 is designed for magnetic surveys so the data requires only a minimal ~10nT (nanoTeslas) adjustment to remove the interference. Originally used by the Navy for locating submarines the P3 has a tail stinger or boom designed to hold the instrument while minimizing magnetic interference from the plane. The Ice Bridge P3 holds two magnetometers. One measures the total magnetic field, the other is a flux gate, with three orthogonal sensors to record plane directional maneuvers, information that is needed for later data corrections.
The Earth’s magnetic field is not constant so collecting data at a magnetic ground station is important in order to gather daily background levels. The magnetic anomaly that we report is the change from this background or anticipated levels. This means that a series of corrections must be applied to all the data collected including removing the Earth’s total magnetic field, daily diurnal fluctuations, and small spikes from the plane radio. What remains is the anomaly, any representation of a magnetic signal from the geology in the area.
Magnetic surveys normally begin with an assessment or compensation flight in a magnetically quiet area, at a high elevation to minimize the effects of high magnetic gradients caused by the geology. This provides the reference points needed for final corrections and processing of the data. This season Ice Bridge has had the opportunity to fly almost continuous missions so the compensation flight has been put on temporary hold. Once that flight is completed, the data will receive a final adjustment.
Today our screens are busy with magnetic shifts tracking on the screen. Seeing the data jump onto the screen is always exciting. The instruments take a reading approximately every meter as we fly above at a rate of close to120 meters a second. The data appears as a wrapping stream of plotted points. When a line travels from one side of the left hand column to the other it shows that the magnetic field has changed by 100 nT. The second column shows greater detail in the measured field with one line showing a change of 10 nT. Peak values of magnetic anomalies appear as a mid-column direction change on the wrapping plot. When the magnetic gradients are high, indicating a distinct geologic boundary, it can appear as a dark block.
What we see on the screen tells us about what happened in the geologic formation of this country millions of years ago. Understanding how the changing rock types affect the flow of ice can help us to predict what might happen in the future.
For more on this project at LDEO: http://www.ldeo.columbia.edu/icebridge
For more about NASA Ice Bridge: http://www.nasa.gov/icebridge/