It takes a lot of instruments to collect ice measurements!

by |April 12, 2011

The IceBridge Gravity & Magnetics Team (Kirsty is on the far left)

The Operation IceBridge (OIB) mission is a truly collaborative project with several agencies and multiple instruments involved in collecting independent measurements. The data is then analyzed concurrently to develop an understanding of the ice processes underway. The measurement of sea ice is an excellent example of how multiple methods of measurement are needed to collect the necessary data.

Sea ice: Uniquely challenging, yet extremely important, sea ice measurements show us both the aerial extent and the thickness of the year’s ice. Aerial extent is important for the albedo, or energy reflection, that the ice returns without absorption. Thickness is used to distinguish multi-year ice from the annual ice, and while both are needed for providing data for models of how it is changing, sea ice thickness is somewhat tricky to measure. Because of the complexities OIB uses two independent sets of instruments to collect sea ice data and then compares the results. The Airborne Topographic Mapper (ATM) instrument collects ice surface elevation, which is then compared with the sea-surface elevation where open water leads exist exposing the water surface (photo). This provides what is called the ‘freeboard’ or the amount of ice that sits above the sea surface. If you know the density of the seawater and the density of the ice you can calculate the ice thickness from the freeboard. This is the same method we use to measure the size of an iceberg, since ice is only 9/10’s the density of water causing approximately 90% of the ice to sit underwater.

Close up of an open lead in the Fram Strait (photo K. Tinto)

There are, however, several tricky parts to this method of measure. First, the further from the open water leads we are, the less accurate the freeboard measure is. Second, it can be difficult to know the density of the ice column. Snow that falls on the ice adds to the elevation, but snow is much less dense than ice. We need to know the differences since we are interested in the extent of the ice under the snow. This is where OIB radars contribute.

There are four different types of radar currently involved in this operation; each has a strength and function. The challenge is to select radar that has the correct sensitivity to measure the small amount of thickness involved. OIB uses two different radar for sea ice measure, accumulation radar, that they use to measure the depth of sea-ice, and the snow radar that reflects off the snow/ice boundary to measure the snow depth above ice. Between the two radars and the ATM measures there is a system of checks and balances on the determination of the freeboard of the ice.

Satellite image of magnetics with the IceBridge hi-res image lines running diagonally across the image (background magnetics image from B. Csatho)

Magnetics: 7 of the first 9 flights of OIB flew northwards out of Thule. This allowed us to take a quick look at the magnetometer data from adjacent lines to see how it was working. Since the magnetometer measures the total magnetic field (including changes in the earth’s field through the day, changes in the field produced by the plane and all its instruments as it flies, and changes in the field caused by the geology that we fly over) we will need to fly a magnetic compensation flight, to determine how the magnetic field of the plane changes with changes in direction. Our schedule has been so packed that compensation flights are scheduled for days when the weather doesn’t allow us to venture over the ice. In the meantime, we can only process the data to remove the effects of the earth’s field changing through time. This is done by subtracting the field measured by the stationary base station from the field measured by the sensor on the plane.

Magnetics and gravity together are key to helping us understand the geology that lies underneath the ice. The gravity can tell us if there is an anomaly, or a change in formation or material under the ice sheet by it’s gravitational pull. Large geologic structures often have more pull, as do small but much denser structures. While gravity is useful for locating these anomalies, the magnetics can help us determine still further what is under the ice by distinguishing between weakly magnetized structures, like mounds of soft sediment, from highly magnetized structures like volcanic basalts which have retained the magnetic signature that was locked into them when they cooled and solidified and may be high in iron content. These distinctions are important since we are looking for geologic features that exist under the Greenland outlet glaciers that will affect the way that they flow into the sea. Mounds of soft sediment will not behave the same in front of a fast flowing glacier as a hardened substrate. In addition these geologic formations are essential for determining: how the sea water underneath interacts with floating glacial tongues; where the ice is currently grounded; or where it might reground were it to melt back. These are all key questions in the OIB mission.

The map shown plots the flight lines over a low resolution regional magnetic anomaly grid that already exists for the area. It is clear that the higher resolution data from the magnetometer on the aircraft fits well with the satellite generated regional grid, suggesting that the instrument is working correctly. Flying the gravimeter and the magnetometer together improves the interpretation of the data from both. A satisfying result this first northern phase of the project.

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