February 2012

Satellites that "feel" the Earth

Mines can monitor ground movement by the millimetre

By Barbara L. Campbell

In Kiruna, Sweden, a city of 18,000 residents are living on top of a rich iron ore deposit – one that Luossavaara- Kiirunavaara Aktiebolag (LKAB) is hoping to access with the extension of its Kiirunavaara facility.

To mine beneath Kiruna, LKAB must be prepared to move the town if subsidence becomes a concern, and well before it causes infrastructure damage. And that is where MacDonald, Dettwiler and Associates Ltd. (MDA) has come in. Based in Richmond, British Columbia, MDA operates a satellite called RADARSAT-2, which circles the Earth many times per day, providing detailed images from 800 kilometres above the planet’s surface.

But these are not ordinary photographs. “It’s like you’re feeling the Earth rather than seeing it,” says Wendy Branson, manager, services operations, at MDA Geospatial Services. Since November 2009, RADARSAT-2 and its synthetic aperture radar (SAR) technology have been monitoring ground movement to help LKAB keep track of the area.

“Because the technique provides a bird’s-eye view of an entire operation, we are able to identify areas that might be unstable,” says Marc Beaudry, a manager at MDA. “Underground mines can be quite sizeable – several hundred square kilometres – so identifying potential areas of instability over such large areas can be compared to finding a needle in a haystack.”

InSAR reaching its potential

SAR is a technology that dates back to the 1960s. It creates images by emitting energy in the microwave range of the electromagnetic spectrum. Working much like an echo, the energy interacts with the Earth’s surface, then bounces back to the satellite. Visually, SAR information shows up as a high-contrast black-and-white image.

By analyzing the phase information of the return signal, the satellite can establish the relative location of the ground. Because RADARSAT-2 passes over the exact same spot in orbit every 24 days, those images of the Earth can be compared directly. If nothing has altered since the previous observation, the phase of the waves from the satellite to its target will be exactly the same.

Differing signals from the same point taken at two different times, however, can be early warning signs of shifts, or subsidence, in the ground. It is especially important to look for safety risks in these areas. The key to such analysis is interferometry – the “In” part of InSAR. InSAR processing techniques combine two or more SAR images of the same area and create another image – an interferogram. Using “stacks” of images, InSAR allows the user to see how changes happen over time. MDA can measure surface changes in millimetres and illuminate instability that is still completely invisible to the naked eye. Image sets can be compared precisely over months or years, allowing users to see the way changes have happened over time.

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