ENGINEERING – Understanding land subsidence with InSAR

In mining, land subsidence is a potential problem with some potentially serious costs. Subsidence can result from h...
In mining, land subsidence is a potential problem with some potentially serious costs. Subsidence can result from human activities, such as subsurface fluid withdrawal or mining, or from natural processes. Failing to understand and plan for subsidence can result in structural damage to a mine, lost or damaged support infrastructure, and sometimes, employee injury or loss of life.

The key to dealing with these issues lies in understanding the geological processes at work around the mine site. Managers need to know where subsidence is a factor, how the ground is moving, and how quickly subsidence may be progressing. This knowledge is crucial for making informed decisions on current and future operations.

AMEC has access to new technology that can provide subsidence data faster and more economically than traditional ground-based observation technique, called InSAR, or Interferometric Synthetic Aperture Radar. AMEC is an international project management and engineering company that has an exclusive agreement with the European Space Agency to trial the use of its satellite earth observation technologies to locate and evaluate land subsidence problems.

InSAR uses satellite measurement data for a specific location acquired at two different times along similar orbits to detect minute changes occurring in ground topography. Satellite data are captured for each piece of the Earth's surface every 28 days. Information from any two passes can be analyzed and compared to detect areas of ground settlement that could harm existing or planned facilities and infrastructure.

The key advantages of InSAR over traditional ground-based observation are cost and scale. Typical approaches require the user to review geotechnical data from previous projects, study geologic maps, do ground reconnaissance, and sometimes engage in geophysical and subsurface drilling and long-term monitoring.

InSAR greatly extends the ability to monitor subsidence because, unlike other techniques that rely on location-specific measurements at a few points, InSAR can produce a complete map of ground deformation. Combined with traditional measurements, InSAR data can improve the accuracy of computer models used to assess potential deformation hazards before, during and after they occur.

InSAR can analyze topography over large-scale or remote areas faster and more economically than traditional techniques. This can relieve the need to send survey crews out into the field, making it easy and safe to acquire information in places considered too remote or unsafe for personnel to enter.

Just as importantly, InSAR accurately maps ground deformation over the entire survey area, not just a few representative points. Past subsidence can also be evaluated using historical data.

The benefits of InSAR are already being experienced by the mining sector around the world. AMEC used InSAR data to prepare a full site analysis of the Palabora mine, near Pretoria, South Africa. AMEC used InSAR images acquired during two consecutive satellite passes to see whether the areas surrounding the open pit mine operation is affected by subsidence related to the underground caving operation. The data identified the extent of movement outside the pit rim as a result of the underground operation and confirmed where more detailed point surveys should be conducted.

Closer to home, AMEC also was hired to prepared a subsidence profile for the closed Hollinger gold mine, near Timmins, Ontario, which has experienced extensive subsidence issues in recent years. AMEC's study of satellite data between 1992 and 2003 found subsidence levels in the range of 25 to 55 mm in some areas previously thought to be stable. This study proved that InSAR data could be a useful prediction tool in the mine owner's subsidence monitoring program.

Using InSAR, companies are gaining a better understanding of subsidence and other ground deformations, so they can account for it in planning their operations and infrastructure.

(Stuart Anderson is senior geotechnical engineer at AMEC, AMEC also announced in December that it has been awarded the contract to provide detailed engineering and procurement services for Adanac's $450-million Ruby Creek molybdenum mine in British Columbia.)


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