Ground Water Canada

Features Mapping Research
Eyes From the Sky

Airborne mapping shows the size and shape of aquifers


August 29, 2014
By Colleen Cross


Topics

This story begins in Denmark, a country acutely aware of its finite ground water resources. Though salt water surrounds the nation, desalination is an expensive, impractical option.

This story begins in Denmark, a country acutely aware of its finite ground water resources. Though salt water surrounds the nation, desalination is an expensive, impractical option.

SkyTEM  
SkyTEM Canada conducted an airborne geophysical survey in the Horn River Basin of B.C. in 2012 to locate sources of near-surface ground water thought to be contained in buried channels. Photo courtesy of SkyTEM Canada Inc.

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In the mid-1990s, the Danish government set out to map the country’s ground water and determine how it could best protect and allocate the resource.

Enter Kurt Sorensen, then-professor in the department of geoscience at Aarhus University in Denmark, who is leading a research effort funded by government and universities to develop a method of airborne electromagnetic (AEM) technology known as time-domain electromagnetic (TEM) technology.

Sorensen and his partners at Aarhus University and the Geological Survey of Denmark use the method to provide a detailed description of the geology in the top 300 to 500 metres of earth. To date, they have mapped about half of the country’s near-surface aquifers.  In addition, they have used the method to model the extent of nitrate in near-surface aquifers and nitrate leaching into connected streams.

SkyTEM Surveys was primarily a research organization until it flew its first commercial flight in 2003. The following year, Sorensen incorporated the company, which now offers several versions of the proprietary technology. The method caught on quickly and the company now has more than 30 employees and offices in South Africa, Australia and Canada.

Time-domain electromagnetic technology is a fast, economical method for mapping ground water over large areas or areas where land access is limited or difficult, says Bill Brown, manager of sales for North America, who works out of the SkyTEM Canada office with two other employees in Ayr, Ont. With traditional mapping and drilling, “if you have a large area, you must drill many holes and spend millions, or drill fewer holes and interpolate between holes. With this method, you fly first, pick the juicy targets and drill fewer holes.”

Various instruments map different depths: SkyTEM 101 maps the top 100 metres and is the smallest and lightest. SkyTEM 304 maps the top 350 metres. SkyTEM 508 and 512 effectively map more than 500 metres down.

A non-metallic frame suspended under a helicopter carries a transmitter coil, which also houses global positioning systems, lasers and tiltmeters. The transmitter, a type of magnetometer, creates a powerful primary current that generates secondary currents in the ground. When the primary current is turned off suddenly, the secondary magnetic field begins to decay below the surface. The decay rate of the secondary field is measured in the receiver coil in the sky. This rate of decay depends on how resistive the ground is. Clay, for example, has low resistivity and sand has high resistivity. Water content affects these resistivity values. The more resistive the ground, the shorter the response as the magnetic field fades more quickly. In this way, the rate of decay and the electrical resistivity of the subsurface layers can be calculated.

The method has its roots in airborne geophysics developed during the Second World War, when the U.S. Navy designed magnetometers to detect submarines, says Brown. In the 1950s and 1960s, the tool was used for mapping minerals in Canada. But whereas this technology was passive, the field of electromagnetics actively sends a signal into the earth.

Broad Applications
The technology has numerous applications for water and the environment, mineral exploration, geotechnical engineering, and oil and gas, including hydraulic fracturing. Application of the technology to mineral exploration, in particular, is growing and represents about half of SkyTEM’s business, says Brown, who notes that’s where the demand is.

SkyTEM and two other companies, Geotech in Aurora, Ont., and La Compagnie Générale de Géophysique-Veritas (CGG) in Ottawa, offer airborne mapping technologies. SkyTEM’s technology is unique in that it detects subtleties in the near-surface – they call it “listening early” – as well as to depth, says Brown. It uses a patented ‘multi-moment’ system that alternates between transmitting low (shallow) and high (deep) signals and takes a sample every three to five metres along the ground.

The company would like to see the technology used more often in Canada by government, industry and communities to map ground water in order to help manage these limited and vulnerable resources sustainably, he says.

The method has already been used in several ground water projects: mapping aquifers in the Galapagos Islands, determining salt water encroachment in Australia, identifying paleochannels in the Ogallala Aquifer in the U.S. Great Plains, and understanding ice and water distribution beneath Antarctica. It also has been applied to environmental and engineering studies.

SkyTEM undertook some of these projects in partnership with U.S. geosciences firm Exploration Resources International Geophysics (XRI). When several employees of the U.S. Geological Survey (USGS) and the U.S. Army Corps of Engineers learned of the work SkyTEM was doing, they were motivated to launch XRI, which provides geophysical investigations, hydrogeologic frameworks, and water resource planning and management to government, oil companies, municipalities and native communities that want to locate, then either tap in to or protect, their water.

Headquartered in Vicksburg, Miss., XRI employs about 50 geophysicists, engineers, technicians and project managers with expertise in geophysics, geology, hydrology, water resource engineering and management, geospatial applications, and geotechnical engineering.

“The survey is only half of the work,” says Jared Abraham, senior research geophysicist with XRI. Technicians must process the data, adjust calibrations and clean up data where parts of it are obstructed. When obstacles create a gap in the picture, they must delete a section and fill it in by other means such as substituting pictures taken from other angles. They then create an inversion model to display the data in a way that is useful for clients. That could mean a 3D, cubed image or a cross-section showing the subsurface geological variations.

The Collaboration Process
The two companies work together in a turnkey operation, says Brown. “We send out the crew, the equipment, the sensor, the helicopter – everything.”

SkyTEM and XRI consult with clients about how deep they want to map. Together they devise a flight plan. The self-contained apparatus is shipped to the work site, where SkyTEM’s two-person geophysical crew from Gatineau, Que., and XRI staff converge. They assemble the pieces into one unit in one to two days then attach the apparatus by hook to the helicopter. A specially trained pilot flies a programmed flight path consisting of parallel lines spaced anywhere from 100 to 200 metres apart. They fly back and forth repeatedly, with the frame about 100 feet above the ground. The crew and XRI staff collect data via computers aboard the helicopter. They stitch the data together and convert that data to a 3D representation of the geology under each flight line.

Although it typically takes about six weeks to fully process the data, the company can provide preliminary data within two days, he says. This is helpful to companies because they can see whether or not there is an aquifer while staff and helicopter are still on site and able to adjust their flight plan if needed.

Mapping Methods not Mutually Exclusive
Airborne mapping can work in tandem with well drilling or surface mapping, says Abraham. “We combine the interpretations with wells. We have a well that’s pre-existing or we drill a new well and we test the aquifer’s properties. Then we take those tests, go back to our 3D interpretations of the aquifer. Now we have a volume and a yield.”

A 2014 paper goes further, inviting the Canadian ground water industry to consider applying airborne technology to regions of exploration to supplement existing knowledge. “The hydrogeological settings in large parts of Canada are similar to those in Denmark,” says Andrea Viezzoli of Aarhus Geophysics in Rethink Ground Water Mapping and Management Strategies for Unconventional Hydrocarbons Using Airborne EM. “We suggest that parts of this [AEM] approach can be readily and successfully applied in Canada.”

Whether or not the Danish technology becomes widely used for ground water applications in Canada remains to be seen, but the industry would be wise to watch the skies for it.