A Bird's-eye View of the Riches Beneath Our Feet

by Charlene Wiles

Developed to locate submarines during the Second World War, aeromagnetic surveys are now being used to help scientists map rock types and measure the concentrations of various elements — such as gold, copper, lead and zinc — in specific geographical areas across Canada.

A survey of this kind is essential for science and exploration. “It’s a roadmap for geoscientists,” explains Warner Miles, acting Section Head of the Regional Geophysics Section at Natural Resources Canada (NRCan) in Ottawa. “Just as you likely wouldn’t go into downtown Toronto without a map to help navigate your way, scientists wouldn’t go into the field without a map guiding them to what they are looking for. Aeromagnetic maps provide a better understanding of the geology in the area.”

Unlike real road maps, aeromagnetic surveys describe what lies beneath ground-level structures. They help identify the type of bedrock as well as the minerals contained within it. To do this, it is necessary to first understand the composition, structure and history of the bedrock. This would be a difficult task for anyone working in the field, because bedrock can often be hidden from view by overlying soil, loose rock, vegetation, water or ice. That’s where magnetic geophysics comes into play, since it can detect the physical properties of buried bedrock.

“All rock contains specific magnetic minerals, and each molecule within the rock acts as a magnet,” explains Warner. Since these variations cause changes in the magnetic field over different types of rock, measurements of the magnetic field can give indications of the particular type of rock that lies beneath the surface.

To conduct an aeromagnetic survey, an aircraft fitted with an instrument called a magnetometer flies back and forth along parallel lines, with a constant line spacing, over the area to be surveyed. This method allows large areas to be covered very quickly. As the plane passes over the area, the magnetometer measures and records the varying magnetic intensities of the rock. These intensities are represented by different colours on a specialized map of the area that is archived in the Canadian Aeromagnetic Data Base.

The maps can then be used in mineral exploration and to help communities, Aboriginal associations and governments make well-informed decisions on land use. The information is available to the general public, but since it is generally used for large-scale projects, NRCan’s main clients for the maps are universities, the provinces and territories, and oil and gas or mineral exploration companies.

Because of the many practical uses of this specialized type of information, the demand for aeromagnetic surveys is strong.

“The surveys are an excellent tool for geologic mapping and resource exploration, and they’re becoming more and more relevant in today’s research,” says Warner. “Because we support programs based on government issues, the number of surveys and the areas being covered depends on the needs of the scientists and their specific research. We are currently involved in 14 different surveys across the country. A few years back, we would have only been working on three or four.”

Stay tuned for more information on these surveys and how they are being used in British Columbia for mineral exploration in areas impacted by the mountain pine beetle.