Technical Notes - Helicopter Electromagnetics

Helicopter Geophysics for Utilities

Dependent on the type of target, there are several helicopter-borne geophysical systems suitable for this type of work:

• Magnetometer arrays (either mounted on the helicopter directly, or suspended in an array below the aircraft) are useful for detecting ferrous metal pipelines as small as four inches in diameter. This has been done with suspended arrays in the past. The new unexploded ordnance (UXO) detection systems we are now offering promise to improve the sensitivity for smaller or deeper pipelines. These UXO systems are mounted on three "stingers" out the side and front of the helicopter, which flies at a few metres above ground. Such low altitudes are not possible with systems slung below the helicopter. The electrical currents injected into pipelines (cathodic protection) create a strong magnetic anomaly, increasing the chance of detection considerably.

• Non-ferrous pipelines or utilities (telecommunications or power) are detectable using electromagnetic (EM) systems. (They generally are not detectable with magnetometers, unless there is a strong DC electric current in them.) Our DighemV helicopter borne EM system is very sensitive to any type of long conductive body. Because the system is five frequency and incorporates EM sensing coils with two different orientations, it can be used to map the ground conductivity in three dimensions at the same time as detecting the utility lines. This information can be used to build a map of ground conditions such as soil types, ground water, and/or bedrock depth.

• The Dighem^v system is suspended about 30m (100ft) below a standard, small-to-mid size helicopter, and carried about 30m (100ft) above the ground surface. The helicopter travels at about 30m/sec, and data are sampled at 10 samples per second. With GPS sensors in the EM system one can expect a positioning accuracy of 5-10m. The UXO systems offer considerably better resolution because of their lower altitude.

• Helicopter-borne EM systems (HEM) are effective methods of mapping large areas very quickly. Coverage in a day depends on many factors, but 25 square kilometres (10 sq miles) coverage at 100m line spacing is normal.

Greg Hodges, Chief Geophysicist, 2002