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Airborne Electromagnetic Method (AEM)

Description of Technology

The airborne electromagnetic method (AEM) provides a rapid assessment of geological conditions as they relate to electrical conductivity or electrical resistivity. AEM uses a specialized transmitter to drive a time-varying current into a transmitter loop on an airborne platform. The transmitter loop generates an EM wave that propagates into the subsurface; as this energy encounters different subsurface layers or materials, it induces eddy currents that generate secondary EM fields. Secondary EM fields are detected by a receiver loop or magnetic antenna as the induced energy diffuses into the ground. The rate of diffusion indicates the resistivity of the subsurface materials.

 

Benefits of the AEM system are numerous, owing to the flexibility and transportability of the system components:

  • Proven technology, with over 1,000,000 line kilometers flown
  • Rapid acquisition over large areas
  • Deep exploration
  • Robust modeling that can be constrained with additional data
  • Highly accurate information based on calibration for each project
  • High sensitivity due to sensor elevation
  • Flexible system design for deeper or shallower focus
  • Available for anywhere in the world

Rapid Acquisition

For regional investigations, AEM provides rapid acquisition over large areas to depths of 1200ft or more. The airborne system that HGI deploys is from SkyTEM.  The system is flown on a helicopter platform above the terrain at heights of 100 to 200ft. Speeds range between 60 and 90 mph. EM soundings are collected approximately every 10ft when flown at 60 mph. The helicopter is typically flown along parallel lines, swathing back and forth across the survey area. Soundings are acquired consecutively along each line to form a profile dataset that is georeferenced with differential GPS. In addition to the AEM, the helicopter can also acquire magnetic, radiometric, and other essential data for your investigation.

 

 

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Image of EM soundings collected approximately every 10ft when flown at 60 mph.

AEM rapid acquisition over a large area. A helicopter is typically flown along parallel lines, swathing back and forth across the survey area.

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Mining Example

AEM data was collected over a copper heap leach pad to investigate hydraulic issues and potential seepage from the pad to a nearby pit wall. The helicopter was flown at 60ft line spacing for a total line length of 123 miles over an area of 1.5 square miles. The SkyTEM system collected 11,395 soundings to create a 3D block model of the pad, pit, and surrounding formations. The block model was sliced from southwest to northeast to show the internal electrical properties to depths of 800 to 1000ft. The pit on the right-hand side of the contoured section below is shown to be electrically conductive. The leach pad, sitting on top of a resistive and impermeable bedrock, shows interesting drainage patterns of PLS. Most of the leachate is at the bottom of the pad, with minor layered sequences towards the middle. A large portion of the leach pad on the northeast side is dry and unleached to eliminate the potential for additional seepage to the pit. A slightly elevated conductivity along the pit wall may be related to natural or induced (i.e., from blasting) fracturing that allows meteoric water to infiltrate and seep into the pit. It is unlikely that the leach pad is contributing to the seep.

 

AEM data collected over a copper heap leach pad to investigate hydraulic issues and potential seepage from the pad to a nearby pit wall.

AEM data collected over a copper heap leach pad to investigate hydraulic issues and potential seepage from the pad to a nearby pit wall.

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