Terrain conductivity measurements (also commonly referred to as Electromagnetic techniques) are characterized by the Geonics EM-34 and EM-31 instrumentation, although other systems are also available. EM surveys measure the apparent conductivity of the subsurface in mS/m (milliSiemens/meter). Variations in conductivity may indicate changes in composition, layer thickness, or moisture content, or the presence of buried metal such as drums, and/or contamination.
EM-31 is used to measure the quadrature and in-phase component (in-phase very sensitive to buried metals). Penetration is typically up to 6 meters in normal subsurface conditions. Horizontal and vertical loop components are recorded with EM-34 using coil separations of 10, 20 and 40 meters. Using different coil separations allow for increased penetration.
The following table summarizes the different combinations of use for the EM34:
|Coil separation (m)||Horizontal component (Depth in meters)||Vertical component (Depth in meters)|
The vertical component is also very sensitive to steeply dipping features such as water soaked fractures. In this configuration the system operates in the slingram mode where a steeply dipping conductor appears as an M-shape feature on the vertical component data.
EM surveys can be performed rapidly and large areas can be covered much quicker than gravity and resistivity surveys. Data can be acquired at single stations and/or continuously along lines. Data is collected on a grid or profile lines, depending on the target characteristics. Station spacing must not be larger than the dimensions of the expected target. Data can be acquired in profiling or sounding mode where sounding gives depth information. However, most EM surveys are conducted in profiling mode as lateral variations in conductivity are more of the interest in most environmental.
- Buried metallic objects
- Buried tanks, drums, utility lines
- Mapping of landfills and trenches
- Mapping of contaminant plumes
- Mapping of groundwater
- Mapping of fractures, faults
- Mapping bedrock topography
- Mineral exploration
|MaxMin EM system transmitter during EM survey to map massive Ni-Cu mineralization|