EM34 Response Curves

The EM34-3 uses electromagnetic signals to determine the apparent conductivity of materials concealed under the earth's surface.  This instrument is manufactured and marketed by Geonics, Ltd.  This instrument is a frequently useful in conducting reconnaissance for groundwater studies, for mapping variations in depth to bedrock, for detecting water-filled karstic openings and detecting landfill leaks.

The EM34-3 can be operated in either a vertical or horizontal dipole at any one of 3 transmission frequencies; a high frequency for shallow exploration, a low frequency for deeper penetration, and an intermediate frequency.  During high frequency measurements, the transmitting and receiving antennas are 10. meters apart.  Antenna separations for other frequencies are 20. meters and 40. meters.  Antenna separation rather than frequency is used to specify depth sensitivity.

These charts were prepared using equations supplied by Geonics in their publications (some available online).

   Horizontal dipole, relative response.  Circular antennae are oriented vertically, meaning the electromagnetic field is projected horizontally, in a loop that travels under the feet of the operators (and also overhead, but there is seldom anything conductive in the air above the survey crew).  This orientation is most sensitive to materials very near the surface, with sensitivity decreasing more rapidly for the 10-meter separation than for the 20-meter or 40-meter separations.

    Vertical dipole, relative response.  Antennae are oriented horizontally (lying flat on the ground), projecting the electromagnetic field vertically into the ground.  The 10-m dipole has a 'sweet spot' centered 3.5 meters under the surface.  The 'sweet spots' for the 20-meter and 40-meter dipoles are 7 and 14 meters respectively.

   Horizontal dipole, cumulative response: Half the signal comes from above the depth corresponding to 0.5, half from below this point.  The apparent conductivity measured by the EM34-3 is a sort of weighted average involving thicknesses, depths to and electrical conductivities of layers under the earth's surface.

   Vertical dipole, cumulative response: Half of the signal for the 40-m dipole comes from materials below 35 meters depth, half from materials above this depth.  By comparing values measured for both dipole orientations and all three dipole separations, one can begin to determine depths at which materials are conductive or resistive to the flow of electricity.  Highly conductive materials might be certain kinds of clay or groundwater containing high levels of dissolved salts.  Highly resistive materials might be dry sand or low-porosity, solid bedrock.

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