Electromagnetic Techniques
Fixed Frequency Electromagnetic (EM) Profiling
Detail
Electromagnetic profiling utilises a time-varying electromagnetic field (the primary field) to induce eddy currents within subsurface conductors. These currents result in a secondary magnetic field that is measured together with the original transmitted signal, using a receiver coil on the EM instrument. The secondary field is then separated into two orthogonal components, the real and imaginary (quadrature) components, representing respectively the vector components of the field in-phase and 90 degrees out of phase with the primary. The quadrature component provides a measure of the apparent ground conductivity whilst the real (in-phase) component is responsive to buried metallic objects.
The depth of penetration attained is dependent on a number of factors including the ground conductivity, the loop spacing and the orientation of the primary field (dipole orientation). A number of EM instruments are available which together provide a depth of investigation range of between 0.5m and 30m. The use of three or more loop spacings in both dipole orientations enables quantitative modelling of the depth to individual conductive layers. This is commonly known as EM depth sounding.
Shown on the right is an EM31 Ground Conductivity Meter manufactured by Geonics Inc. of Canada. This instrument gives a maximum depth of investigation of approximately 6m in vertical dipole mode (dependent on ground conductivity). Data from the EM31 instrument is logged automatically using an on-board data logger and can be periodically downloaded to a field computer during the day for quality checking and initial data processing.
