Electromagnetic
Our experienced staff is fluent in the wide variety of electromagnetic (EM) survey methods; from shallow subsurface EM conductivity mapping, using instruments such as the Geophex GEM-3 and Geonics EM -31, -34, or -38, to deep investigations for ground water, geothermal, and mineral resources using the time-domain EM, CSAMT (controlled source audio-magnetotelluric), and magnetotelluric (MT) methods. Each method, however, involves the measurement of one or more electric or magnetic field components induced in the subsurface by a primary field produced from a natural (transient) or an artificial alternating current source. The primary field spreads out in space both above and below the ground and induces currents in subsurface conductors, in accordance with the laws of EM induction. These currents give rise to secondary EM fields, which distort the primary field. In general, the resultant field, which may be picked up by a suitable receiving coil, will differ from the primary field in intensity, phase, and direction and reveal the presence of the conductor.
Electromagnetic methods are sensitive to variations in electrical properties of subsurface materials and can map out regions with enhanced conductivity due to the presence of fluids, metals, or other variations. Electromagnetic induction instruments induce currents in conductors within the subsurface without having to make direct contact with the ground. Such instruments can collect data rapidly. Electromagnetic methods can be utilized to:
- Locate buried tanks and pipelines,
- Delineate landfill boundaries,
- Map conductive soil and groundwater contamination,
- Map soil moisture and salinity in agricultural areas,
- Characterize subsurface hydrogeology (map buried channel deposits, groundwater exploration, locate conductive fault and fracture zones),
- Characterize geological structure.
Typically electromagnetic methods are classified according to the operational mode of the source; continuous wave field methods (frequency domain), transient field methods (time-domain), and magnetotelluric methods. Frequency domain EM methods use a fixed transmitter or mobile loop source to broadcast a continuous signal, usually sinusoidal, at certain frequencies. Field data are typically collected in profiling mode for mapping lateral variations in ground conductivity. A transmitting coil induces an electromagnetic field and a receiving coil, at a (usually) fixed separation, measures the amplitudes of the in-phase and quadrature (ground conductivity) components of the electromagnetic field. The various instruments have different coil spacings and operating frequencies. Coil spacing and frequency affect the depth of signal penetration. Both single frequency and multi-frequency instruments have been developed for commercial use.
Time-domain electromagnetic methods use transient source signals rather than continuous waves, and they measure the decay of the secondary EM field after the primary has been turned off. With highly conductive bodies, the eddy currents decay more slowly than for weak conductors. Measurements of the decay rate of the eddy currents can be used to locate subsurface conductors and estimate their conductivity. TDEM methods use a transmitter coil and a receiver coil with a variety of transmitter and receiver coil sizes and geometries used.
Schematic of the time-domain electromagnetic method; a large transmitter loop generates the primary signal and a small receiver coil measures the resulting decay of the secondary electromagnetic field to produce a sounding of ground conductivity.
Magnetotelluric methods make use of EM energy from natural transient phenomena. Natural EM energy has mainly two sources; solar wind induced flow of charged particles in the ionosphere that are used in the magnetotelluric method and distant thunderstorm activity that are used in the audio-magnetotelluric method. The magnetic disturbances penetrate the ground and induce time-variable telluric currents that will generate secondary electromagnetic fields, which can be measured at the surface.