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Deep EM methods measure the magnetic and electric fields associated with natural or artificially generated subsurface currents. The nature of the energy source defines whether an EM method is passive or active. Passive EM methods employ natural energy as incoming plane (EM) waves as the source recorded by receivers. Active EM methods require a controlled or artificial energy source (e.g., transmitter) and receivers. Passive and active EM instrumentation can be mixed and matched in a modular fashion.

Passive EM methods used by Cordillera Geo-Services include AMT (audiomagnetotellurics) and MT (magnetotellurics). Both AMT and MT target different depths of investigation in sectors like hydrocarbon, geothermal, and mineral exploration; and are typically used to map conductive targets. AMT and MT utilize natural variations in the Earth’s magnetic field as a source. Natural AMT and MT signals come from various induced currents caused by thunderstorms and activity in the ionosphere. AMT operates at a frequency band of 1-70 kHz, allowing an investigation depth of 50 m to ~1 km. MT works with either broadband and super-broadband MT receivers that operate at a lower frequency band of 0.0001-1 kHz and 0.0001-10 kHz, respectively, reaching depths of investigation from 300 m to 200 km and 1 km to 200 km. Both passive methods require the use of three induction coil magnetometers or receivers to measure the magnetic field’s components and four non-polarizable electrodes to measure the electric field’s components, which are in turn logged by a digital data acquisition unit. Multi-station AMT/MT surveys are suitable for conventional or time-lapse 2D and 3D acquisition.

Active EM methods, so-called controlled-source electromagnetic (CSEM) methods, require receivers to measure the magnetic and electric fields, as in a passive EM system; however, the energy source is a high-power transmitter that generates artificial EM fields with prescribed signal characteristics rather than natural EM fields. In a CSEM survey, the current is injected into the subsurface via a grounded horizontal electrical dipole antenna. CSEM surveys map buried resistors. The Earth’s response is measured by one or more electric and magnetic field receivers located at various distances from the source in a 2D or 3D configuration. The Long-offset transient electromagnetic (LOTEM) method is a worldwide-used CSEM method for deep studies. It is ideal for geothermal exploration, enhanced oil recovery (EOR), developing and producing geothermal fields, carbon storage and monitoring, and mineral exploration. LOTEM method surveys comprise a high-power land transmitter station and several receiver stations. Conventional or time-lapse 2D and 3D surveys are ideal for monitoring or exploration projects.