Electromagnetic (EM) surveying uses a transmitting ‘loop’ to generate primary magnetic field that can induce an electric (eddy) current into conductive bodies. When the primary EM field is turned off, the induced field decays, and itself generates a secondary EM field. In Australia, the most common type of EM survey is a time-domain EM (TDEM) survey, which measures the decay in the secondary EM field with respect to time. This decaying field is recorded by either wire coils or magnetometers.
Ground EM surveying
EM surveys can be carried out in the air, on the ground or within a drillhole. For mineral exploration, EM methods are typically used to detect semi-massive to massive conductors associated with sulphide mineralisation. However, EM methods are also excellent at mapping regolith and paleodrainage features. Airborne EM surveying is a great tool for regional mineral exploration. The technique can cover a lot of ground quickly, and typically also acquires magnetic information. Surveys carried out on the ground typically record much longer and have more power (depth penetration).
Ground EM surveys can be carried out in either a moving loop (MLEM) or fixed loop (FLEM) configuration. Each configuration has its advantages over the other, and which configuration to use will depend on the size of the survey area, geological knowledge and the depth of investigation. If you already have a drillhole, a sensor can be lowered down the hole to detect conductive material in the near vicinity of the drillhole. In all cases, the recorded EM signal can be modelled to provide accurate drill targets.
If you would like to more information of EM surveying or are thinking of carrying out an EM survey or already have EM data would like to discuss processing, modelling or interpreting the data, please feel free to contact us.