Frequency Domain Electro-magnetics



Electro-Magnetic (EM) surveying uses the principle of electromagnetic induction to measure changes in the electrical conductivity of the subsurface. Bulk subsurface conductivity is affected by variations in porosity and permeability, degree of saturation and fluid type, whilst discrete variations is often an indication of buried metallic objects. As such mapping variations in conductivity allows for both the identification of bulk conductive anomalies and discrete anomalies within a homogenous background.

With the FEM method a primary magnetic field with an alternating electric current of known frequency and magnitude is situated at the ground surface through a transmitting coil. This induces turbulent electric currents (eddy currents) in the earth, the magnitude of which is directly proportional to the ground conductivity. These eddy currents result in an alternating secondary magnetic field that is detected by a receiving coil.

The nature and size of FEM instrumentation provides a means of quickly and efficiently collecting densely sampled data which is geo-referenced to provide accurate conductivity maps of the site being investigated.

Data Analysis & Presentation

An FEM reading is related to the electrical conductivity of the subsurface directly below the instrument, measured in apparent conductivity. The apparent conductivity can be correlated to conductivity measurement in milliSiemen/metre (mS/m). The results are presented as "heat maps" showing the variation of conductivity from the various transmit frequencies chosen. These are analysed for targets of interest with spatial positioning, the approximate dimensions and depth being extracted.

The depth of exploration realised with the FEM method is governed by the transmit frequency as well as the overall subsurface conductivity. Expressed in terms of skin depth, a maximum depth of penetration achievable typically is 15 metres below the surface level.