Electro-magnetic – VLF and Slingram

Slingram is a pair of moving transmitter and receiver coils connected by a cable at a fixed distance, normally 40 m or 60 m, with penetration depths corresponding to half the coil space. The oscillating primary field from the transmitter coil induces electrical currents in the subsurface that in turn, generates a secondary magnetic field which is picked up by the receiver coil. This secondary magnetic field is further divided into in-phase (real) and quadrature (imaginary) components, which is dependent on the subsurface electrical conductivity distribution and can be related to e.g. mineralizations and fracture zones. The interpretation of this data is mainly qualitative and an estimation of the thickness, dip of the conductive zone can be made from observation of the shape of the anomaly.

For very small coil separations, the inphase component of the signal becomes very small and the quadrature component becomes directly related to the conductivity of the ground. This enables a direct reading of the mean ground conductivity using ground conductivity meters. This method is very useful for shallow investigations (< 10 m), making it ideal for many geotechnical and groundwater contaminant surveys.

Very Low Frequncy (VLF) surveys makes use of an existing external field generated by permanent military transmitters. The receiver is small and portable enough for one person to carry and the method is hence quick and cheap.