The EM31 equipment is a simple "Slingram" method consisting of a magnetic dipole (a current loop) transmitter (Tx) and a coplanar magnetic dipole receiver (Rx) operating at a fixed frequency of 9.8 kHz and with a fixed distance between Tx and Rx of 3.66 m. When a current is injected into the Tx coil a primary magnetic field is generated. When the primary magnetic field interacts with the electrical conductors in the earth secondary currents are induced in them. These secondary currents in turn generate a secondary magnetic field that adds to the primary field at the position of the receiver.
The outputs of an EM-31 survey are the conductivity (quadrature) and In-phase components of the secondary magnetic field. The secondary magnetic field is a complicated function of the intercoil spacing, the operating frequency, and the ground conductivity. The relationship is simplified when certain constraints, technically defined as "operation at low induction number", are met. When the low induction number constraints are not satisfied the measured quadrature and In-phase responses deviate from expected values.
The Geonics LtdEM31-MK2 maps geologic variations, groundwater contaminants, or any subsurface feature associated with changes in ground conductivity. Using a patented electromagnetic inductive technique that allows measurements without electrodes or ground contact. With this inductive method, surveys can be carried out under most geologic conditions including those of high surface resistivity such as sand, gravel, and asphalt. Ground conductivity (quad-phase) and magnetic susceptibility (in-phase) measurements are read directly from an integrated DL600 data logger (which can be easily removed from the console for data transfer). Real Time (RT) graphical presentation of the data during collection is possible by connecting a computer directly to the RS232 output port on the front panel with an optional RS232 interconnect cable (Geonics, 2020).
The effective depth of exploration is about six metres, making it ideal for geotechnical and environmental site characterization. Important advantages of the EM31-MK2 over conventional resistivity methods are the speed with which surveys can be performed, the precision with which small changes in conductivity can be measured and the continuous readout and data collection while traversing the survey area. Additionally, the in-phase component is particularly useful for the detection of buried metallic structure and waste material (Geonics, 2020).
Advantages
The Frequency Domain Conductivity Mapper is used by Geophysics HM Ltd for environmental, geological and geotechnical investigations. FDEM is successfully utilized for soil type mapping, landfill extent delineation, contamination mapping, utility locates, UST mapping, land reclamation and due diligence investigations. The main advantages of the system are:
Mapping soils, fill, contamination, waste and debris up to 6 meters depth
Easy to deploy over unfavorable terrain conditions
Fast data acquisition and large surface coverage
Excellent resolution for large metallic objects such as UST and buried metallic drums
Rapid and efficient data interpretation and presentation
Good resolution and delineation of buried structures, debris, landfill and contamination
Geophysics HM
