Frequency Domain Electromagnetic Methods (FDEM)

Frequency Domain Electro Magnetic induction (FDEM) involves generating an electromagnetic field which induces current in the earth which in turn causes the subsurface to create a magnetic field. By measuring this magnetic field, subsurface properties and features can be deduced. This method measures the magnitude and phase of induced electromagnetic currents, which are related to the subsurface electrical conductivity. Electrical conductivity is a function of the soil and rock matrix, percentage of saturation, and the conductivity of the pore fluids. EM instruments provide two measurements simultaneously, the electrical conductivity data and the in-phase component, which responds to magnetic susceptibility and metal.

FDEM has distinct advantages over many other techniques. Because no contact with the ground is required, FDEM can cover a large area quickly and therefore economically. In certain cases, depths of up to 50 meters can be mapped.

Common applications of FDEM include the following:

• groundwater investigations
• mapping lateral changes in natural geologic conditions
• detecting and mapping contaminant plumes
• mapping buried wastes, metal drums, tanks, and metal utilities

One of our goals at Geo-Sense is to reduce client-costs without compromising on data quality. This is why we constantly seek out the most sophisticated and state-of-the-art equipment. For FDEM (Frequency Domain Electro Magnetic) surveys we have chosen to use the Geophex instruments. The advantage of these systems lies in their ability to collect data from a number of different pre-programmed frequencies simultaneously. In reality, this means that data can be collected from different depths at the same time and be compiled into a depth slice of the survey area. This is extremely useful and allows the coverage of large areas, quickly, thus reducing the time and cost of the survey. Because data is collected from different depths concurrently, there is usually no need for more than one pass with the instrument in the survey area.

For our purposes, we employ the GEM-2 sensor and are currently working on the development of a unique GEM-5 array.