Electromagnetics

Electromagnetic (EM) measurements are primarily used to detect and map lateral changes in natural geologic and hydrogeologic conditions. The method is also applicable to detecting and mapping contaminant plumes and can be used for locating and mapping buried wastes, metal drums and tanks, and metal utilities.   There are two general types of EM measurements: Frequency Domain and Time Domain.

Frequency domain electromagnetics measures the magnitude and phase of induced electromagnetic currents, which are related to the subsurface electrical conductivity (ASTM D6639-01)   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.

Applications

• Frequency domain electromagnetic measurements are primarily used for profiling to detect and map lateral changes in natural geologic and hydrogeologic conditions
• Electromagnetic measurements are also applicable to detecting and mapping contaminant plumes
• Electromagnetic measurements can be used for mapping buried wastes, metal drums, tanks, and metal utilities

Advantages

• Measurements are relatively easy to make
• Data do not require extensive processing and corrections
• Does not require ground contact
• Electromagnetic measurements provide excellent lateral resolution
• Continuous data may be acquired to depths of 50 feet with hand-carried or vehicle-mounted equipment
• Provides measurements with depths ranging from a few feet to 250 feet

Limitations

• Limited vertical resolution
• Susceptible to interference from nearby metal pipes, cables, fences, vehicles and induced noise from power lines

Time domain electromagnetics (TDEM) measures the bulk electrical resistivity of subsurface conditions by inducing pulsed currents in the ground with a large transmitter coil.  The decay of the induced currents over time results in a decaying secondary magnetic field at a rate that is governed by the electrical resistivity of the earth.  The decay of the secondary magnetic field is then monitored at increments in time with a separate receiver coil.

Applications

• TDEM measurements are primarily used to determine depth and thickness of geologic strata and hydrogeologic conditions
• TDEM measurements can also be applied to detection and mapping of landfill leachate plumes, seepage from brine pits, saltwater intrusion and mineral exploration

Advantages

• Depth range of approximately 20 to 3,000 feet
• Requires less space that resistivity measurements to reach the same depth

Limitations

• Deeper measurements require a large transmitter coil (1,000 x 1,000 feet or more) for which space may not be readily available
• Susceptible to interference from nearby metal pipes, cables, fences, vehicles and induced noise from power lines
• Not effective for shallow (less than 20 feet) measurements