Transient Electromagnetic or Time-Domain EM (TEM)

- Aug 19, 2019-

TEM

The transient electromagnetic (TEM) method, alternately called time-domain EM (TDEM) or pulse EM (PEM), is a commonly-used, non-intrusive, geophysical method for obtaining subsurface resistivity-conductivity data.


Because rock conductivity strongly correlates to rock properties, TEM is an effective way to map changes within rock or soil, for example, clayey layers restricting groundwater flow, conductive leachate in groundwater, and seepage in earthen embankments.


TEM methods have been used in mineral exploration for more than half a century and are now used for an extremely broad range of applications in exploration, engineering, and environmental investigation.

The depth of investigation can vary from 10s of meters to over 1000 meters (30 to 3,000 feet), depending upon the size of the transmitter loop being used, available power from the transmitter, and ambient electromagnetic noise.


How it works:


TEM requires
a specialized transmitter to drive a time-varying current into a transmitter loop, usually an ungrounded loop of wire laid on the surface. The transmitter loop generates an EM wave that propagates into the subsurface. As the EM energy encounters different subsurface materials, it induces eddy currents that generate secondary EM fields. These secondary EM fields are picked up at the surface by a receiver loop or magnetic antenna and recorded as the induced energy diffuses into the earth. The rate of diffusion indicates the resistivity of the subsurface materials.


Applications:

TEM techniques are used to map geologic structure in search of geothermal sources, groundwater, and aggregate deposits. Environmental and engineering uses range from delineating salt-water intrusion and contaminant migration to determining permafrost and depth to bedrock.