Electromagnetics (TDEM):

Time Domain ElectroMagnetic induction (TDEM) involves generating an electromagnetic field which induces a series of currents in the earth at increasing depths over time. These currents, in turn, create magnetic fields. By measuring these magnetic fields, subsurface properties and features can be deduced at great depths.

Common applications of TDEM include the following. Identifying aquifers and aquatard and detecting the presence of water or salt water intrusions in groundwater investigations. Mapping aggregate deposits for quarry operators. Detecting metal objects and mapping leachate in environmental investigations. Mapping permafrost and other geologic features in geotechnical engineering.

The figure above illustrate how TDEM works. A current is created in the transmitter wire loop which generates a magnetic field. The magnetic field induces weaker eddy currents immediately below the transmitter loop, approximating a mirror image. As the initial near-surface eddy currents decay, their magnetic fields, in turn, induce weaker eddy currents at greater depths which also decay. This process continues over time with ever weakening currents at ever increasing depths as is depicted in the above figure.

The magnitude and rate of decay of those eddy currents depend on the conductivity of the medium, (i.e. the electrical conductivity of the soil) and on the geometry of the conductive layers. In resistive media the currents will decay very rapidly. In conductive media the currents will decay more slowly. A conductive layer at depth may “trap” currents in that layer, while currents elsewhere decay more rapidly.

The TDEM receiver measures the magnetic fields created by the eddy currents in the subsurface. In TDEM the current generated by the transmitter is a sharp pulse, or transient signal. The induced eddy currents and their magnetic fields echo this, being sharp pulses occurring moments later. Measurements of the magnetic field are typically made in the time range from 10 micro-seconds to 10 milli-seconds following the “turn-off” of the primary field. Because measurements are made while the transmitter current is turned off, the more sensitive measurement of the magnetic field generated by the subsurface can be made. Measurements are made in 20 to 30 discrete “time gates”, or time intervals, following the current pulse generated by the transmitter. For deeper exploration (thousands of feet) in conductive sections, measurement times can extend up to one second. These measurements of the induced currents' magnetic fields tell us great deal about subsurface conditions.

TDEM has similar applications to FDEM. It advantage lies in its ability to determine depths and map much deeper features. However, TDEM surveys are not as rapid as FDEM and thus large areas can not be mapped as economically.