Time-domain electromagnetic method (TDEM or TEM – transient electromagnetic or PEM – pulse electromagnetic) is a popular non-intrusive geophysical exploration technique for determining subsurface resistivity data.
Measuring the resistivity and/or conductivity of soils can tell a lot about its properties. That is why TEM methods are widely used in rocks mapping, seepage location, determination of groundwater flows etc.
Advantages of TDEM methods over DC methods:
TEM does not require long arrays (e.x. in order to get the information on subsurface at depth of 30 m, only a 15 m loop is required; DC measurements require arrays that are 3-5 times greater than investigation depths);
TEM is less sensitive to lateral resistivity changes;
TEM has a good depth resolution (especially for location of resistive layers within a conductive section; section under thick nonconductive layer can be easily mapped).
How it works
Equipment set consists of transmitter with ungrounded transmitting loop of wire and receiver with receiving loop or magnetic antenna. Larger loops allow greater depths of investigation.
TEM measurements are made at one station at a time. They last quickly since each transient is equivalent to a wide spectrum of individual frequency measurements.
At first, transmitter emits a square-wave signal to subsurface. Then the signal is cut off quickly.
Induced current in subsurface occurs to preserve the magnetic field produced by original current (eddy current). Eddy currents generate secondary electromagnetic fields. Receiving loop or antenna record this secondary EM field.
Secondary EM fields attenuate with time– that is why the terms “transient” and “time-domain” are used to describe the method. Diffusion of the signal in subsurface indicates its resistivity.
Induced currents in good conductors decay slowly, in poor conductors – quickly, in very poor conductors – are not be capable to be measured.
Each decay is recorded in different “time gates” after the pulse. This time interval varies from microseconds to 100 milliseconds. Depending on the application, “time gates” differ.
Depths of investigation can vary from 10 cm to 1000 m depending on the size of transmitting loop, transmitter power and other factors like noise.
geological structures mapping (deposits location, groundwater and geothermal sources search);
environmental and engineering tasks (contamination migration, bedrock location, permafrost detection, salt-water intrusions mapping);
communications search (location of conductive objects like pipelines, storage tanks, UXO, wells etc).