Time domain
TEM/TDEM — Transient / Time-Domain Electromagnetics
Switch the current off and watch the ground decay
Frequency band
Equivalent 1 Hz - 100 kHz
Depth range
20 - 1,500 m
Resolution
Good for conductors
Source
Switched transmitter loop
Typical cost
Moderate
Survey speed
Moderate - loop layout dominates
Best terrain
Open ground; loops need clear space
Domain
Time domain
Overview
Current in a transmitter loop is abruptly switched off. The collapsing field induces eddy currents that diffuse downward and outward, and the decaying secondary field is measured between transmitter pulses with no primary field present.
How the physics works
Measurement in the off-time removes the primary field entirely, which is why TEM is so sensitive to good conductors. Late-time decay of a half-space follows t^-5/2, while a discrete conductor decays exponentially with its own time constant.
Where it is used
- Massive sulphide and graphite conductor detection
- Groundwater and aquifer mapping
- Mapping saline intrusion and contamination plumes
- Void and abandoned mine working detection