Time Domain Electromagnetic (TDEM Survey)

Time Domain Electromagnetics (TDEM) systems are composed of a transmitter instrument, a transmitting coil or wire, a receiver coil or antenna, and a receiver instrument. The system operates by utilizing a transmitter that sends an alternating current through a square loop of insulated electrical cable placed on the ground. This current alternates between equal periods of being on and off, with base frequencies ranging from 3 to 75 Hz, thereby generating an electromagnetic field. The cessation of current flow occurs over a very short duration (a few microseconds) known as the ramp time, during which the magnetic field varies with time.

The variable nature of the primary electromagnetic field induces a secondary electromagnetic field in the ground beneath the loop, which is an exact replica of the transmitter loop. This secondary field starts to decay immediately, generating additional currents that move downward and outward into the subsurface, akin to a series of smoke rings. The secondary currents are measured only during the time-off interval by a receiver located at the center of the transmitter loop.

The depth of the TDEM investigation is determined by the time interval following the current shutoff, as the receiver senses currents at progressively deeper levels over time. The strength of these currents at specific times and depths is dictated by the bulk conductivity of the subsurface rock units and the fluids they contain. Based on subsurface resistivity, the induced current, receiver sensitivity, and the geometry between the transmitter and receiver, TEM/TDEM measurements allow for geophysical exploration from just a few meters below the surface to depths of several hundred meters.