TDEM is a geophysical technique used to detect geological conductors, airborne anomalies, examine Brownfields, map intrusions and uncover a wide range of minerals, conductivities and environments.
The method works by employing a transmitter that drives an alternating current through a square loop of insulated electrical cable laid on the ground. The current consists of equal periods of time-on and time-off, with base frequencies that range from 3 to 75 Hz, producing an electromagnetic field. Termination of the current flow occurs over a very brief period of time (a few microseconds) known as the ramp time, during which the magnetic field is time-variant.
The time-variant nature of the primary electromagnetic field creates a secondary electromagnetic field in the ground beneath the loop that is a precise image of the transmitter loop itself. This secondary field immediately begins to decay, in the process generating additional currents that propagate downward and outward into the subsurface like a series of smoke rings. Measurements of the secondary currents are made only during the time-off period by a receiver located in the center of the transmitter loop.
The depth of the investigation depends on the time interval after shutoff of the current, since at later times the receiver is sensing currents at progressively greater depths. The intensity of the currents at specific times and depths is determined by the bulk conductivity of subsurface rock units and their contained fluids. Depending on subsurface resistivity, current induced, receiver sensitivity and transmitter-receiver geometry, TEM/TDEM measurements allow geophysical exploration from a few metres below the surface to several hundred metres of depth.
In this article, Kevin Vine, President of multiVIEW, describes a variety of utility locating technologies including Ground Penetrating Radar (GPR) and Electromagnetic (EM) Induction, and the activities that they are best suited to such as locating telecommunication lines, finding Underground…
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