The EM61, one of the newest instruments from GEONICS, is a time-domain metal detector which detects both ferrous and non-ferrous metals. A powerful transmitter generates a pulsed primary magnetic field in the earth, which induces eddy currents in nearby metallic objects. The eddy current decay produces a secondary magnetic field measured by the receiver coil.
By taking the measurement at a relatively long time after the start of the decay, the current induced in the ground has fully dissipated and only the current in the metal is still producing a secondary field. The responses are recorded and displayed by an integrated data logger.
The EM61 detects a single 200-litre (55 gal) drum at a depth of over 3 metres beneath the instrument, yet is relatively insensitive to nearby cultural interference, such as fences, buildings and power lines. The response is a single, sharply defined peak, greatly facilitating quick and accurate location of the target. Depth of the target can usually be estimated from the width of the response.
The system can be pulled around as a trailer with odometer mounted on the axle to trigger the data logger or it can be carried by a single operator with a shoulder harness.