borehole EM

All electromagnetic (EM) techniques operate under the principle of electromagnetic induction. Time domain electromagnetics (TDEM), alternately called transient electromagnetics (TEM) or pulse electromagnetics (PEM), is a commonly used and non-intrusive geophysical technique employed to detect and define conductive units within the subsurface. TDEM surveys are a particularly effective exploration tool for massive sulphide type deposits. The TDEM method utilizes an alternating pulsed, primary current with a controlled turn-off time and a sophisticated receiver to measure the rate of decay of the induced secondary field across a series of time windows during the off-time. The advantage of this technique is that the secondary field is measured while the primary field is off. Therefore, measurements can be made with greater sensitivity and variations due to the spatiality of the primary field can be avoided. In a time-domain EM system, the amplitude of the secondary EM field decays with time after the primary field shuts off. The form and rate of decay of this secondary transient EM field can be used to deduce characteristics of the form, size, and conductivity of the conductive body in the earth. The greater the conductivity or size of the conductive body in the earth, the greater will be the secondary EM field.

The TDEM system is comprised of a portable generator connected to a specialized transmitter that drives a precise/time-varying current into an ungrounded transmitter loop of wire laid on surface. A receiver and a surface coil (or borehole probe) are then moved along surface lines (or down boreholes).