Transient Electro-Magnetics

Applications

Method

Transient Electro-magnetic (TEM) surveying uses the principle of electro-magnetic induction to measure changes in the electrical conductivity of the subsurface. An electric current of known frequency and magnitude is transmitted through a large coil which induces a magnetic field in the space surrounding the coil including the subsurface which in turn induces electrical currents (eddy currents) in the ground material. The transmitting current is then stopped and the rate of decay of the eddy currents is measured. The rate of decay is plotted as a curve and then inverted and analysed to determine the subsurface material conductivity with depth.

Electrical current is transmitted and recorded using coincident square loops. Loop size dictates vertical resolution and maximum depth of investigation with a larger loop providing deeper imaging with a lower vertical resolution. At each sounding location multiple readings are taken and stacked. Where considerable noise is observed in the generated decay curve often from electrical infrastructure such as power lines, the soundings are repeated with an increased number of stacks.

Data Analysis & Presentation

The acquired TEM data is displayed as a decay curve plotting the received voltage against time. The decay curves are inverted and constrained to obtain TEM soundings plotting conductivity with depth. Inversion of the TEM data can be constrained using the ERI data where both have been collected. Other sources for constraining the inversion may include downhole conductivity logs and groundwater monitoring wells which can be used if nearby and made available.

Where multiple adjacent TEM sounds have been made, continuous profiles can be generated showing the variation with conductivity with depth and laterally along the profile.