Electromagnetic Techniques
Time Domain Electromagnetic Sounding (TDEM)
Detail
This active method measures the bulk electrical resistivity of the ground by inducing eddy currents in subsurface conductors using pulsed electromagnetic energy transmitted from a square loop of wire laid on the ground. The decay of these induced currents results in a decaying secondary magnetic field which is measured over up to 30 time increments ('gates') immediately after termination of the transmitter pulse. Measurement of the secondary field can be made using either the transmitter loop or more commonly with a separate receiver coil located at the centre or to the side of the transmitter loop.
In the case of horizontally layered materials the induced current loop will diffuse outwards and downwards with time whilst gradually decaying in amplitude. The speed of this diffusion and the amplitude of the secondary magnetic fields are related
to the conductivity of individual subsurface layers. As a consequence thin resistive layers are generally invisible to TDEM soundings. Use is made of a very early time (capable of 1.2msec turn-off time) NanoTEM system with Zonge GDP-32 receiver owned by Zetica Ltd. This system provides improved resolution of the near surface as compared with 'traditional' TDEM systems. The depth of investigation of a TDEM survey is dependent on the moment of the transmitted signal together with the conductivity of the subsurface layers. A larger moment (achieved through an increase in the loop size and/or transmitter current) and an increase in ground resistivity will result in increased signal penetration.
Modelling of TDEM sounding data can be carried out using commercially available software programs as well as proprietary software. Background noise levels are commonly measured at each sounding location so that the quality of the data can be effectively monitored. Data falling below the measured noise level is discarded from the modelling process. A hand-held conductivity meter is used to log existing core from calibration boreholes and provide control for the modelling process.
