Electromagnetic Techniques
Time Domain Electromagnetic Profiling
Detail
Time domain electromagnetic profiling utilises a pulsed electromagnetic wave to induce secondary EM fields within subsurface conductors. The duration of these secondary fields, following shut off of the transmitter pulse, is dependent on the conductivity of the target, such that fields induced in metallic targets dissipate much slower than those resulting from ground conductivity. The secondary response is measured over a set time interval during the off period between pulses using one or more receiver coils. By measuring this response after dissipation of the ground component, the response from metallic targets is enhanced. TDEM profiling is consequently highly sensitivity to both ferrous and non-ferrous metals.
The EM61 comprises a portable coincident loop time-domain transmitter and receiver together with an additional receiver loop. The latter enables depth-to-target estimations and discrimination between near surface and deeper target response.
The EM61 uses a pulse frequency of 150Hz and a high powered transmitter providing a maximum depth of investigation of up to 5m (depending on target size). Output from the instrument is the integral of the time-gated secondary response, expressed in millivolts (mV).
Commercial software exists for estimating depth to source using the raw data from the two receiver loops. When combined with the high sampling rate of the EM61, this enables rapid screening of a site for potential hazards such as unexploded ordnance (UXO) and buried drums.
