Time-Domain ElectroMagnetic (TDEM) methods are based on the principle of using electromagnetic induction to generate measurable responses from sub-surface features. When a steady current in a cable loop is terminated a time varying magnetic field is generated.  As a result of this magnetic field, eddy currents are induced in underground conductive materials.  The decay of the eddy currents in these materials is directly related to their conductive properties, and may be measured by a suitable receiver coil on the surface.

In order to identify a specific feature, it is necessary that its inherent electrical conductivity contrast significantly with the conductivity of surrounding materials. In most successful TDEM applications, the targets sought possess enhanced conductivities relative to their host material, as demonstrated in the image below.

1. Mineral exploration - metallic elements are found in highly conductive massive sulfide ore bodies.
2. Groundwater investigations - groundwater contaminants such as salts and acids significantly increase the groundwater conductivity.
3. Stratigraphy mapping - rock types may have different conductivities.
4. Geothermal energy - geothermal alteration due to hot water increases the conductivity of the host rock.
5. Permafrost mapping - there is a significant conductivity contrast at the interface between frozen and unfrozen ground.
6. Environmental - locate hazards such as drums and tanks.

1. TDEM systems may be used in many different configurations such as large loop Turam style, moving loop Slingram style, in-loop soundings, and borehole. Paper by Balch and Smith on measuring In-phase response for borehole surveys with the PROTEM system.
2. A pulsed transmitter waveform allows the receiver to measure the electromagnetic response during the transmitter off-time without the presence of the primary field.
3. No direct electrical contact with the ground is required so that surveys can be equally effective in frozen environments.
4. The same basic techniques can be used to investigate the top few metres of ground or to depths over 1000 metres.
5. Generally fast and cost effective for the amount of data generated.

Fugro Ground Geophysics operates the following TDEM acquisition systems

• SmarTEM^v
• Geonics digital Protem
• Sirotem Mk.3
• Zonge GDP-16

All the above receivers provide the capability of simultaneous acquisition of 3-component data and real-time quality control permitting high production rates and economical survey charges. The SmarTEM receiver is particularly useful in high noise environments, with the ability to customise noise filtering during acquisition. The multi-component data improves interpretation of both surface and borehole surveys. For borehole work, the Geonics BH43 single component downhole probe or the BH43-3D multi-component probe is available with cable lengths of up to 2000 metres. A range of transmitter units are available, the choice of which depends on the particular application, i.e. shallow investigation requiring a very fast turnoff time or larger generator powered units for deep penetration.

The Zonge GDP-16 receiver is also available for TDEM surveys. This is a highly flexible unit which in addition to TDEM has the capability of being used for other geophysical methods such as IP and CSAMT. The Zonge GGT-30 transmitter offers very high power for maximum depth penetration.

Geophysicists are assigned to all field crews in order to ensure efficient field operations and to monitor data quality. Custom developed 32 bit data processing software is used to rigourously analyse data in the field and generate profiles. Interpretation of the data is available using various computer modeling routines such as Grendl, Beowulf and Spiker.