Ground GeophysicsSurface EM

Time-domain electromagnetic (TEM) surveys involve induction of electromagnetic (EM) fields in the sub-surface via a square or rectangular transmitter loop which has no electrical connection with the earth. In turn, the sub-surface EM fields induce secondary EM fields in a receiver coil and a receiver attached to this coil measures a transient decay which is diagnostic of ground conditions.

In ground EM surveys the receiver is at the surface and can measure both vertical and horizontal components of the B-field and/or dB/dt field.

Due to the physics of the technique it is inherently better at detecting conductors in resistive environments rather than resistors in conductive environments.

The depth of exploration for ground EM is down to 1000m.

Please click here for a flyer about ground EM.

APPLICATIONS

Ground EM has been used for the following applications:

Mineral Exploration
Detecting and mapping sub-surface electrical conductors such as a massive sulphide ore body.

Geothermal Energy
Ability to map contrasting conductivity associated with geothermal resources.

Groundwater
Detecting and mapping groundwater that is more conductive than its host.

Environmental
Locating buried hazards and groundwater contaminants.

Structure and Lithology
Mapping geological structure and lithology.

Hydrocarbon Exploration
The technique has been used for hydrocarbon exploration.

TECHNIQUES

An EM field is generated using a transmitter coil - typically at 1 Hz - and the induced secondary EM response from subsurface conductors is measured during the off-time. Time-domain EM techniques measure the transient decay in the off-time. The techniques are used to generate a two dimensional conductivity map. A three dimensional map can be created using specialised modelling software.

Various sensors are available that can measure B-field and/or dB/dt field. The main configurations are:

Fixed Loop
This technique involves laying out a large transmitter loop and recording the EM response along grid lines both inside and outside the loop.

Moving Loop
This technique uses a transmitter loop and an EM receiver. The receiver takes a measurement or series of measurements inside the loop and then the loop is moved and the process repeated.

Slingram
This technique is the same as moving loop but EM measurements are made both inside and outside the loop.

EQUIPMENT SPECIFICATIONS

GPX Surveys use SmarTEM receivers manufactured by Electromagnetic Imaging Technology (EMIT). Data can be collected using three component B-field sensors and/or dB/dt coils.

A variety of transmitters is available depending on the depth of penetration required. Click on the links for specifications

Transmitters
Zonge Engineering NT-20, ZT-20, ZT-30, GGT-10 & GGT-30

EM Receivers
EMIT SmarTEM receiver with
- EMIT three component fluxgate magnetometer (B-field) and/or
- GPX-BA coil (dB/dt) or
- coincident loop receiver (dB/dt)
- Zonge TEM/3 coil receiver (dB/dt)