Ground GeophysicsDownhole EM

Downhole EM surveys are used to map subsurface geology and locate conductive targets that may have been missed by a drill-hole.

The technique uses a surface loop to induce an EM field in the subsurface. The EM response is measured using a downhole probe that is lowered into a drill hole. All three components of the B-field or dB/dt field are measured by the probe and the readings can be used to model the location and distance to target.

APPLICATIONS

Mineral Exploration
Downhole EM surveys are used to help locate conductive targets that may have been missed by a drill-hole. Conductive targets include ferrous metals such as nickel and chrome, and massive sulphide ore bodies.

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.

TECHNIQUE

An EM field is generated by a surface transmitter at 25Hz or 30 Hz and the secondary EM response from the subsurface conductors is measured during the off-time using a three component downhole probe. The secondary field is created by subsurface conductors which could be of economic interest, either if the target is conductive or the target is hosted in conductive rock.

If a conductor is mapped the transmitter loop can be repositioned over the conductive target to generate a stronger EM response. Downhole measurements are made every 5 to 25 metres up to a depth of 2000m.

EQUIPMENT SPECIFICATIONS

GPX Surveys has a range of downhole EM equipment that enables us to survey to a depth of up to 2000m. Click on the links for specifications.

Transmitters
Zonge Engineering GGT-10 & GGT-30

Downhole EM Receivers
EMIT SmarTEM receiver with
- EMIT Atlantis three component fluxgate magnetometer probe (B-field) or
- Geonics BH43-3D three component coil receiver probe (dB/dt)