The main goal of electromagnetic geophysical prospecting in the mining industry, which is based on variations in relative subsurface conductivity, is to detect deposits that conduct electricity. These formations are detected using the principle of induction, as they would be by a current transformer.
 

The usual process is to put an transmitter on the surface. It produces a strong primary magnetic field that varies over time and penetrates more or less deeply into the subsurface. “Eddy currents” are induced in conductors that are excited by this primary field. These in turn produce a so-called “secondary” electromagnetic field that will be detected on the surface or in boreholes by a probe connected to a receiver.
 

What Types of Conductors Can Be Detected with Electromagnetic Geophysics?

• Superficial: overburden, lake bottoms, streambeds;
• Present in the rock: graphite, sulphide deposits, magnetite deposits, shear and fault zones, serpentine peridotite;
• Artificial: metal tanks, metal conduits and waste, pipelines, railways, high voltage lines.

In What Fields Are Electromagnetic Geophysical Surveys Used?
 

Electromagnetic prospecting applies to various fields: mining, environmental, civil engineering and hydrogeology. In geology, it is useful for structural studies to detect faults or fractures, to look for cavities, to estimate the thickness of alluvial cover, and to look for streams of groundwater, or for graphite, sulphide or magnetite deposits.

 

What Methods Are used in Electromagnetic Geophysical Prospecting?

Very Low Frequency (VLF) Electromagnetism
 

This passive method uses permanent transmission antennas to detect surface conductors, and can also be used as a tool to establish the boundaries of structures, faults and fractures. Since it doesn’t use a local transmitter, it can be used in many environments.
 

The transmitter used already exists for communication with submarines. The frequencies are around 20 kHz, corresponding to very low frequencies for radio transmissions.

 

EMH-EMV
 

EMH-EMV electromagnetic prospecting is used to map conductors at shallow (no more than around one hundred metres) depths. The method is relatively simple and inexpensive to carry out.

 

TDEM on the ground and in borehole
 

TDEM is used to measure the conductive response of the subsurface and to locate conductive bodies. Often, the conductivity of base metal sulphide deposits will contrast strongly to that of the other rocks in their vicinity, making this method very effective for locating these types of deposits. The conductance of a deposit will depend on its mineralogy, texture and thickness. The shape of TDEM response is also influenced by the conductor’s positioning and size.
 

In addition to economic sulphide deposits, other conductive subsurface materials include barren sulphides, graphitic sediments and saline fluids.

 

Who needs a TDEM survey?

This type of survey is useful for mining and exploration companies searching for metal deposits, as well as in many environmental and engineering applications.
 

How does a TDEM survey work?
 

To conduct a TDEM survey, technicians lay loops of wire on the surface of the earth, and a generator is connected to a transmitter that produces a current through these loops. This current generates a changing magnetic field (the primary field) that induces currents in the conductive bodies in the subsurface. The induced currents in turn produce their own magnetic field (the secondary field) which is measured by sensors at surface and/or in boreholes to define the electromagnetic signature of the ground.
 

A typical surface TDEM survey will have readings taken every 50 m along lines that are spaced 100 m apart. Borehole surveys take readings every 5-10 m down the hole. Typical TDEM survey transmitter loops can vary in size from 50 m x 50 m loops which move with each measurement station, up to multiple large fixed-position loops which can commonly be 1 km x 2 km each.

 

What TDEM survey types are available?
 

There are several different types of TDEM configurations, most of which can be conducted as a surface survey or in a borehole. The basic types include:

• Moving loop: The transmitter loop is moved with each measurement station.
• Standard Fixed Loop: A single transmitter loop is deployed and stays in one position while the sensor moves.
• InfiniTEM: A pair of transmitter loops with opposite polarity are deployed and the sensor moves. This powerful system is the industry leader for deep investigation of steeply dipping conductors.
• OMNIVision: A high-powered solution using three loops that are switched on and off in different arrangements to provide industry-leading deep investigation for conductors of all dips.

What kind of deposits can be found by TDEM?

Base-metal sulphide deposits such as Ni-Cu-PGE and VMS as well as uranium deposits are the most common targets for TDEM surveying.


Consult Abitibi Geophysics’s innovations in electromagnetic geophysical prospecting: Transmitteur TerraScope®,  InfiniTEM® and InfiniTEM XL, OmniVision and ARMIT-3.

At Abitibi Geophysics, we seek out the most innovative technologies on the market and are constantly developing methods and equipment to help you discover new deposits in complex geological conditions. Contact us to learn more about our services.
 

Sources : (In French only)

Méhodes Électromagnétiques

Cours de prospection électromagnétique

Méthodologie d’interprétation en électromagnétisme aéroporté