Electromagnetic Methods in Geophysical Exploration

 

Electromagnetic (EM) techniques are a class of geophysical methods that use induction (i.e. generation of either time or frequency varying electromagnetic fields) to induce secondary currents in the ground. These secondary fields are then measured and used to determine a variety of characteristics related to the target, including dip, strike, conductivity-thickness, etc. Typically, EM methods are applied for direct detection of massive sulphides from a) surface, b) boreholes and c) underground. A lesser application is to map lithologies based on contrasts in electromagnetic properties (resistivity). Quantec Geoscience performs all of the survey types described below and can be reached at info@QuantecGeoscience.com for further details.

 

Frequency and Time Domain Methods

MaxMin is the most common ground frequency domain electromagnetic method. FEM is limited to shallow conductivity mapping. Using a frequency range from 222 Hz  to +14kHz. this method is applicable to shallow conductors and applicable for investigation limits of approximately 125 meters.

Very low frequency (VLF) is a frequency domain method that is used for shallow conductors with investigation limits of a few 10’s of meters. VLF is often used in conjunction with magnetics as a geological mapping tool. VLF means very low frequency, (+/- 22kHz) -- this is true for communication frequencies, however as a geophysical frequency the VLF band is very high frequency and susceptible to current channeling and skin depth conductors, with minimal penetration capabilities in conductive cover.

Time domain electromagnetics (TEM or TDEM) is effective for mapping conductive material, using frequencies from < 0.125 to + 2048 Hz. Current  investigation limits are approximately 500 meters (surface survey) and >2000 meters (borehole survey). For borehole EM (BHEM) applications, note that detection around the hole is from 150 to 250 metres based on a strong primary signal being generated from surface.

TEM / TDEM can be applied using various surface loop configurations, including fixed loop, in-loop and moving loop for various applications and to achieve various survey objectives, including increasing depth of penetration, increasing survey productivity, etc. The following image shows the survey configuration for an in-loop profiling survey. Note that this method is traditionally used for horizontal (sub-horizontal) targets.

 

 

For More Information

For more information on TEM / TDEM methods, please click TEM Method – Overview and Case Histories or TEM – Borehole Methods.

For case histories, please select from the list below:

TEM – Case History – Outlining an Orebody

TEM – Case History – Targeting of Voisey’s Bay Nickel and Copper Deposits

TEM – Case History - Perserverence