Very Low Frequency EM

Very Low Frequency Electromagnetics (VLF) is a geophysical ground probing technology that utilizes VLF signals in the 15 to 30 kHz range normally used for communication with submarines. The signal generated is suitable for making geophysical measurements globally.

VLF is an electromagnetic method that relies on transmitted currents inducing secondary responses in conductive geologic units. A VLF anomaly represents a change in the attitude of the electromagnetic vector overlying conductive materials in the subsurface.

Today, the VLF method is having a revival, thanks to both technology improvements and software developments. It is a very inexpensive means to collect high quality and informative geophysical data.

VLF Applications:

Mineral Exploration:

• Mapping geologic structure, including the apparent dip of fault zones and shear zones. The data can be interpreted to identify the dip of these structures for reliable drilling
• Identifying conductive ground which might correspond to sulphide concentrations
• Mapping overburden in preparation to drilling and further sampling.

Ground Water Exploration:

• Detecting conductive bodies of fluid in bedrock or in the vicinity of fractures
• Mapping contaminants. contaminants often have lower resistivity than surrounding fluids

Depth of Investigation:

Depth of investigation is controlled by the electrical “skin depth” of the local geology. It varies from shallow to in some cases > 100m depending upon the overall background resistivity of the subsurface. Typically 20-75 meters can be expected. Conductive overburden suppresses signals and depth penetration may be severely limited at times. VLF works best where rocks are resistive and overburden is minimal or is highly resistive.  

Instruments and Software:

IES uses GEM Systems GSM-19V portable VLF system.