The Terraquest XDS VLF-EM System: Discussion of the 3-component passive VLF EM system and exploration case studies By Peter Walker and Jeremy S. Brett

VLF-EM is a passive electromagnetic method that employs energy in the 3-30 KHz band, typically broadcast by naval transmitters. Under good conditions, VLF-EM can be sensitive to conductivity variation to a depth of 100 meters.

Standard VLF measurements are typically made by tuning the system to a particular transmitter. Provided the transmitter is on, and provided the strike of the conductivity variation is in-line with the direction to the transmitter, useful data can be acquired. Unfortunately, these conditions are not always met, and VLF data must often be discarded.

The XDS-VLF EM system has been developed by Terraquest to overcome these difficulties. While it can acquire data in the standard mode, tuned to a particular VLF transmitter, it also has a broadband mode spanning from 12 to 35 KHz. Thus, XDS is not sensitive to geological strike and the foibles of a particular transmitter. This increases the reliability of the XDS measurement in comparison to the standard method. The XDS receiver consists of three orthogonal coils and is sensitive to geological variation independent of the strike to the transmitter and the orientation of the aircraft.

VLF data can be of great utility, particularly when interpreted in conjunction with magnetic and radiometric data to confirm or identify new structures. Results from a number of case histories have shown: how VLF conductors and associated probable structures can be identified in the Athabasca Basin, how structures associated with uranium mineralization in the Otish Basin; can be traced; and how VLF data can be employed in gold, nickel-copper/chromite and diamond exploration.

Model XDS
Manufacturer Terraquest Ltd.
Primary Source Magnetic field component radiated from government VLF radio transmitter
Parameters Measured X, Y, Z components, absolute field
Broadband 12 to 35 KHz
Gain Constant Gain Setting