The Airborne VLF-EM System

Airborne VLF-EM data are mainly used for geological mapping, interpretation of large scale geological features, such as faults and shears, and locating graphite and base metal conductors.  McPHAR uses the TOTEM-2A Airborne VLF-EM system to acquire these data.  

The TOTEM-2A receives the magnetic component of fields radiated from VLF transmitters in the 15 to 25 kHz frequency range. These transmitters are located around the world for the purposes of navigation and communication with submarines.  The parameters normally measured are the change in total field and the vertical quadrature component and the total field gradient from dual sensors. The sign of the quadrature polarity is also recorded.

The TOTEM-2A can be configured to measure two VLF transmitting stations simultaneously, which results in greatly improved data quality for two important reasons. First, the likelihood of finding conductive bodies that strike in different directions is enhanced. Secondly, responses from more than one transmitter can be correlated to verify the results of each measurement, thereby resulting in better confidence in the data and better interpretations.

The frequency of the Line and Ortho channels can be selected individually by means of three digital thumbwheel switches, in 100 Hz steps, throughout the full range of 15.0 to 24.3 kHz. As VLF stations are located at 100 Hz increments, the acceptance bandwidth around the nominal frequency is sufficient to accommodate modulation plus some offset without causing severe modulation noise.

The TOTEM-2A has the capability to detect conductors with strikes in almost any direction, because its efficiency of operation makes it feasible to read two transmitted signals.

QUANTITIES MEASURED BY THE TOTEM-2A

The TOTEM-2A VLF can process simultaneously dual frequency information from each of two spatially separated sensors, thereby permitting gradient measurement at two primary field azimuths.

The horizontal magnetic field component Ho of the propagating field from VLF radio transmission stations is utilized by the TOTEM-2A as the source of primary excitation. This component continues to exist beneath the earth's surface as Hd, decreasing at a rate dependent on the ground conductivity.

Conductivity variations in the horizontal plane, due to localized mineralization or other geological structure, causes a secondary field to be generated.

The TOTEM-2A avoids the need for independent orientation, and amplitude or phase reference by effectively measuring the change in magnitude and the ellipticity (polarization) of the total field (primary plus secondary). Because anomalous secondary fields are generally quite small in comparison with the primary field, these measurements equate very closely with measurement of the in-phase horizontal and quadrature vertical secondary fields. The nominal 100% primary magnitude is determined in an anomaly free zone. 

To avoid interference from aircraft or bird roll, pitch and yaw, it is necessary to make field measurements with an orthogonal (xyz) array of three antennae. The three directional antennae components are designated as LINE, ORTHO and ERECT. LINE couples maximally with a field in the nominal direction of flight (in-line with flight). ORTHO couples maximally with a field in the nominal direction at 90* to the direction of flight (orthogonal to flight). ERECT couples maximally with a nominally vertical field.

The field parameters which are actually computed by the TOTEM-2A are determined relative to the principal axis signal, which may be designated as either Line or Ortho.  The TOTEM-2A can process VLF signals from two primary field azimuths simultaneously, one in each of the two principal axis channels.