Herz TOTEM-2A multi-channel VLF Electromagnetic  continuous measuring survey instrument

Airborne VLF data are mainly used for the interpretation of large scale geological features such as faults and conducting rock units, although under favourable circumstances, smaller conductors such as those due to sulphide mineralization may be revealed.  VLF data are regarded as an excellent, cost-effective complement to airborne magnetic data.

Nose-stinger installation of TOTEM-2A receivers

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.
 
 

TOTEM-2A VLF receiver consisting of 19-inch rack-mounted console, the receiving coils and preamplifer unit

A TOTEM-2A system includes a sensor comprising three mutually orthogonal ferrite-cored coils (air coils optional) and a preamplifier mounted on an assembly, which can be inserted inside an airfoil (either a stinger or towed bird).  A cable connects the sensor to an electronics console which provides analog output for recording.

The TOTEM-2A employs state-of-the-art, unique digital and linear integrated circuits to implement the functions of crystal-controlled phase-locked loop frequency synthesizers, dual frequency heterodyne conversion and proprietary time domain sampling vector computation techniques.

Simultaneous dual frequency operation ensures that transmitters can be selected to provide good coupling with conductors of any orientation. The gradient mode measurements enhance interline contouring and delineation of multiple conductors.

Because of its simplicity, size and ease of operation, this VLF system is an ideal add-on to existing airborne and mobile geophysical exploration systems.

OUTSTANDING FEATURES

Automatic measurement of two VLF stations:

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.

Accurate frequency selection:

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.  By special modification the range can be extended to 10 to 30 kHz, however, by doing so the TOTEM-2A will not then be direct reading.

Wide signal bandwidth:

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.

Detects geological strikes in almost any direction:

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.
 

Primary fields: 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.

Secondary field:

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.

PARAMETERS MEASURED

Total field:

The change in magnitude of the vector sum of the principal axis field and the in-phase components of the other two orthogonal fields.  (100% primary field magnitude is considered to be above the absolute magnitude of the total field established in an anomaly free zone.)

Vertical quadrature:

Magnitude of that component of the Erect axis field in quadrature with the principal axis field.

Horizontal quadrature:

Magnitude of that component of field, on the Line or Ortho axis orthogonal to the principal, in quadrature to the principal axis field.

Gradient:

As the TOTEM-2A can process VLF signals from two spatially separated sensors simultaneously, it can, therefore, perform gradient measurements.

Gradient is defined as the difference in magnitude of the total field at the lower sensor with the total field at the upper sensor.  Total field is by definition independent of sensor orientation, hence the magnitude of the gradient anomaly will not be modulated by relative misorientation between upper and lower sensors.

PRIMARY SOURCE	Magnetic field component radiated from VLF radio transmitters (one or two simultaneously)
PARAMETERS MEASURED	Total field, vertical quadrature, horizontal quadrature and gradient
FREQUENCY RANGE	15 kHz to 25 kHz; front panel selectable for each channel in 100 Hz steps
SENSITIVITY RANGE	130 µV m to 100 mV m at 20 kHz, 3 dB down at 14 kHz and 24 kHz
VLF SIGNAL BANDPASS	-3 dB at ±80 Hz; < 4% variation at ±50 Hz
ADJACENT CHANNEL REJECTION	300 to 800 Hz = 20 to 32 dB;  800 to 1500 Hz = 32 to 40 dB;  > 1500 Hz > 40 dB (for < 2% noise envelope)
OUT OF BAND REJECTION	10 kHz to 2.5 kHz = 5 x 10-4 Am to 5 x 10-1 Am < 2.5 kHz rising at 12 dB octave;  30 kHz to 60 kHz = 5 x 10-4 Am to 8 x 10-3 Am > 60 kHz rising at 6 dB octave (for no overload condition)
OUTPUT FILTER	Time constant 1 sec. for 0% to 50% or 10% to 90%, noise bandwidth 0.3 Hz (second order LP)
INTERNAL NOISE	1.3 µV m rms (ambient noise will exceed this)
ELECTRIC FIELD REJECTION	< 0.5% error for 20 m tow cable
SFERICS FILTER	Reduces noise contribution of impulse interference
CONTROLS	Power switch, frequency selector switches (Line and Ortho), meter switch (total quad), and sferics filter switch
DISPLAYS	Meters (Line and Ortho), sferics light, overload light
INPUTS	Power: 23 to 32 VDC; fused 0.5 Amps  Signal: Sensor upper;  Sensor lower
OUTPUTS	Total, quad, gradient, multiplexed (line and ortho); Span: ± 1.0 V  Audio monitor, stereo line and ortho; Approx. 3 V p/p
DIMENSIONS AND WEIGHT	Console: 480 mm wide x 45 mm high x 340 mm deep, 19” rackmount (19x1.75 x 13.4 in.), 3.8 kg (8.3 lb.)  Sensor and Preamplifier Assembly: 150 mm diameter x 460 mm long, (5.9 x 18.1 in.), 1.5 kg (3.3 lb.)
ENVIRONMENTAL	Operating temperature:      Console:  0 - 50ºC       Preamp/Antenna:  -10 - 50ºC  Storage temperature:      -40 - 70ºC  Relative Humidity:      0 - 99% non-condensing  Altitude:      0 - 20,000 ft.

SPECIFICATIONS SUBJECT TO CHANGE WITHOUT NOTICE

MANUFACTURED AND SUPPORTED BY RMS INSTRUMENTS