VLF electromagnetic survey uses powerful VLF transmitters located around the world for navigation and submarine communications. For survey in North-East America, the transmitting station in Cutler, Maine (NAA, 24.0 kHz) is normally used.
This radio transmitter emits powerful radio-waves (around 2 Megawatts) at 24 kHz that induce electrical currents, in the medium they travel through, which intensity depends on different factors such as the distance of the emitting source, the conductivity (and permittivity), volume and depth of the medium bodies. The induced currents produce a secondary magnetic field at the same frequency which is detected at the near surface through alteration of the original primary field. This secondary field is tilted up on the near side of the conductor and down to the other, as shown by its quadrature. The total field is composed of both primary and secondary fields (Applied Geophysics, 2nd ed., Telfort).
The VLF electromagnetic system used for this type of survey is the TOTEM-2A, which 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, with a sampling interval of 0.1 second.
The VLF receiver measures the vertical, transversal and longitudinal quadrature components and total field as a ratio of the total primary field.
System specifications:
- Capability of simultaneous measurement of two VLF stations;
- Accurate frequency selection: from 15 kHz to 25 kHz; selectable for each channel in 100 Hz steps;
- Sensitivity range: from 130 μV m to 100 mV m at 20 kHz; 3dB down at 14 kHz and 24 kHz;
- VLF signal bandpass: -3dB at ± 80 Hz; < 4 % variation at ± 50 Hz;
- Internal Noise: 1.3 μV m RMS.
Due to its simplicity, its small size and ease of use, the Herz TOTEM -2A system is a great addition to any helicopterborne survey.
For more information, please contact our Airborne Survey Team
