The calibration method used in the digital Dighem^v has been developed as a significant improvement over the practices described by Fitterman (1998), which practices were originally developed through experimentation and consultation between Fugro and the United States Geological Survey. The problems defined by Fitterman, including jig calibration and conductive ground response, are obviated by calibration at high altitude using internal, rigidly mounted, automatically triggered and measured calibration coils.

Calibration of the system during the survey will use the Fugro AutoCal automatic, internal calibration process. At the beginning and end of each flight, and at intervals during the flight, the system will be flown up to high altitude to remove it from any "ground effect" (response from the earth). Any remaining signal from the receiver coils (base level) will be measured as the zero level, and removed from the data collected until the time of the next calibration. Following the zero level setting, internal calibration coils, for which the response phase and amplitude have been determined at the factory, are automatically triggered - one for each frequency. The on-time of the coils is sufficient to determine an accurate response through any ambient noise. The receiver response to each calibration coil "event" is compared to the expected response (from the factory calibration) for both phase angle and amplitude, and the applied phase and gain corrections adjusted to bring the data to the correct value.

In addition, the output of the transmitter coils are continuously monitored during the survey, and the applied gains adjusted to correct for any change in transmitter output (due to heating, etc.)

Because the internal calibration coils are calibrated at the factory (on a resistive halfspace) ground calibrations using external calibration coils on-site are not necessary for system calibration. A check calibration may be carried out on-site to ensure all systems are working correctly. All system calibrations will carried out in the air, at sufficient altitude that there will be no measurable response from the ground.

The internal calibration coils are rigidly positioned and mounted in the system relative to the transmitter and receiver coils. In addition, when the internal calibration coils are calibrated at the factory, a rigid jig is employed to ensure accurate response from the external coils.

Using real time Fast Fourier Transforms and the calibration procedures outlined above, the data will be processed in real time from measured total field at a high sampling rate to in-phase and quadrature values at 10 samples per second.

Greg Hodges, Chief Geophysicist, 2001