The calibration method used in the Dighem^VRES 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 be done principally using the internal calibration coils. A system calibration measurement will be conducted at the beginning and end of each flight, and at intervals during the flight. For this, 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 recorded with the data. 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. These responses are recorded with the data. 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 system phase and gain adjusted to bring the data to the correct value.

Because the internal calibration coils are calibrated at the factory (on a resistive halfspace) ground calibrations on-site are not necessary for data calibration. A check calibration will be carried out to ensure all systems are working correctly. All data calibrations will carried out after completion of the flight, using the in-flight calibration events. These will have been conducted 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.

References:

Fitterman, D.V., (1998). Sources of calibration errors in helicopter EM data. Exploration Geophysics 29, 65-70.