Sources of calibration errors in helicopter EM data

Abstract

The procedures used to calibrate helicopter electro-magnetic systems are analysed to determine the errors produced both by mispositioning the Q-coil and by the effects of conducting ground at the calibration site. In general, errors produced by mispositioning the Q-coil can be greater than 2 percent/cm of displacement for movement in-line with the bird axis. Displacement in other directions produces smaller errors. The use of calibration jigs to stabilise Q-coil position is strongly recommended. The effect of conductive ground at the calibration site depends upon the conductivity of the ground, the transmitter frequency, and the coil geometry. As transmitter frequency increases, conductivity effects appear at higher resistivities for typical five-coil-pair birds with frequencies ranging from 900 Hz to 56 kHz. In general, these effects start to appear in the quadrature channels as resistivities drop below 100 ohm-m. Inphase channels are not affected until resistivities are less than 10 ohm-m. The horizontal coplanar coil geometry is more susceptible to conductivity effects than the vertical coaxial coil geometry, which is relatively immune to these effects at resistivities greater than 10 ohm-m for frequencies up to at least 7200 Hz.