Technical Papers - Airborne Electromagnetics

Advances in Time-Domain EM Technology

Abstract

The introduction of digital systems in the 1980s has resulted in the signal to noise ratios obtainable with airborne TEM systems being significantly reduced. This results in a greater depth of penetration. The digital primary-field compensation algorithms mean that data collected in the transmitter pulse (in the transmitter on-time) can be used for geological mapping. This is particularly useful in resistive areas, where the conductivity can be mapped down to 25 mS/m. The on-time data can also be used to deconvolve for the shape of the waveform, allowing the measured data to be transformed into data which would have been collected if the system were transmitting another waveform. When the data is transformed to that which would be measured if the waveform were a square wave and the receiver a magnetometer, then the subsequent data can be used for calculating and mapping the conductivity as a function of depth.

The greater capability of the computers acquiring the digital data has allowed for larger volumes of data to be collected, so that multicomponent measurements are now standard. The extra components provide much more diagnostic information and make interpretation significantly easier. The fidelity of the data can also be improved by using all three components.

The systems have also been extended to operate at lower frequencies or longer time bases, allowing the response to be measured much later in time. This allows for more conductive bodies to be detected, or for bodies to be detected under thicker conductive overburden.

The ancillary magnetic data collected with EM systems has also improved, either by moving the sensor closer to the ground and/or closer spatial sampling of the data.

The number of possible applications of airborne EM has increased; for example the polarisability of the ground can now be estimated from data acquired with an airborne TEM system. It has been found that the polarisability can only be measured when the ground is highly polarisable, but this occurs surprisingly frequently.

Paper published in "Proceedings of Exploration 97; Fourth Decennial International Conference on Mineral Exploration" edited by A.G. Gubins, 1997, GEO F/X, p. 497-504.

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