Technical Papers - Airborne Electromagnetics

The Use of B-field Measurements in an Airborne Time-Domain System:
Part I - Benefits of B-field Versus dB/dt Data

Richard Smith and Peter Annan, Fugro Airborne Surveys, Ottawa. 

Abstract

Standard airborne electromagnetic (AEM) systems measure the response of the ground using an induction coil, which in the time domain outputs a voltage proportional to the time rate of change of the magnetic field dB/dt. A recent development of the GEOTEM system allows B-field data also to be acquired. These two forms of data (dB/dt and B-field) are compared by examining the power spectra, model data and field examples.

The power spectrum of the B-field excitation waveform has significantly more energy than the dB/dt power spectrum at low frequencies, while at high frequencies the dB/dt spectrum has more energy. The impact of this different distribution of energy can be seen in model data: the ratio of the signal to noise for the good conductors is significantly enhanced on B-field data and reduced on dB/dt data. In fact for very good conductors (between 2000 S and 100 000 S) the dB/dt response is reduced to such an extent that it is below the noise level, while the B-field response is still significantly above the signal level. On the other hand, the signal to noise ratio for poor conductors is greater in the dB/dt data than the B-field data.

Field and model data show the B-field response of good conductors (about 100 S) is greater than the response of poorer conductors, whereas for dB/dt data it is less. Thus, the B-field profiles are generally easier to interpret than the dB/dt data, as the larger amplitudes are generally more indicative of conductive zones.

Two field examples (from Frobisher Bay and the Ottawa Valley) illustrate a number of other points. Spheric noise appears reduced on the B-field data. This implies that it may be possible to acquire AEM measurements in certain areas and at certain times when dB/dt data would be too noisy. Also, the range of values cover by the B-field data is much less than that covered by the dB/dt data. This makes it easier to display B-field data on an image or a profile. 

Published in Exploration Geophysics, 1998, Volume 29, pages 24-29. For more information on this and other ASEG publications, see http://www.aseg.org.au/

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See also Part II of this paper.