1887
Volume 29, Issue 1-2
  • ISSN: 0812-3985
  • E-ISSN: 1834-7533

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 . A recent development to the GEOTEM system allows -field data to also be acquired. These two forms of data ( and -field) are compared by examining power spectra, model data and field examples.

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

Field and model data show that the -field response of good conductors (about 100 S) is greater than the response of poorer conductors, whereas for data it is less. Thus, the -field profiles are generally easier to interpret than data, as the larger amplitude zones 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 in the -field data. This implies that it may be possible to acquire AEM measurements in certain areas and at certain times when data would be too noisy. Also, the range of values covered by the -field data is much less than that covered by the data. This makes it easier to display -field data on an image or a profile.

© 1998 ASEG
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1998-03-01
2026-01-19

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References

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  • Article Type: Research Article

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