The use of B-field measurements in an airborne time-domain system: Part I. Benefits of B-field versus dB/dt data

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 to the GEOTEM system allows B-field data to also be acquired. These two forms of data (dB/dt and B-field) are compared by examining 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 the noise for 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 noise level. On the other hand, the signal-to-noise ratio of poor conductors is greater in the dB/dt data than the B-field data. Field and model data show that 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 dB/dt 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 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 covered 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.