Technical Papers - Airborne Electromagnetics

On the Effect of Varying the Pulse Width to Detect High Conductance Bodies

Richard Smith, Fugro Airborne Surveys, Ottawa. 

Abstract

Theory, modelling and field observations have shown that good conductors have the strongest time-domain AEM response if the duration of the transmitter pulse is twice the time constant of the conductor. Because good conductors have long time constants, recent time-domain AEM system modifications have resulted in the pulse width increasing to 4 ms and more recently, 6 ms. The theoretical analyses, modelling and data observations have all assumed that the secondary voltage response from the conductor is only measured in the off-time. However, very high conductance bodies have off-time responses which are asymptotically close to zero, so these bodies are undetectable, irrespective of the pulse width.

However, theoretical analysis shows that high conductance bodies do have a significant voltage response during the transmitter on time. The shape of this measured response is a half-sine pulse identical in shape to the transmitter current waveform. The on-time amplitude of a horizontal high conductance sheet is independent of the pulse width. As the conductance increases, the amplitude decreases, but does not disappear into the noise until the conductance is greater than about 3x10⁵ S. This conductance is significantly greater than the upper limit of conductances detectable with off-time AEM systems (1000 S).

Measuring the secondary magnetic field rather than the voltage response results in a finite, non-decaying response on the off-time. The magnitude of the late-time response is proportional to the width of the pulse, assuming the peak dipole moment is the same. Thus, the off-time magnetic-field response of very high conductance bodies can be enhanced with a longer transmitter pulse. For extremely high conductances, the response at all delay times is zero.

For a conductive half-space, the off-time voltage response is significant; however, the on-time voltage response is even greater. Like the thin sheet case, the magnetic-field response is finite in the off-time and this response decays more slowly than the voltage response. Because the magnitude of the measured response decreases as the inverse square root of the conductivity, it is possible to detect half-spaces as conductive as 4x10⁶ S.

Abstract of a paper published in Exploration Geophysics, 1998, Volume 29, pages 42-45. For more information on this and other ASEG publications, see http://www.aseg.org.au/

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