Technical Papers - Airborne Electromagnetics

Tracking the Transmitting-Receiving Offset in Fixed-Wing Transient EM Systems: Methodology and Application

Richard S. Smith, Fugro Airborne Surveys
2060 Walkley Road, Ottawa, Ontario, Canada, K1G 3P5 

Abstract

The response of a fixed-wing, towed-bird, airborne electromagnetic (EM) system is determined by the system geometry. Successful systems are ones which are able to obtain a good measure of the ground response when the system geometry is varying; hence the reason that most fixed-wing systems measure the quadrature frequency domain, or off-time transient response. With the advent of wideband multicomponent measurement systems, the primary-field components at the sensor can be determined with a high degree of accuracy. Using this primary-field information as the input to a non-linear inversion algorithm, it is possible to determine the vertical, longitudinal and transverse offsets of the bird from the transmitting loop. The result is a dynamic and continuing measure of system geometry that can be used to augment data processing and interpretation schemes.

When calibration data is acquired at altitudes high above the ground surface, the estimated bird offsets will be correct as the primary field values will be unaffected by any response from the ground. This is confirmed by independent estimates made using a laser range finder. At survey altitudes, laser range finders cannot be employed practically, as readings are required at short time intervals and this is difficult because the aircraft and bird move about too much. However, the primary-field values can be used to estimate the bird position, as this position varies dynamically throughout a flight. In two examples presented, the estimated positions are not significantly effected by the ground response, and are therefore reasonable estimates. For a typical flight, the bird position is confined to within a few metres of the mean position when using a bird and/or tow cable with reasonably high coefficients of drag.

The dynamic bird position was used as input to a conductivity/depth estimation algorithm and compared with the results obtained when only the nominal bird position is used. In this particular example, the impact was not considerable.

Reprint of article published in volume 32 of Exploration Geophysics (2001), the journal of the ASEG. 

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