Tracking the transmitting-receiving offset in fixed-wing transient EM systems: methodology and application

Abstract

With the advent of wideband three-component airborne electromagnetic (AEM) systems, the primary-field components at the sensor (bird) can be determined with a high degree of accuracy. Using all three components of the primary-field information, it is possible to determine the vertical, longitudinal and transverse offsets of the bird from the transmitting loop. At altitudes high above the ground surface, the primary-field values will be unaffected by any response from the ground, so the estimated offsets should be fairly accurate. This has been confirmed by independent measurements from a laser range finder. At survey altitudes, laser range finders are not practical, so the primary-field method has been used to dynamically estimate the bird position. In two examples presented, the estimated positions are not affected significantly by ground response, and therefore appear to be reasonable estimates. For a typical flight using a bird and tow cable with reasonably high drag coefficients, the bird position is confined to within a few metres of the mean position. The dynamic bird position was used as one of the inputs to a conductivity/depth estimation algorithm and the conductivity/depth results are compared with the results obtained when the nominal bird position was used. In this particular example, using the estimated position rather than the nominal position resulted in minimal improvement of the conductivity/depth section.