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

Abstract

Fixed-wing transient airborne electromagnetic methods are now capable of measuring the on-time response and accurately estimating the three components of the primary field. Using the primary fields as the input to an inversion algorithm, it is possible to determine the offset of the receiver sensor (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. Independent estimates of the offsets using a laser range finder show that the primary-field estimates are accurate at high altitudes away from any ground effect. Because laser range finders cannot be employed practically at survey altitude, the primary-field method can also be used to obtain a dynamic estimate of the bird position close to the ground. Typically, 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. Empirical observation suggests that the estimated positions are not significantly effected by the ground response, and are therefore good estimates. In one example, the dynamic positions are used as input to a conductivity/depth estimation algorithm and compared with the results obtained when using only the nominal bird position. The results are comparable.