Limitations of two approximate methods for determining the AEM bird position in a conductive environment

Abstract

The full potential of airborne electromagnetic (AEM) applications requiring sub-metre depth accuracy cannot be achieved if variable geometry between the receiver and transmitter is neglected. Methods involving photography, laser ranging, and aerodynamic computations have too many limitations to be of general use. We utilise the primary-field technique for estimating the dynamic bird position, assuming a free-space approximation. This procedure is expected to be accurate at high altitude or over a very resistive ground. In these circumstances, averaged bird offsets, relative to the transmitter, can be compared to the nominal receiver bird offset. When using AEM for bathymetric mapping of shallow seawater, the primary field is distorted by conductive seawater, and an inductive-limit approximation is used to correct the primary field. We compare the bird offsets obtained from both uncorrected (free-space) and corrected (inductive-limit) primary fields from survey data recorded over seawater, in order to understand the limitations of these two methods. Primary field values at the receiver increased when flying from a resistive ground to a conductive ground. The increment was greater for the vertical component than the inline component and was more pronounced at 25 Hz than at 12.5 Hz. Our analysis shows that for 25 Hz survey data over seawater, the inductive-limit approximation is an improvement on the free-space approximation, but still does not give the same range of receiver offsets as those obtained from high-altitude primary field data. Averaged bird offsets at high altitude were found to differ from the expected nominal receiver offsets. At 25 Hz, there is only agreement within the upper limits of experimental error. At 12.5 Hz, only the horizontal offsets are in agreement within experimental error. The accuracy of bird offsets at survey altitude is estimated by comparing them with those obtained at high altitude. Differences greater than several metres were observed. If errors greater than several metres in the bird position cannot be tolerated, then the values derived from the primary field method should not be used and alternative methods must be investigated.