Inversion of Spectrem AEM data for conductivity and system geometry

Abstract

We evaluate the use of data from the fixed-wing Spectrem 2000 airborne electromagnetic (AEM) survey flown for ore body detection, regolith mapping and assessment of aquifers. Since the position and orientation of the receiver bird are not measured in the Spectrem system, the primary field at the bird cannot be known and removed precisely. In typical Spectrem data processing it is estimated and removed by subtracting out the last time window, relying on the assumption that the entire secondary field has decayed to zero by late-time. This is not generally a good assumption under the conductive regolith conditions and prevents the simultaneous fitting of both X- and Z-component data to within the noise envelope. In order to successfully invert the AEM data, and produce conductivity's depth maps and sections, we first reinstate the removed primary field and convert the data from ppm units to Teslas. We then inverted the total (primary plus secondary) field data, to solve for a 1D conductivity model as well as the horizontal and vertical offsets and pitch of the receiver bird using Geoscience Australia's sample-by-sample inversion algorithm. High-altitude data and synthetic forward models were also analysed to better understand the system's noise. Spectrem has flown many kilometres in other parts of the world but not that much in Australia. Our results highlight many similarities between Spectrem 2000 data and other systems previously flown over the same areas. Through our further processing and inversions we have resolved conductivity's depth structures very similar to those previously obtained from other well-established AEM systems flown under Australian conditions. We also present sections of AEM with project logged drilling core from both reverse circulation and diamond drill as means of cross validation of our derived inverted models with other data sets; which is always desirable.