Abstract¶
The expensive and time-consuming nature of magnetotelluric (MT) surveys has motivated the development of airborne natural source EM (NSEM) systems, which includes Z-axis Tipper EM (ZTEM), quantum audio magnetotellurics (QAMT) and MobileMT. These systems compute transfer functions from airborne magnetic data, and horizontal field measurements at a base station located on the Earth’s surface Available literature offering in-depth analysis of the factors that influence airborne NSEM data and inversion results remains sparse; especially for systems that measure electric fields at the base station. In our work, we characterize the nature of QAMT data, and by extension MobileMT data. We demonstrate the impact of the conductivity at the base station on ZTEM and QAMT anomalies. And we investigate the impact of the starting and reference model on ZTEM and QAMT inversion results when the conductivity at the base station differs significantly from the host conductivity within the survey region. Our analysis determined that QAMT data are directly sensitive to the conductivity at the base station, and that QAMT anomalies are produced by anomalous magnetic fields arising from 3D structures within the survey region. Like ZTEM, models recovered through QAMT inversion depend significantly on the choice in starting and reference models. When the conductivity near the base station differs significantly from the background conductivity within the survey region, target structures are likely recovered erroneously. When the true host conductivity within the survey region is used as the starting and reference models, both ZTEM and QAMT inversions recover conductive and resistive structures appropriately regardless of base station conductivity. However structures are also recovered near the base station. And these structures likely assist in fitting signatures produced by targets within the survey region, thus reducing our confidence in the recovered model.
