Semi-airborne electromagnetic exploration of deep sulfide deposits with UAV-towed magnetometers - Part 2: Inversion Resolution analysis

Modern measurement systems for geophysical exploration are increasingly based on uncrewed aerial vehicles. For performing semi-airborne electromagnetics, they offer, with total payload limitations of commonly 25 kg, a cost-efficient alternative to helicopters as a carrier for receiver systems with reduced logistical efforts and greater flexibility in survey layouts. We evaluate the inversion results of the controlled-source electromagnetic (EM) data recorded with two different sensors, a scalar magnetometer (SM) and a vector magnetometer (VM), acquired in two mining areas, namely, the Hope deposit in Namibia and the Poderosa mine in the Eastern Iberian pyrite belt, Spain. The SM is more sensitive at lower frequencies, whereas the VM is more sensitive at higher frequencies. The Hope demonstration site is easily accessible with almost no vegetation and topography, allowing for optimized regular survey layouts and dense data sets to cover the region of interest. In contrast, the Poderosa test site is characterized by poor accessibility, topographic undulations up to 300 m, and thick vegetation. For simulating and inverting scalar data projected to the local total magnetic field direction, we consider a resource-saving simulation approach using mesh rotation. We further extend the capabilities of the open-source tools custEM/pyGIMLi to invert the combined data sets of both receiver systems in 2.5D and three dimensions. As the two applied sensors are sensitive in different frequency ranges, combining the two data sets in inversion with a common frequency range of 1–1024 Hz provides the advantage of enhanced near-surface resolution together with maximum investigation depths up to 1 km. We assess the reliability of the inversion results by performing misfit and resolution analysis as well as comparing the inverted conductive zones with additional information about the two deposits. The results from both test sites demonstrate the capability of our measurement design to recover comparatively small but elongated conductors associated with known massive sulfide deposits.