Torus-nuclear magnetic resonance: Quasicontinuous airborne magnetic resonance profiling by using a helium-filled balloon

The application of surface nuclear magnetic resonance (NMR) measurements, although proven to be valuable for various hydrogeological issues, is in practice often limited to single 1D surveys because measurement progress is slow. First, large stacking rates are necessary to overcome the low signal-to-noise ratio and, second, time and effort are required to move the equipment and to place the measurement loops in the field. We have evaluated a novel approach to cope with the latter. We have assessed and tested a data acquisition scheme based on a torus-shaped helium-filled balloon carrying the loop and moving it rapidly from one to the next measurement position along the profile lines. We have referred to this system as Torus-NMR. We have showed the feasibility of the system to deliver reliable results using common processing and inversion. Surface NMR field data are acquired at successively overlapping positions along a test profile using the Torus-NMR system. To take advantage of the faster loop setup, the overall measurement time of the single NMR soundings is reduced by reducing the number of stacks, whereas the number of soundings is increased to obtain highly overlapping coverage. Regarding a single measurement position, a significant loss of vertical resolution must be accepted that can, however, be compensated to some extent by inverting the complete profile data set simultaneously using a laterally constrained inversion. We have found that for simple subsurface models, e.g., a layered earth with up to three layers, the proposed scheme provided reasonable results, which were demonstrated using synthetic and real field data. We do not expect the Torus-NMR concept to replace 2D data acquisition schemes if high spatial resolution is required. However, we expect fields of application that aim at rapid lateral overview of how specific hydrogeological parameters of shallow aquifers are distributed over large catchment-scale areas.