Imaging shallow three dimensional water-bearing structures using magnetic resonance tomography

Abstract

The technique of surface NMR has been applied to image 1D, 2D and recently 3D subsurface structures. Although limited resolution is reported for imaging deep 3D structures using a coincident loop configuration, high resolution is obtained for shallow 2D structures by including separated loop configurations. We adapt the concept of separated transmitter and receiver loops to obtain increasing resolutions for imaging 3D shallow structures. We present a numerically efficient approach to calculate the 3D kernel with sufficient accuracy and a small number of elements. Using synthetic data, we show that including separated loop layouts enhances the 3D image reconstruction. To evaluate our 3D inversion approach, a field campaign including surface NMR with various layouts and ground penetration radar (GPR) measurements was conducted on top of a frozen artificial barrier lake in the Harz Mountains (Germany, Lower Saxony) with a well known geometry. We show that the results obtained from measurements using the coincident loop layout give a rough approximation of the lake's bathymetry, and including separated loop layouts provides a more detailed view into the subsurface. In particular, the obtained image matches not only the known water content of 100%, but the geometry known from construction plans and estimated from GPR profiles. In addition to the 3D assessment, a 2D profile is extracted from the 3D dataset to demonstrate the need for 3D inversion.