Full Envelope Inversion of Surface-NMR Data for Retrieval of Hydro-geophysical Characterization

In contrast to other geophysical approaches which only provide information on the lithological characteristics of aquifers indirectly, the surface-NMR technique yields hydro-geophysical parameters such as water content and relaxation time. In surface-NMR inversion process, from data space point of view, two inversion techniques (i.e., initial amplitude inversion (IAI) and time step inversion (TSI)) exist. In IAI, the initial amplitudes derived from free inductions decays are used to estimate water content. In TSI, the data set transferred into a water-content domain followed by mono-exponential fitting to the water-content domain to both estimate water content and relaxation time as a function of depth. Both methods take advantage of sub-data sets instead of the full data. In this paper, a novel inversion algorithm based on the application of the full decay cube, including the entire times and pulse moments, to determine simultaneously water content and relaxation time using mono-or multi-exponential behavior, is developed. To verify and compare the functionality of the inversion algorithms, some numerical experiments using synthetic and real data set are provided. The numerical results reveal that the comprehensive inversion algorithm outperforms IAI in terms of stability and resolution properties. The results indicated that using full data set yields improved stability and depth resolution compared to those of sub-data set.