Smooth and block inversion of surface NMR amplitudes and decay times using simulated annealing

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This paper introduces a new 1-D inversion scheme for surface nuclear magnetic resonance (SNMR) amplitudes and decay times, using the optimized random search algorithm simulated annealing (SA). As an alternative to the smooth inversion used in other SNMR inversion techniques, the new scheme can also use block inversion, similar to 1-D geoelectrics. In smooth inversion, the number and thickness of inversion layers as well as the degree and type of the smoothness constraint, for example, the parameter of regularization, is most decisive on the results of the inversion. Therefore, the results are highly ambiguous, depending on the type and degree of regularization. To improve the mapping of aquifers with sharp boundaries and to overcome the ambiguity of the smooth inversion, a different approach has therefore been chosen, using block inversion. Using both smooth and block inversions, respectively, the results for two synthetic sets of SNMR data and for field data from a site in northern Germany with well-known hydrogeological properties are discussed. The results of block inversion and smooth inversion using a regularization that favors more block-like structures correspond to the sharp aquifer boundaries of the synthetic models and the site. The conventional smooth inversion gives only a rough estimate of the aquifer location.