Enhancing SNMR model resolution by selecting an optimum combination of pulse moments, stacking, and gating

Authors: E. Dalgaard, M. Müller-Petke and E. Auken
Journal name: Near Surface Geophysics
Issue: Vol 14, No 3, June 2016 pp. 243 - 253
DOI: 10.3997/1873-0604.2016004
Language: English
Info: Article, PDF ( 2.02Mb )
Price: € 30

Summary:
For the surface nuclear magnetic resonance sounding method, we investigate the tradeoff between stacking and the number of different pulse moments by analyzing the a posteriori model covariance matrix. It is shown that a better determination of the model parameters is obtained by increasing the number of pulse moments compared with increasing the stack size until a certain point. From this point, the determination does not change. A distribution of pulse moments based on the resolution of the surface nuclear magnetic resonance sounding kernel is calculated and compared with a standard logarithmic distribution. Our results show that the resolution-derived distribution performs better overall, and the logarithmic distribution is only slightly better for very shallow layers. Finally, we use gating of the free induction decays in order to suppress noise, and we show that, by using a logarithmic distribution of gates, we obtain maximum resolution of the models by only seven gates per decay.