Correlating intensity of pulse moment with exploration depth in surface NMR

Abstract Because of its selective sensitivity to groundwater, surface nuclear magnetic resonance (SNMR) is a method widely applied in hydrogeology and engineering geology. However, the accurate numerical relationship between pulse moment intensity and exploration depth of this method has lacked research. Optimum pulse moment is a concept proposed to describe this correlation. By analyzing, using least-square fitting, various factors that may influence this correlation, an empirical relation for the optimum pulse moment has been obtained. The relation indicates a well-known law: to get a deeper exploration depth, a larger optimum pulse moment is needed. Moreover, from the relation, we found that the increase rate of exploration depth diminishes with the pulse moment increasing. Additionally, the correlation illustrates that the optimum exploration depth of the surface NMR method is not simply proportional to the loop diameter. Hence, to further extend the exploration depth, both the pulse moment intensity and the size of transmitter loop need to be increased simultaneously. Based on a synthetic example, we demonstrate that the optimum pulse sequence which is established from our empirical formula has a better inversion result and higher signal-to-noise ratio (SNR) when the probable depth range of the target aquifer is known.