Modeling of Surface Nuclear Magnetic Resonance Based on Prepolarization and Its Application in Urban Shallow Measurements

Surface nuclear magnetic resonance (SNMR) technology is a geophysical method to directly measure the water content and saturated porosity of aquifers by exciting the nuclear magnetic resonance (NMR) phenomenon of hydrogen nuclei in groundwater. With the natural Earth magnetic field B-0 as the background detection field, the SNMR signals, which usually exist only within a range of tens of nanovolts, are very likely to be submerged in widespread environmental noise. The prepolarization (PP) method with artificial application of an active field to enhance NMR signals has been gradually applied to the SNMR field, and this method is expected to overcome the existing problems and to gain further development in more high-noise detection environments, e.g., urban engineering detection. However, when PP is introduced to SNMR, difficulties in data interpretation focus on the theoretical formula derivation and model construction. In this article, we studied the state of the detected target under a PP field. Based on currently available measurement processes, we established the corresponding PP-SNMR forward equations and modeling for groundwater. It was proven by simulation and actual measurements that the research in this study can interpret actual field detection data. In addition, compared with the conventional PP-SNMR theory, the proposed improved method can effectively avoid detected aquifer misestimation. The results gained in this study compensate for the shortcomings of the current PP-SNMR theory, which is of significance to the development and application of high-power PP-SNMR technology.