Interpretation of Prepolarization SNMR Based on Instantaneous Polarization and Its Application in Urban Environment

The surface nuclear magnetic resonance (SNMR) technique is the only method that can directly detect aquiferous layers. By detecting and interpreting multiple parameters such as water content and relaxation time, it can provide rich information related to pore structure and hydraulic conductivity, and directly and effectively predict water source geological hazards such as groundwater outbursts and collapse of cavern areas. The conventional SNMR method has a weak signal, so the prepolarization (PP) SNMR signal enhancement technique has been rapidly developed to improve the adaptability of this method in urban and underground engineering fields. However, the present PP SNMR modeling and data interpretation methods are based on the conventional steady-state background field SNMR theory, and their accuracy is limited. In this article, the researchers construct an instantaneous polarization (IP) model for the accurate interpretation of the PP SNMR response through an in-depth analysis of the active PP field source properties, which not only effectively improves the inversion accuracy of the subsurface aquiferous layer, but also helps to separate the longitudinal relaxation parameters of the aquiferous layer carried by the variable PP field itself. The method is used for the interpretation of experimental data from known urban hydrological sites, and the results are generally consistent with the real situation, which confirms the effectiveness of the method proposed in this article.